Abdulai, A.L. Parzies, H., Kouressy, M., Vaksmann, M., Asch, F., Brueck, H., 2012
Yield stability of photoperiod-sensitive sorghum [Sorghum bicolor L. (Moench)] accessions under diverse climatic environments. International Journal of Agricultural Research, 7, 17-32.

Abstract
Climate variability is a characteristic feature of the tropics where the summer monsoon starts from MaylJune and ends mostly in October, thus producing an unprectably variable length of growing season. This results in serious challenges for the mainly subsistent small holder farmers in the arid to semi-arid zones of the tropics. A study was conducted to determine the attainable grain yield and yeld stability of 10 well characterized and extensively cultivated tropical sorghum accessions across 18 environments comprised of 3 dates of sowing at 3 sites (along a latitudinal gradient covering 3 agro-ecolopcal zones) over 2 years in Mali. For each year and site combination, sorghum accessions and dates of sowing were arranged in a split plot and tested in a Randomized Complete Block (RCB) design. Appropriate cultural practices and timing were used to minimize effects of biotic factors. In addition to Grain yeld, yeld penalty associated with delayed sowing was determined. Two static and five dynamic indices were used to assess the stability of grain yeld for genotypes across environments. Mean grain yeld ranged from 0 to 248 g m^-2 across environments, from 74 to 208 g m^-2 across the 10 genotypes and generally reduced with delayed sowing. A genotype combining photoperiod sensitivity and stay-green traits was revealed as the most stable. The similarities and hfferences were observed among the stability indices used in terms of ranking of the genotypes. Implications of these for adaptation to climate change are discussed

Rajaona, A., Brueck, H., Asch, F. 2011
Effect of pruning history on growth and dry mass partitioning of Jatropha curcas L. on a plantation site in Madagascar. Biomass and Bioenergy 35, 4892-4900.

Abstract
While technical aspects of oil processing of seeds of jatropha are under intensive investigation, comparably little is known about the performance of jatropha in the field. We investigated the effects of water availability (rainfed versus irrigated) and pruning-induced differences in plant stature on growth, biomass partitioning, and canopy size at a plantation site in Madagascar in 2010. Plants of different pruning types differed in trunk height (43 versus 29 cm) and primary branches total length (171 versus 310 cm). The two pruning types had effects on dry mass formation and leaf area projection (LAP) during the vegetation period. Trees which had a shorter trunk and longer lateral branches produced more biomass and had a higher LAP. Total dry mass formation varied from 489 to 912 g m−2 and LAP from 3.26 to 7.37. Total aboveground biomass increased from 2.3 ± 0.5 to 4.89 ± 1.4 kg tree−1 and from 4.6 ± 1.8 to 8.9 ± 1.0 kg tree−1 for the pruning types with shorter and longer lateral branches, respectively. Growth of twigs and leaves was positively correlated with total length of branches. Relative dry mass allocation to branches, twigs and leaves, length of twigs per cm of branches and specific leaf area (13.57 ± 0.72 m2 kg−1) were not affected by pruning and water supply. Trees with shorter branches had higher LAD. Results indicate that pruning type should be considered as a management tool to optimize biomass production. Detailed studies on effects of canopy size and shape on radiation interception and growth are required to improve the productivity of jatropha.
Full paper here

Asch, F., Brueck, H. 2011
Rice crop innovations and natural-resource management — A glimpse into the future. Africa Rice Congress, Bamako 2010 – Proceedings –

Abstract
Rice is and will be the major global food crop. Cultivars, rice-based cropping systems and the rice itself will have to undergo adaptations and improvements in order to meet future demands for both food security of the growing population and environmental conservation. Growing more food will increase the pressure on natural resources such as land, water and nutrients, which must be used efficiently and sustainably. The challenge posed by imminent climate change forces the speeding-up of the innovation process, which will require collaboration by a large number of scientific disciplines and stakeholders. Rice’s path into the future will have to follow several parallel lanes. On one hand, we cannot slacken our efforts to improve existing cropping-systems management to decrease the gap between potential and current productivity. On the other hand, we need to increase our knowledge base of the genomic, proteomic and metabolic make-up of rice to pave the way for future innovations through genetic-engineering based on in-depth knowledge of physiological processes. A third highly important approach is to maximize productivity in clearly defined high-input environments, such as irrigated rice and intensive rainfed production, using a strong systems approach. Another parallel approach must focus on the low-intensity production systems and those environments most vulnerable to changes in climate. Here, in contrast to the intensive systems, genotypic elasticity and region-specific management options need to be exploited to ensure a secure level of production in highly variable environments and those undergoing transition. Finally, existing networks addressing some or all of these options should be more tightly knit to increase information flow among, and the innovative power of, the scientists involved. This includes a strong focus on scientific capacity-building through North–South collaboration in research and education, with a strong role of the Consultative Group on International Agricultural Research in streamlining the combined efforts.
Full paper here

Gorim, L., Asch F., 2011
Effects of composition and share of seed coatings on the mobilisation efficiency of cereal seeds during germination. Journal of Agronomy and Crops Science - DOI: 10.1111/j.1439-037X.2011.00490.x.

Abstract
Cereal production systems are increasingly threatened by suboptimal water supply or intermittent drought spells early in the planting season. Seed coated with hydrophilic materials or hydro-absorbers that increase the amount of water available for germination and seedling development is a promising approach to improving stand establishment under changing conditions. Barley, rye and wheat grains with combinations of hydro-absorber, humic acid and Biplantol® in different shares of the total seed mass were germinated in plates at 25 °C on moist filter paper. Germination rates, resource partitioning and mobilization efficiency were assessed and compared with those of uncoated seeds. Results show a strong influence of coat thickness and composition on the germination rate and the efficiency of mobilization of carbohydrates stored in the endosperm. In general, coating significantly reduced germination rate and total germination as compared to uncoated seeds in all cereals tested. Differences in coating thickness had a distinct effect on germination rate for most combinations of coatings and species. Germination rates increased with increasing coat size. This effect was most pronounced for coatings containing hydro-absorbers and least pronounced for coatings containing humic acid or Biplantol®. Coating generally increased the amount of carbohydrates partitioned to the roots, and thick coating increased the efficiency of grain reserve mobilization compared with the uncoated seeds. Differences between species and the implications for coating-related changes in germination metabolism are discussed.
Link to full paper online

Germer, J., Sauerborn, J., Asch, F., de Boer, J., Schreiber, J., Weber, G., Müller, J., 2011
Skyfarming an ecological innovation to enhance global food security. Journal für Verbraucherschutz und Lebensmittelsicherheit - DOI 10.1007 /s00003-011-0691-6

Abstract
Population growth increases the demand for food and thus leads to expansion of cultivated land and intensification of agricultural production. There is a definite limit to both of these options for food security and their multiple negative effects on the environment undermine the aim for sustainability. Presently the impact of the Green Revolution on crop production is levelling off at high yields attained and even the potential of large scale irrigation programmes and transgenic crops seem to be limited in view of the expected increase in demand for food. Moreover, climate change threatens to affect agricultural production across the globe.
Skyfarming represents a promising approach for food production that is largely environment independent and therefore immune to climate change. Optimal growing conditions, shielded from weather extremes and pests are aimed at raising plant production towards the physiological potential. Selecting rice as a pioneer crop for Skyfarming will not only provide a staple for a large part of the global population, but also significantly reduce the greenhouse gas emission caused by paddy cultivation. Multiplication of the benefits could be achieved by stacking production floors vertically.
In Skyfarming the crop, with its requirements for optimal growth, development and production, determines the system's design. Accordingly, the initial development must focus on the growing environment, lighting, temperature, humidity regulation and plant protection strategies as well as on the overall energy supply. For each of these areas potentially suitable technologies are presented and discussed. Paper here online

Shrestha, S., Asch, F., Dingkuhn, M., Becker, M., 2011.
Cropping calendar options for rice – wheat production systems at high-altitudes. Field Crops Research 121, 158-167

Abstract
The onset of rains during dry to wet transition fallow periods in rice-wheat production systems in Nepal cause substantial losses of soil nitrogen if the system is improperly managed. To make use of available nutrients and water, this transition period can either be shortened by early rice planting, or extended by late planting, allowing a third crop to be grown. Shifting planting dates would require rice genotypes adapted to the different environment. Crop duration is influenced by both vegetative and reproductive development, which in turn is influenced by the photothermal environment and genotypic responses to it. An experiment was conducted to derive genotypic photo-thermal constants from phenological observations on diverse rice cultivars, which were then applied to the concept of the phenological model RIDEV to design cropping calendar options. Environmental effects on variation of crop duration were determined by planting at different dates. The risk of yield losses to sterility caused by low temperatures was estimated by simulation. Thirty-one different genotypes of rice were planted at 8 dates in 15-day intervals starting 27 April 2004 at the experimental field of the Regional Agriculture Research Station, Lumle, Nepal. The shortest duration to flowering was observed for planting dates in late May and early June. Simulation of flowering dates with RIDEV yielded correct results only for the early planting dates. For later planting dates simulated flowering dates showed an increasing deviation from the observed. In most cultivars, minimum air temperature below 18°C during booting to heading stages caused near-total spikelet sterility and a specific delay in flowering. However, the chilling tolerant cultivars Chomrong and Machhapuchre-3 cultivated at high altitude showed less than 30% spikelet sterility even at 15°C. Simulating crop durations with the derived thermal constants allowed evaluating the different calendar options for high altitude systems.

Tatar, Ö., Brueck, H., Gevrek, M.H., Asch, F., 2010.
Physiological responses of two Turkish rice (Oryza sativa L.) varieties to salinity. Turkish Journal of Agriculture and Forestry 34 (6), 451-459

Abstract
Effects of salinity (60 mM NaCl) on two Turkish rice varieties (Kıral and Yavuz) were studied in comparison to two international check varieties (IR4630-22-2 tolerant and IR31785-58-1-2-3-3 susceptible) in two hydroponic and one out-door soil-based pot experiments. Partitioning of dry matter, leaf chlorophyll concentration, K+ and Na+ uptake, and proline accumulation in the leaves were investigated under both control and salt stress conditions. Dry matter accumulation decreased with salt stress in all varieties with the decreases being more pronounced in IR31785 and Kıral. The results show that the varieties tested expressed different adaptation mechanisms under salt stress, however the increase of leaf proline accumulation was a general indicator for responses to salt stress in all cultivars. - full paper here online -

Dimpka, C., Weinand, T., Asch, F., 2009.
Plant–rhizobacteria interactions alleviate abiotic stress conditions. Plant, Cell, and Evironment, 32 (12), 1682-1694

Abstract
Root-colonizing non-pathogenic bacteria can increase plant resistance to biotic and abiotic stress factors. Bacterial inoculates have been applied as biofertilizers and can increase the effectiveness of phytoremediation. Inoculating plants with non-pathogenic bacteria can provide ‘bioprotection’ against biotic stresses, and some root-colonizing bacteria increase tolerance against abiotic stresses such as drought, salinity and metal toxicity. Systematic identification of bacterial strains providing cross-protection against multiple stressors would be highly valuable for agricultural production in changing environmental conditions. For bacterial cross-protection to be an effective tool, a better understanding of the underlying morphological, physiological and molecular mechanisms of bacterially mediated stress tolerance, and the phenomenon of cross-protection is critical. Beneficial bacteria-mediated plant gene expression studies under non-stress conditions or during pathogenic rhizobacteria–plant interactions are plentiful, but only few molecular studies on beneficial interactions under abiotic stress situations have been reported. Thus, here we attempt an overview of current knowledge on physiological impacts and modes of action of bacterial mitigation of abiotic stress symptoms in plants. Where available, molecular data will be provided to support physiological or morphological observations. We indicate further research avenues to enable better use of cross-protection capacities of root-colonizing non-pathogenic bacteria in agricultural production systems affected by a changing climate.

Asch, F., Huelsebusch, Chr., 2009.
Agricultural research for the Tropics: caught between energy demands and food needs. Journal of Agriculture and Rural Development in the Tropics and Subtropics 110 (1), 75–91

Abstract
The use of plant biomass for fuel is almost as old as mankind. However, a continuously growing population and the increasingly rapid exploitation of both fossil fuels and natural resources such as soil, water and biodiversity, have stimulated a debate of how to balance the needs and demands for food, feed, non-food raw materials and most recently energy in agricultural systems. Against the background of the current population growth, mankind faces the problem that the global system is closed and the available resources are finite. Energy is the only resource constantly supplied to the system from outside. All energy resources available on earth are in one way or the other transformations
of one of the four following: a) solar energy - which can be exploited directly, is transformed into biomass by photosynthesis, and drives the global wind and water cyle, b) tidal force owing to gravitational pull between earth and moon, c) the earth’s internal heat exploited as geothermic energy and d) nuclear energy. Of these, solar, tidal and geothermic energy are energy sources, which are not finite in time periods humans can still grasp. Based on data on fossil fuel reserves and consumption figures....(continue)

Asch, F., Bahrun, A., Jensen, C.R., 2009.
Root–shoot communication of field-grown maize drought-stressed at different rates as modified by atmospheric conditions. Journal of Plant Nutrition and Soil Science 172, 678-687

Abstract
Maize is often grown in drought-prone environments and, thus, drought resistance is an important trait. In order to minimize production losses, plants need to respond and adapt early and fast to moisture loss in the root zone. From experiments under controlled conditions, constituents of the xylem sap, such as the plant hormone abscisic acid (ABA), or xylem pH have long been recognized to act as signals in root-shoot communication. To investigate early signals of field-grown maize under conditions of progressive drought, a field trial was set up in a field lysimeter for two consecutive years. Although the experimental set-up was very similar in the two years, plant responses to moisture loss were significantly different in both, the cascade of events and the intensity of responses. The main difference between the two years was in atmospheric vapor-pressure deficit (VPD), accelerating the drying rate of the soil in the second year. In contrast to observations during the first year, the sudden increase in VPD in the second year caused a strong, transient peak in xylem sap ABA concentration, but no change in xylem pH or leaf ABA concentration was observed. Whereas the water relations of the maize plants remained stable in the first year, they were severely unbalanced in the second. It is argued that the strong xylem-ABA signal triggered a change from adaptation mechanisms to survival mechanisms. Modulations due to VPD of constituents of the signal cascade induced by drought are discussed with regard to possible resistance strategies, their initiation, and their modification by combining primary environmental signals.

Badridze, G., Weidner, A., Asch, F., Börner, A., 2009.
Variation in salt tolerance within a Georgian wheat germplasm collection. Genetic Resources and Crop Evolution, 56 (8), 1125-1130.

Abstract
Bread wheat Triticum aestivum L. possesses a genetic variation for the ability to survive and reproduce under salt stress conditions. Durum wheat (T. durum Desf.) is in general more sensitive in comparison to bread wheat, however, exceptions can be found showing the same extent of salt tolerance. Endemic wheats in general are characterised by a high adaptability to their environment. The level and variability of salt tolerance were assessed in a germplasm collection of 144 winter and spring wheat accessions from Georgia comprising Triticum aestivum L., T. durum Desf., T. dicoccon Schrank, T. polonicum L. and Georgian endemics: T. carthlicum Nevski, T. karamyschevii Nevski, T. macha Dekapr. et Menabde, T. timopheevii (Zhuk.) Zhuk. and T. zhukovskyi Menabde et Ericzjan. The accessions were tested for salt tolerance at the germination stage. Large variability in salt tolerance within the Georgian germplasm was found among the different wheat species. The endemic hexaploid winter wheat T. macha and the endemic tetraploid wheat T. timopheevii were among the most tolerant materials, thus presenting promising donors for salt tolerant traits in future breeding efforts for salinity tolerance in wheat.

Becker, M., Asch, F., Chiem, N. H., Ni, D. V., Saleh, E., Tanh, K. V., Tinh, T. K. 2008.
Decomposition of organic substrates and their effect on mungbean growth in two soils of the Mekong Delta. J. Agric. Rural Dev. Trop. Subtr. 109, 95-107. 

Abstract
Agricultural land use in the Mekong Delta of Vietnam is dominated by intensive irrigated rice cropping systems on both alluvial and acid sulfate soils. A generally observed decline in productivity is linked on the alluvial soils to low N use efficiency and low soil organic matter content while on acid sulfate soils to acidity, Al toxicity and P deficiency. Faced with productivity declines, farmers increasingly diversify their cropping system by replacing the dry season rice by high-value horticultural crops grown under upland conditions. However, upland cropping is likely to further exacerbate the soil-related problems. Organic substrates from decentralized waste/water management are widely available and may help alleviate the reported soil problems. During the dry season of 2003/2004, the effect of the application of various types and rates of locally available waste products on crop performance was evaluated at both an alluvial and an acid sulfate soil site. The C and N mineralization dynamics of nine organic substrates from waste and wastewater treatment were determined by anaerobic (N) and aerobic (C) incubation in the laboratory. The response of diverse 12 week-old field-grown upland crops (dry matter accumulation) to substrate application (1.5 – 6.0 Mg ha^-1) was evaluated on in a degraded alluvial and an acid sulfate soil. In the alluvial soil, largest mineralization rates were observed from anaerobic sludge. Biomass increases in 12 week-old upland crops ranged from 25-98% above the unfertilized control and were generally highest with legumes and lowest with vegetables. In the acid sulfate soil, highest net-N release rates were observed from aerobic composts with high P content. Crop biomass was related to soil pH and exchangeable Al³⁺ and was highest with the application of aerobic composts, with vegetables responding more than tubers or legumes. We conclude that the use of organic substrates in the rice-based systems of the Mekong Delta needs to be soil specific.

Becker, M., Asch, F., Maskey, S. L., Pande, K. R., Shah, S. C., Shresth, S. 2007. Effects of transition season management on soil N dynamics and system N balances in rice–wheat rotations of Nepal. Field Crops Research 103, 98-108.

Abstract
In the low-input rice–wheat production systems of Nepal, the N nutrition of both crops is largely based on the supply from soil pools. Declining yield trends call for management interventions aiming at the avoidance of native soil N losses. A field study was conducted at two sites in the lowland and the upper mid-hills of Nepal with contrasting temperature regimes and durations of the dry-to-wet season transition period between the harvest of wheat and the transplanting of lowland rice. Technical options included the return of the straw of the preceding wheat crop, the cultivation of short-cycled crops during the transition season, and combinations of both. Dynamics of soil N_min, nitrate leaching, nitrous oxide emissions, and crop N uptake were studied throughout the year between 2004 and 2005 and partial N balances of the cropping systems were established. In the traditional system (bare fallow between wheat and rice) a large accumulation of soil nitrate N and its subsequent disappearance upon soil saturation occurred during the transition season. This nitrate loss was associated with nitrate leaching (6.3 and 12.8 kg ha⁻¹ at the low and high altitude sites, respectively) and peaks of nitrous oxide emissions (120 and 480 mg m⁻² h⁻¹ at the low and high altitude sites, respectively). Incorporation of wheat straw at 3 Mg ha⁻¹ and/or cultivation of a nitrate catch crop during the transition season significantly reduced the build up of soil nitrate and subsequent N losses at the low altitude site. At the high altitude site, cumulative grain yields increased from 2.35 Mg ha⁻¹ with bare fallow during the transition season to 3.44 Mg ha⁻¹ when wheat straw was incorporated. At the low altitude site, the cumulative yield significantly increased from 2.85 Mg ha⁻¹ (bare fallow) to between 3.63 and 6.63 Mg ha⁻¹, depending on the transition season option applied. Irrespective of the site and the land use option applied during the transition season, systems N balances remained largely negative, ranging from −37 to −84 kg N ha⁻¹. We conclude that despite reduced N losses and increased grain yields the proposed options need to be complemented with additional N inputs to sustain long-term productivity.

Becker, M., Asch, F. 2005. Iron toxicity in rice -  conditions and management concepts. J. Plant Nutri. Soil Sci. 168, 558-573.

Abstract
Iron toxicity is a syndrome of disorder associated with large concentrations of reduced iron (Fe²⁺) in the soil solution. It only occurs in flooded soils and hence affects primarily the production of lowland rice. The appearance of iron toxicity symptoms in rice involves an excessive uptake of Fe²⁺ by the rice roots and its acropetal translocation into the leaves where an elevated production of toxic oxygen radicals can damage cell structural components and impair physiological processes. The typical visual symptom associated with these processes is the bronzing of the rice leaves and substantial associated yield losses.
The circumstances of iron toxicity are quite well established. Thus, the geochemistry, soil microbial processes, and the physiological effects of Fe²⁺ within the plant or cell are documented in a number of reviews and book chapters. However, despite our current knowledge of the processes and mechanisms involved, iron toxicity remains an important constraint to rice production, and together with Zn deficiency, it is the most commonly observed micronutrient disorder in wetland rice. Reported yield losses in farmers' fields usually range between 15% and 30%, but can also reach the level of complete crop failure.
A range of agronomic management interventions have been advocated to reduce the Fe²⁺ concentration in the soil or to foster the rice plants' ability to cope with excess iron in either soil or the plant. In addition, the available rice germplasm contains numerous accessions and cultivars which are reportedly tolerant to excess Fe²⁺. However, none of those options is universally applicable or efficient under the diverse environmental conditions where Fe toxicity is expressed. Based on the available literature, this paper categorizes iron-toxic environments, the steps involved in toxicity expression in rice, and the current knowledge of crop adaptation mechanisms in view of establishing a conceptual framework for future constraint analysis, research approaches, and the targeting of technical options.
- full paper here online -

Asch, F., Becker, M., Kpongor, D. S. 2005. A quick and efficient screen for resistance to iron toxicitiy in lowland rice. J. Plant Nutri. Soil Sci. 168, 764-773.

Abstract
Iron (Fe) toxicity is a major stress to rice in many lowland environments worldwide. Due to excessive uptake of Fe²⁺ by the roots and its acropetal translocation into the leaves, toxic oxygen radicals may form and damage cell structural components, thus impairing physiological processes. The typical visual symptom is the bronzing of the rice leaves, leading to substantial yield losses, particularly when toxicity occurs during early vegetative growth stages. The problem is best addressed through genotype improvement, i.e., tolerant cultivars. However, the time of occurrence and the severity of symptoms and yield responses vary widely among soil types, years, seasons, and genotypes. Cultivars resistant in one system may fail when transferred to another. Better targeting of varietal improvement requires selection tools improving our understanding of the resistance mechanisms and strategies of rice in the presence of excess iron. A phytotron study was conducted to develop a screen for seedling resistance to Fe toxicity based on individual plants subjected to varying levels of Fe (0-3000 mg L^-1 Fe supplied as Fe(II)SO₄), stress duration (1-5 d of exposure), vapor-pressure deficit (VPD; 1.1 and 1.8 kPa), and seedling age (14 and 28 d). Genotypes were evaluated based on leaf-bronzing score and tissue Fe concentrations. A clear segregation of the genotypic tolerance spectrum was obtained when scoring 28 d old seedlings after 3 d of exposure to 2000 mg L^-1 Fe in a high-VPD environment. In most cases, leaf-bronzing scores were highly correlated with tissue Fe concentration (visual differentiation in includer and excluder types). The combination of these two parameters also identified genotypes tolerating high levels of Fe in the tissue while showing only few leaf symptoms (tolerant includers). The screen allows selecting genotypes with low leaf-bronzing score as resistant to Fe toxicity, and additional analyses of the tissue Fe concentration of those can identify the general adaptation strategy to be utilized in breeding programs.
- full paper here online -

Asch, F., Dingkuhn, M., Sow, A., Audebert, A. 2005. Drought-induced changes in rooting patterns and assimilate partitioning between root and shoot in upland rice. Field Crops Research 93, 223-236.

Abstract
Drought is a major stress affecting rainfed rice systems. Root characteristics such as root length density, root thickness, and rooting depth and distribution have been established as constituting factors of drought resistance. Deep rooting cultivars are more resistant to drought than those with shallow root systems. The present study sought to quantify the effects of different levels of drought on dry matter partitioning and root development of three rice cultivars (CG14 [Oryza glaberrima], WAB56-104 [O. sativa tropical japonica, improved] and WAB450-24-3-2-P18-HB [CG14 x WAB56-104 hybrid]. Two experiments on assimilate partitioning under different levels of drought stress were conducted under rain shelters at the West Africa Rice Development Association, Mbe, Ivory Coast. PVC tubes (diameter = 0.2m, height 0.6m) containing about 25 kg of sandy loam were used for the drought stress experiments. For rooting depth and root distribution studies, the tubes were subdivided into four compartments of  0.15 m each. In the first trial, tubes with WAB56-104 were gradually droughted to five levels of soil moisture content that were kept constant thereafter. In the second trial, plants of all cultivars were subjected to three drought treatments: (1) constant soil moisture content at field capacity (about 22% moisture content), (2) constant soil moisture content of 14% (about –0.5 MPa soil matrix potential) and (3) constant soil moisture content of 9% (about –1 MPa soil matrix potential). Rice reacted to drought stress with reductions in height, leaf area and biomass production, tiller abortion, changes in root dry matter and rooting depth and a delay in reproductive development. Assimilate partitioning between root and shoot, determined from changes in dry matter, was not affected by drought when the plants were gradually stressed. In no case additional biomass was partitioned to the roots, on the contrary, dry matter partitioning to the root completely ceased under severe stress. Due to the irrigation technique used, vertical soil moisture distribution varied little, but roots grew deeper under drought stress. This was particularly the case for the upland adapted WAB56-104. Implication for modeling of drought responses in upland rice systems are discussed.

Andersen, M. N., Asch, F., Wu, Y., Jensen, C. R., Naested, H., Mogensen, V. O., Koch, K. E., 2002. Soluble Invertase Expression Is an Early Target of Drought Stress during the Critical, Abortion-Sensitive Phase of Young Ovary Development in Maize. Plant Physiol. 130, 591-604.

Abstract
To distinguish their roles in early kernel development and stress, expression of soluble (Ivr2) and insoluble (Incw2) acid invertases was analyzed in young ovaries of maize (Zea mays) from 6 d before (-6 d) to 7 d after pollination (+7 d) and in response to perturbation by drought stress treatments. The Ivr2 soluble invertase mRNA was more abundant than the Incw2 mRNA throughout pre- and early post-pollination development (peaking at +3 d). In contrast, Incw2 mRNAs increased only after pollination. Drought repression of the Ivr2 soluble invertase also preceded changes in Incw2, with soluble activity responding before pollination (-4 d). Distinct profiles of Ivr2 and Incw2 mRNAs correlated with respective enzyme activities and indicated separate roles for these invertases during ovary development and stress. In addition, the drought-induced decrease and developmental changes of ovary hexose to sucrose ratio correlated with activity of soluble but not insoluble invertase. Ovary abscisic acid levels were increased by severe drought only at -6 d and did not appear to directly affect Ivr2 expression. In situ analysis showed localized activity and Ivr2 mRNA for soluble invertase at sites of phloem-unloading and expanding maternal tissues (greatest in terminal vascular zones and nearby cells of pericarp, pedicel, and basal nucellus). This early pattern of maternal invertase localization is clearly distinct from the well-characterized association of insoluble invertase with the basal endosperm later in development. This localization, the shifts in endogenous hexose to sucrose environment, and the distinct timing of soluble and insoluble invertase expression during development and stress collectively indicate a key role and critical sensitivity of the Ivr2 soluble invertase gene during the early, abortion-susceptible phase of development.
- full paper here online -

Bahrun, A., Jensen, C. R., Asch, F., Mogensen, V. O., 2002. 
Drought-induced changes of xylem pH, ionic composition, and [ABA] act as early signals in field-grown maize (Zea mays L.). Journal of Experimental Botany, 53, 251-263.

Abstract
Early signals potentially regulating leaf growth and stomatal aperture in field grown maize (Zea mays L.) subjected to drought were investigated. Plants grown in a field-lysimeter on two soil types were subjected to progressive drought during vegetative growth. Leaf ABA content, water status, extension rate, conductance, photosynthesis, nitrogen content, and xylem sap composition were measured daily. Maize responded similarly to progressive drought on both soil types. Effects on loam were less pronounced than on sand. Relative to fully-watered controls, xylem pH increased by about 0.3 units and conductivity decreased by about 0.25 mS cm^-1 one day after withholding irrigation (DAWI). Xylem nitrate, ammonium, and phosphate concentrations decreased by about 50 % at 3-5 DAWI, potassium concentration decreased by about 50 % at 7-11 DAWI and xylem ABA concentration increased by 40-60 pmol ml^-1 at 5-7 DAWI. Midday leaf water potential, photosynthesis and leaf nitrogen content were significantly decreased in droughted plants. Leaf extension rate decreased 5 DAWI, after the changes in xylem chemical composition had occurred. Xylem nitrate concentration was the only ionic xylem sap component significantly correlated to increasing soil moisture deficit and decreasing leaf nitrogen concentration. Predawn leaf ABA content in droughted plants increased by 100-200 ng g^-1 dry weight at 7 to 8 DAWI coinciding with a decrease in stomatal conductance before any significant decrease in midday leaf water potential was observed. Based on the observed sequence, a chain of signal events is suggested eventually leading to stomatal closure and leaf surface reduction through interactive effects of reduced nitrogen supply and plant growth regulators under drought.

Asch, F. Andersen, M. N., Jensen, C. R., Mogensen, V. O., 2001. 
Ovary Abscisic Acid Concentration does not Induce Kernel Abortion in Field-grown Maize Subjected to Drought. European Journal of Agriculture 15, 119-129.

Abstract
This study investigated the effects of drought of different duration and severity on ovary ABA concentration and yield components in field-grown maize (Zea mays L. cv. Loft). The study was conducted in a field lysimeter of the Royal Veterinary and Agricultural University (KVL) in Højbakkegaard (55°40' N; 12°18' E; 28 masl), Denmark in 1997. Irrigation was withheld at four different dates to induce drought of different duration and severity at the reproductive stage of the plants. Plots were re-watered shortly after silking and kept at field capacity for the remainder of the season. Soil water status, plant height, and early morning leaf water potential were monitored during the treatment. Ovary abscisic acid concentration was determined at four dates before and after fertilization. Final grain yield, total dry matter, harvest index, mean kernel weight, kernel weight distribution, and kernel number per cob were determined at maturity. Plant height was significantly (p < 0.05) reduced by 40% and 25% respectively in the two most severe drought treatments. In the two shorter drought treatments no effect of drought stress on plant height or biomass was observed. Leaf water potential decreased slowly as a function of relative available soil water content and resulted in -0.4 MPa at the end of the longest and -0.12 MPa at the end of the shortest stress period. Under fully watered conditions, plot yields averaged 1400 g·m^-2 for total dry matter (DM) and 700 g·m^-2 for grain yield, with a harvest index of about 0.5. Initiation of a drying cycle close to flowering did not change yields. Long drying cycles resulted in significant ( p< 0.05) yield reductions up to 70 % of the fully watered controls. Kernel number per cob was reduced up to 60% under long drought conditions and not affected under short term drought. Drought imposed about two weeks prior to fertilization resulted in 30% reduction in kernel number per cob, but this effect was balanced by an increase of 25 % in mean kernel weight. Long and severe drought increased ovary ABA concentration prior to fertilization, whereas short term drought did not. At fertilization no increase of ovary ABA as compared to fully watered controls was found in any treatment. It is concluded that drought induced grain yield losses in field grown maize cannot be attributed to kernel size reduction or kernel abortion due to ovary ABA concentrations as reported by some authors for studies on maize and wheat under controlled conditions, as ovary ABA concentrations peaked before zygote formation and endosperm development.

Asch, F., Wopereis, M. S. C., 2001. Responses of field-grown irrigated rice cultivars to varying levels of floodwater salinity under semi-arid conditions. Field Crops Research 70, 127-137.

Abstract
Shallow saline water tables, naturally saline soils and variations in climatic conditions over the two growing seasons, create a harsh environment for irrigated rice production in the Senegal River Delta. At the onset of the growing season, salts accumulated by capillary rise in the topsoil are released into the soil solution and floodwater. Rice fields often lack drainage facilities, or drain from one field to the other, thus building-up salt levels during the season. Salt stress may, therefore, occur throughout the growing season and may coincide with susceptible growth stages of the rice crop. The objectives of the present study were to (i) determine varietal responses to seasonal salinity in both the hot dry season (HDS) and the wet season (WS) and (ii) derive guidelines for surface water drainage at critical growth stages. We evaluated responses of three rice cultivars grown in the region, to floodwater salinity (0-1, 2, 4, 6, 8 mS cm-1), applied either at germination, during two weeks at crop establishment, during two weeks around panicle initiation, or during two weeks around flowering. Floodwater electrical conductivity (EC) reduced germination rate for the most susceptible cultivar by as much as 50% and yield by 80% for the highest salinity level imposed. . Salinity strongly reduced spikelet number per panicle, 1000 grain weight and increased sterility, regardless of season and development stage. The strongest salinity effects on yield were observed around panicle initiation (PI), whereas plants recovered best from stress at seedling stage. Floodwater EC < 2 mS cm-1 hardly affected rice yield. For floodwater EC levels > 2 mS cm-1, a yield loss of up to 1 t ha-1 per unit EC (mS cm-1) was observed for salinity stress around PI (at fresh water yields of about 8 t ha-1). Use of a salinity tolerant cultivar reduced maximum yield losses to about 0.6 t ha-1 per unit EC. It is concluded that use of salinity tolerant cultivars, drainage if floodwater EC > 2 mS cm-1 at critical growth stages, and early sowing in the wet season to avoid periods of low air humidity during the crop cycle, are ways to increase rice productivity in the Senegal River Delta.

Asch, F., Dingkuhn, M., Dörffling, K., 2000. 
Salinity increases CO₂ assimilation but reduces growth in field-grown, irrigated rice. Plant and Soil 218, 1-10.

Abstract
Salinity is a major yield-reducing factor in coastal and arid, irrigated rice production systems. Salt tolerance is a major breeding objective. Three rice cultivars with different levels of salt tolerance were studied in the field for growth, sodium uptake, leaf chlorophyll content, specific leaf area (SLA), sodium concentration and leaf CO₂ exchange rates (CER) at photosynthetic active radiation (PAR)-saturation. Plants were grown in Ndiaye, Senegal, at a research station of the West Africa Rice Development Association (WARDA), during the hot dry season (HDS) and the wet season (WS) 1994 under irrigation with fresh or saline water (flood water electrical conductivity = 3.5 mS cm^-1). Relative leaf chlorophyll content (SPAD method) and root, stem, leaf blade and panicle dry weight were measured at weekly intervals throughout both seasons. Specific leaf area was measured on eight dates, and CER and leaf sodium content were measured at mid-season on the first (topmost) and second leaf.
Salinity reduced yields to nearly zero and dry-matter accumulation by 90% for the susceptible cultivar in the HDS, but increased leaf chlorophyll content and CER at PAR-saturation. The increase in CER, which was also observed in the other cultivars and seasons, was explained by a combination of two hypotheses: leaf chlorophyll content was limited by the available N resources in controls, but not in salt-stressed plants; and the sodium concentrations were not high enough to cause early leaf senescence and chlorophyll degradation. The growth reductions were attributed to loss of assimilates (mechanisms unknown) that must have occurred after export from the sites of assimilation. The apparent, recurrent losses of assimilates, which were between 8% and 49% according to simulation with the crop model for potential yields in irrigated rice, ORYZA_S, might be partly due to root decomposition and exudation. Possibly more importantly, energy-consuming processes drained the assimilate supply, such as osmoregulation and interception of sodium and potassium from the transpiration stream in leaf sheaths, and subsequent storage there.

Asch, F., Dingkuhn, M., Miezan, K., Dörffling, K., 2000. 
Leaf K/Na ratio predicts salinity induced yield loss in irrigated rice. Euphytica 113, 109-118

Abstract
Salinity is a major constraint to irrigated rice production, particularly in semi-arid and arid climates. Irrigated rice is a well suited crop to controlling and even decreasing soil salinity, but rice is a salt-susceptible crop and yield losses due to salinity can be substantial. The objective of this study was to develop a highly predictive screening tool for the vegetative growth stage of rice to estimate salinity-induced yield losses.
Twenty-one rice genotypes were grown over seven seasons in a field trials in Ndiaye, Senegal, between 1991 and 1995 and were subjected to irrigation with moderately saline water (3.5 mS cm^-1, electrical conductivity) or irrigation with fresh water. Potassium/sodium ratios of the youngest three leaves (K/Na_Leaves) were determined by flame photometry at the late vegetative stage. Grain yield was determined at maturity. All cultivars showed strong log-linear correlations between K/Na_Leaves and grain yield, but intercept and slope of those correlations differed between seasons for a given genotype and between genotypes. The K/Na_Leaves under salinity was related to grain yield under salinity relative to freshwater controls. There was a highly significant correlation (p< 0.001) between K/Na_Leaves and salinity-induced grain yield reduction: the most susceptible cultivars had lowest K/Na_Leaves and the strongest yield reductions. Although there were major differences in the effects of salinity on crops in both the hot dry season (HDS) and the wet season, the correlation was equally significant across cropping seasons. The earliest possible time to establish the relationship between K/Na_Leaves under salinity and grain yield reduction due to salinity was investigated in an additional trial in the HDS 1998. About 60 days after sowing, salinity-induced yield loss could be predicted through K/Na_Leaves with a high degree of confidence (p< 0.01).
A screening system for salinity resistance of rice, particularly in arid and semi-arid climates, is proposed based on the correlation between K/Na_Leaves under salinity and salinity-induced yield losses.

Asch F., Sow A., Dingkuhn M., 1999. 
Reserve mobilization, dry matter partitioning and specific leaf area in seedlings of African rice cultivars differing in early vigor. Field Crops Res. 62, 191-202.

Abstract
Rice breeding for weed competitiveness requires improved screening tools. An ongoing breeding program uses O. glaberrima as genetic donor for growth vigor and weed competitiveness. This study investigates morphophysiological characteristics relevant to seedling vigor, namely, the kinetics of reserve mobilization, dry matter partitioning among organs, and specific leaf area (SLA). Five diverse cultivars (O. glaberrima upland rice, improved and traditional tropical O. sativa japonica upland rices, an improved O. sativa indica cultivar and an interspecific progeny) were grown in the screenhouse for 18 d on wet soil. Dry matter fractions of plant organs were measured daily, leaf area (LA) and SLA were measured 9, 14 and 18 days after seed soaking (DAS). SLA measurements were repeated using crowded populations simulating farmers' seedbed nurseries.
Seedlings achieved photo-autotrophic growth between 7.6 and 9.3 DAS, and had compensated for respiration losses during heterotrophic growth at 10.3 by 12.4 DAS, with O. glaberrima and improved indica cultivars requiring shorter periods than tropical japonica cultivars. For all cultivars, dry matter partitioning coefficients (PC) for roots were initially high but dropped temporarily to near zero during the transition from heterotrophic to autotrophic growth. The O. glaberrima cultivar had the largest PC for laminae and the largest SLA among the cultivars.
It is concluded that the superior early growth vigor of the O. glaberrima cultivar, as documented in previous studies, is partly due to (1) early onset of autotrophic growth, (2) high PC for leaves, and (3) large SLA. Based on the previous observation that SLA is correlated with weed competitiveness, it appears that SLA measured at about 14 d after germination might be employed as a rapid screen for weed competitiveness.

Dingkuhn, M., Asch, F., 1999. 
Phenological Responses of Oryza sativa, O. glaberrima and Inter-specific Rice Cultivars on a Toposequence in West Africa. Euphytica 110, 109-126.

Abstract
Phenological properties of rice cultivars, particularly crop duration, determines their yield potential, local agronomic suitability and ability to escape from drought. Crop duration of a given cultivar depends mainly on photoperiod (PP) and temperature, but is also affected by the crop establishment practice and environmental stresses.
A sample of 84 ecologically and genetically diverse rice cultivars was sown on five dates between May and September 1997 on the flooded-lowland (transplant), hydromorphic and upland levels of a toposequence at 7^o 52' N in Cote d'Ivoire, in order to characterize the cultivars' phenological responses. In the upland, life-saving sprinkler irrigation was applied when drought symptoms were visible. A non-replicated design augmented with four replicated checks (four replications per ecosystem) was used. Phenology was characterized by date of emergence, first heading, 50% flowering and maturity. The period from emergence to flowering was subdivided into three phases following a simple model used at IRRI to characterize germplasm for photoperiodism. For each ecosystem and cultivar, the basic vegetative period (BVP) was estimated by subtracting 30 d from the duration to flowering at the sowing date associated with the shortest duration, and expressed in degree-days (dd), assuming a base temperature of 10 ^oC. The PP-sensitive phase (PSP) was estimated by subtracting BVP+30 d from the time to flowering. PP-sensitivity (PS) was calculated from the apparent change in PSP between 12.0 and 12.5 h mean astronomic daylength during the PSP, by regression across dates.
Cultivars differed strongly in BVP (300 to 1200 dd) and PS (0 to 1000 dd). The BVP was generally longer in the lowland than in the hydromorph, and mostly longer in the upland than in the hydromorph, possibly due to transplanting shock (lowland) and drought (upland). Many cultivars, particularly upland-adapted japonicas, had a greater PS in the lowland than in the upland. Principal-component and cluster analyses based on BVP and PS in each of the three ecosystems established three large and three small groups of cultivars having common phenological responses. The linkage groups were associated with ecotypes (lowland vs upland, traditional vs improved) and genetic groups (O. sativa japonica and indica, O. glaberrima, inter-specific progenies). The groups were seen to represent past selection strategies by farmers and recent breeding strategies, with respect to achieving yield stability in the various ecosystems. For example, indigenous selection strategies for O. sativa upland rices seem to have favored a long BVP, whereas from O. glaberrima, which generally has a superior initial vigor, cultivars with a short BVP have been selected. The authors conclude that the modern upland rice breeding strategy for the region on the basis O. sativa, which aims at drought escape using a short BVP, is paralleled by existing indigenous O. glaberrima materials. Efforts to utilize these materials for breeding are ongoing.

Asch F., Dingkuhn, M., Wittstock, C., Dörffling, K., 1999. 
Sodium and Potassium Uptake of Rice Panicles as Affected by Salinity and Season in Relation to Yield and Yield Components. Plant and Soil 207, 133-145.

Abstract
Salinity is a major yield reducing stress in many arid and/or coastal irrigation systems for rice. Past studies on salt stress have mainly addressed the vegetative growth stage of rice, and little is known on salt effects on the reproductive organs. Sodium and potassium uptake of panicles was studied for eight rice cultivars in field trials under irrigation with saline and fresh water in the hot dry season (HDS) and the wet season (WS) 1994 at WARDA in Ndiaye, Senegal. Sodium and potassium content was determined at four different stages of panicle development and related to salt treatment effects on yield, yield components and panicle transpiration.
Yield and yield components were strongly affected by salinity, the effects being stronger in the HDS than in the WS. The cultivars differed in the amount of salt taken up by the panicle. Tolerant cultivars had lower panicle sodium content at all panicle development stages than susceptible ones. Panicle potassium concentration decreased with panicle development under both treatments in all cultivars, but to a lesser extent in salt treated susceptible cultivars. Grain weight reduction in the early panicle development stages and spikelet sterility increase in the later PDS were highly correlated (p < 0.01) with an increase in panicle sodium concentration in both seasons, whereas reduction in spikelet number was not. The magnitude of salt-induced yield loss could not be explained with increases in sodium uptake to the panicle alone. It is argued that the amount of sodium taken up by the panicle may be determined by two different factors. One factor (before flowering) being the overall control mechanism of sodium uptake through root properties and the subsequent distribution of sodium in the vegetative plant, whereas the other (from flowering onwards) is probably linked to panicle transpiration.

Asch, F., Dörffling, K., Dingkuhn, M., 1995. 
Response of Rice Varieties to Soil Salinity and Air Humidity: a possible Involvement of Root-borne ABA. Plant and Soil 177, 11-19

Dingkuhn, M., Sow, A., Samb, A., Diack, S., Asch, F., 1995. 
Climatic Determents of Irrigated Rice Performance in the Sahel. I. Photothermal and Micro-climatic Responses of Flowering. Agricultural Systems 48, 385-410

Abstract
In the Sahel, variable crop duration of irrigated rice poses serious timing problems for intensification of production. Photothermal effects on phenology have been studied to develop simulation tools for breeding and cropping systems research. Forty-nine genotypes were planted at monthly intervals in various rice-garden trials. Environment variability among seasons, sites, and within the crop canopy was characterized to develop a field-based, photothermal model for flowering. Basic concepts were the summation of heat units and a linear thermal response of development having upper (Topt) and lower (Tbase) response limits. Photoperiodism was model by a slope constant (CPP) and a basic vegetative phase. Photoperiodism and transplanting shock acted as modifiers of heat requirements (Tsum), thereby having greater effects on duration at low than at high temperatures. Tbase, Topt, Tsum, BVP and CPP were considered genotypic constants and calibrated by optimization. Daily input for the model was the physiologically relevant temperature Tphys at the shoot apex. Tphys depended on apex submergence, water temperature and diurnal temperature patterns.
Diurnal temperature segments exceeding the Tbase-Topt range were disregarded. Mean water temperature was below air temperature, particularly at high leaf area indices and on dry days. Mean air temperature was closer to minimum than to the maximum when amplitudes were high or days short. Minimum  temperatures below 18° C at booting stage resulted in near total spikelet sterility and a specific delay in heading. The model was validated for a site thermally different from the site of calibration.