Realistic Physiological Options to Increase Grain Legume Yield under Drought.

Increasing yield resiliency under water deficits remains a high priority for crop improvement. In considering the yield benefit of a plant trait modification, two facts are often overlooked: (1) the total amount of water available to a crop through a growing season ultimately constrains growth and yield cannot exceed what is possible with the limited amount of available water, and (2) soil water content always changes over time, so plant response needs to be considered within a temporally dynamic context of day-to-day variation in soil water status. Many previous evaluations of drought traits have implicitly considered water deficit from a "static" perspective, but while the static approach of stable water deficit treatments is experimentally congruous, the results are not realistic representations of real-world drought conditions, where soil water levels are always changing. No trait always results in a positive response under all drought scenarios. In this paper, we suggest two key traits for improving grain legume yield under water deficit conditions: (1) partial stomata closure at elevated atmospheric vapor pressure deficit that results in soil water conservation, and (2) lessening of the high sensitivity of nitrogen fixation activity to soil drying.

Limited-transpiration trait in response to high vapor pressure deficit from wild to cultivated species: study of the Lens genus.

Lentil (Lens culinaris Medik.) is commonly grown in drought-prone areas where terminal heat and drought are frequent. The limited-transpiration (TRlim) trait under high vapor pressure deficit (VPD) could be a way to conserve water and increase yield under water deficit conditions. The TRlim trait was examined in cultivated and wild lentil species together with its evolution throughout the breeding pipeline. Sixty-one accessions representing the six wild lentil species (L. orientalis, L. tomentosus, L. odemensis, L. lamottei, L. ervoides, and L. nigricans) and 13 interspecific advanced lines were evaluated in their transpiration response to high VPD. A large variation in transpiration rate (TR) response to increased VPD was recorded among wild lentil accessions, with 43 accessions exhibiting a breakpoint (BP) in their TR response to increasing VPD, with values ranging from 0.92 kPa to 3.38 kPa under greenhouse conditions. Ten genotypes for the interspecific advanced lines displayed a BP with an average of 1.95 kPa, much lower than previously reported for cultivated lentil. Results from field experiments suggest that the TRlim trait (BP=0.97 kPa) positively affected yield and yield-related parameters during the years with late-season water stress. The selection of TRlim genotypes for high VPD environments could improve lentil productivity in drought-prone areas.

Microsphere stem blockage as a screen for nitrogen-fixation drought tolerance in soybean.

Symbiotic nitrogen-fixation of soybean (Glycine max [Merr.] L) commonly decreases in response to soil drying in advance of other plant processes. While a few soybean lines express nitrogen-fixation drought tolerance, breeding for genetic variation is hampered by laborious phenotyping procedures. The objective of this research was to explore the potential of an initial screen for nitrogen-fixation drought-tolerant genotypes based on a possible relationship with xylem-vessel diameter. The hypothesis was that nitrogen-fixation drought tolerance might result from fewer, large-diameter xylem vessels in the stem that are vulnerable to disrupted flow as water deficit develops. The disrupted flow could cause nitrogen products to accumulate in nodules resulting in negative feedback on nitrogen-fixation rate. The proposed screen involved exposing de-rooted shoots to a suspension containing microspheres (45-53 µm diameter) and recording the decrease in transpiration rate as a result of microsphere xylem-blockage. Two soybean populations were tested. One population was progeny derived from mating of two parents with high and low nitrogen-fixation drought sensitivity. A high correlation (R2  = 0.68; P < 0.001) was found in this population between decreasing transpiration rate resulting from the microsphere treatment and increasing sensitivity of nitrogen-fixation to soil drying. The second tested population consisted of 16 genotypes, most of which had been previously identified in germplasm screens as expressing nitrogen-fixation drought tolerance. Nearly half of the lines in this second population were identified in the screen as showing minimum blockage of transpiration when exposed to the microspheres. Overall, these results showed the potential of using the microsphere screen to identify candidate genotypes expressing nitrogen-fixation drought tolerance.

An exploration of the variability of physiological responses to soil drying in relation with C/N balance across three species of the under-utilized genus Vigna.

The genus Vigna (Fabaceae) is an agriculturally important taxon, which includes several crop species such as cowpea (Vigna unguiculata L.), mung bean (Vigna radiata) and azuki bean (Vigna angularis). Most studies have focused on cowpea (V. unguiculata (L.) as a drought-resistant crop, although insights on the mechanisms that confer this species the ability to grow in dry environment are still not fully resolved. The diversity of this rich genus has been overlooked in many physiological studies. This study explores the physiological mechanisms of response to soil drying (N2 fixation, transpiration rate and changes in C and N allocation) across three species of the Vigna genus: V. radiata, V. unguiculata, V. vexillata (tuber cowpea). A significant variability among the studied Vigna accessions was found for the threshold in decline of N2 fixation with soil drying. Less variability was observed in the transpiration threshold. Through the analysis of leaf traits variation under well-watered and water-deficit conditions, we were able to relate the variability in N2 fixation and transpiration response to C/N metabolism modifications resulting in different allocation of carbon and nitrogen to leaves under water deficit.

Abscisic Acid and Sulfate Offer a Possible Explanation for Differences in Physiological Drought Response of Two Maize Near-Isolines.

The hypothesis was tested that differences in response to water-deficits between low osmotic potential (LOP) and high osmotic potential (HOP) maize (Zea mays L.) near-isolines were associated with differences in transpiration rate sensitivity to abscisic acid (ABA) and/or sulfate. In a series of four experiments, decreases in transpiration rate (DTR) of whole plants and fully expanded leaves were measured in response to treatments of 1.0 µM ABA and 15 mM MgSO4 singly and in combination following long (2 day) and short (180 min) exposures. There was little evidence that intact plants grown on soil were responsive to the treatments. For hydroponically grown plants subjected to long exposure, there was similarly no response to treatments. Further, the short exposure of hydroponically grown plants to solely ABA or a combination of chemicals resulted in no sensitivity in DTR for either of the near-isolines. On the other hand, when these plants were fed sulfate, the transpiration was stimulated by about 20% for the LOP and 60% for the HOP. Detached leaves proved to be the most sensitive to treatment. Treatment with the two chemicals singly caused essentially equivalent DTR in the two near-isolines. However, treatment with ABA plus sulfate resulted in different DTR between the two near-isolines with values of 65% for the LOP and 16% for the HOP near-isoline. Overall, these results showed that the short exposure treatment of hydroponically grown plants or detached leaves supported the hypothesis of different transpiration rate sensitivities of the near-isolines in response to ABA and sulfate treatments.

Sleep tight and wake-up early: nocturnal transpiration traits to increase wheat drought tolerance in a Mediterranean environment.

In wheat, night-time transpiration rate (TRN) could amount to 14-55% of daytime transpiration rate (TR), depending on the cultivar and environment. Recent evidence suggests that TRN is much less responsive to soil drying than daytime TR, and that such 'wasteful' water losses would increase the impact of drought on yields. In contrast, other evidence indicates that pre-dawn, circadian increases in TRN may enable enhanced radiation use efficiency, resulting in increased productivity under water deficit. Until now, there have been no attempts to evaluate these seemingly conflicting hypotheses in terms of their impact on yields in any crop. Here, using the Mediterranean environment of Tunisia as a case study, we undertook a simulation modelling approach using SSM-Wheat to evaluate yield outcomes resulting from these TRN trait modifications. TRN represented 15% of daytime TR-generated yield penalties of up to 20%, and these worsened when TRN was not sensitive to soil drying TR. For the same TRN level (15%), simulating a predawn increase in TRN alleviated yield penalties, leading to yield gains of up to 25%. Overall, this work suggests that decreasing TRN but increasing pre-dawn circadian control would be a viable breeding target to increase drought tolerance in a Mediterranean environment.

The importance of slow canopy wilting in drought tolerance in soybean.

Slow canopy wilting (SW) is a water conservation trait controlled by quantitative trait loci (QTLs) in late maturity group soybeans [Glycine max (L.) Merr.]. Recently, two exotic (landraces) plant introductions (PI 567690 and PI 567731) were identified as new SW lines in early maturity groups. Here, we show that the two PIs share the same water conservation strategy of limited maximum transpiration rates as PI 416937. However, in contrast to PI 416937, the transpiration rates of these PIs were sensitive to an aquaporin inhibitor, indicating an independence between limited maximum transpiration and the lack of silver-sensitive aquaporins. Yield tests of selected recombinant inbred lines from two elite/exotic crosses provide direct evidence to support the benefit of SW in drought tolerance. Four SW QTLs mapped in a Pana×PI 567690 cross at multiple environments were found to be co-located with previous reports. Moreover, two new SW QTLs were mapped on chromosomes 6 and 10 from a Magellan×PI 567731 cross. These two QTLs explain the observed relatively large contributions of 20-30% and were confirmed in a near-isogenic background. These findings demonstrate the importance of SW in yield protection under drought and provide genetic resources for improving drought tolerance in early maturity group soybeans.

Unraveling the genetic architecture for carbon and nitrogen related traits and leaf hydraulic conductance in soybean using genome-wide association analyses.

BACKGROUND: Drought stress is a major limiting factor of soybean [Glycine max (L.) Merr.] production around the world. Soybean plants can ameliorate this stress with improved water-saving, sustained N2 fixation during water deficits, and/or limited leaf hydraulic conductance. In this study, carbon isotope composition (δ13C), which can relate to variation in water-saving capability, was measured. Additionally, nitrogen isotope composition (δ15N) and nitrogen concentration that relate to nitrogen fixation were evaluated. Decrease in transpiration rate (DTR) of de-rooted soybean shoots in a silver nitrate (AgNO3) solution compared to deionized water under high vapor pressure deficit (VPD) conditions was used as a surrogate measurement for limited leaf hydraulic conductance. A panel of over 200 genetically diverse soybean accessions genotyped with the SoySNP50K iSelect BeadChips was evaluated for the carbon and nitrogen related traits in two field environments (Athens, GA in 2015 and 2016) and for transpiration response to AgNO3 in a growth chamber. A multiple loci linear mixed model was implemented in FarmCPU to perform genome-wide association analyses for these traits. RESULTS: Thirty two, 23, 26, and nine loci for δ13C, δ15N, nitrogen concentration, and transpiration response to AgNO3, respectively, were significantly associated with these traits. Candidate genes that relate to drought stress tolerance enhancement or response were identified near certain loci that could be targets for improving and understanding these traits. Soybean accessions with favorable breeding values were also identified. Low correlations were observed between many of the traits and the genetic loci associated with each trait were largely unique, indicating that these drought tolerance related traits are governed by different genetic loci. CONCLUSIONS: The genomic regions and germplasm identified in this study can be used by breeders to understand the genetic architecture for these traits and to improve soybean drought tolerance. Phenotyping resources needed, trait heritability, and relationship to the target environment should be considered before deciding which of these traits to ultimately employ in a specific breeding program. Potential marker-assisted selection efforts could focus on loci which explain the greatest amount of phenotypic variation for each trait, but may be challenging due to the quantitative nature of these traits.

Increasing Photosynthesis: Unlikely Solution For World Food Problem.

Increasing the photosynthesis rate of plants has been recently revitalized as an approach for increasing grain crop yields and solving world food crises. The idea that photosynthesis is the key to increasing grain crop yields is not new. Considerable research in the 1970s and 1980s showed that carbon input was not limiting for crop growth and yield. Instead, the availability and uptake of water and nutrients were found to be critical for increasing grain yield, and that conclusion still applies today. In this Opinion article, nitrogen limitation is given particular attention because of its quantitative linkage with vegetative and reproductive growth and its essential role as a quantitative component of seeds.

Resources for Crop Production: Accessing the Unavailable.

An acute imbalance between human population and food production is projected, partially due to increasing resource scarcity; dietary shifts and the current course of technology alone will not soon solve the problem. Natural ecosystems, typically characterized by high species richness and perennial growth habit, have solved many of the resource-acquisition problems faced by crops, making nature a likely source of insights for potential application in commercial agriculture. Further research on undomesticated plants and natural ecosystems, and the adaptations that enable them to meet their needs for N, P, and water, could change the face of commercial food production, including on marginal lands.

Effective Water Use Required for Improving Crop Growth Rather Than Transpiration Efficiency.

The phenomenological expression showing crop yield to be directly dependent on crop transpiration use efficiency (TE) has encouraged continued focus on TE as a viable approach to increasing crop yields. The difficulty in the phenomenological perspective is that research tends not to match up with the underlying mechanistic variables defining TE. Experimental evidence and the mechanistic derivation of TE by Tanner and Sinclair showed that the common focus on increasing the intrinsic ratio of leaf CO2/H2O exchange has limited opportunities for improvement. On the other hand, the derivation showed that daily vapor pressure deficit (VPD) weighted for the daily cycle of transpiration rate has a large, direct impact on TE. While VPD is often viewed as an environmental variable, daily weighted VPD can be under plant control as a result of partial stomatal closure during the midday. A critical feature of the partial stomatal closure is that transpiration rate is decreased resulting in conservation of soil water. The conserved soil water allows late-season, sustained physiological activity during subsequent periods of developing water deficits, which can be especially beneficial during reproductive development. The shift in the temporal dynamics of water use by water conservations traits has been shown in simulation studies to result in substantial yield increases. It is suggested from this analysis that effective water use through the growing season is more important for increasing crop yield than attempts focused on improving the static, intrinsic TE ratio.

Aquaporin Activity to Improve Crop Drought Tolerance.

In plants, aquaporins (AQP) occur in multiple isoforms in both plasmalemma and tonoplast membranes resulting in regulation of water flow in and out of cells, and ultimately, water transfer through a series of cells in leaves and roots. Consequently, it is not surprising that physiological and molecular studies have identified AQPs as playing key roles in regulating hydraulic conductance in roots and leaves. As a result, the activity of AQPs influences a range of physiological processes including phloem loading, xylem water exit, stomatal aperture and gas exchange. The influence of AQPs on hydraulic conductance in plants is particularly important in regulating plant transpiration rate, particularly under conditions of developing soil water-deficit stress and elevated atmospheric vapor pressure deficit (VPD). In this review, we examine the impact of AQP activity and hydraulic conductance on crop water use and the identification of genotypes that express soil water conservation as a result of these traits. An important outcome of this research has been the identification and commercialization of cultivars of peanut (Arachis hypogaea L.), maize (Zea mays L.), and soybean (Glycine max (Merr) L.) for dry land production systems.

Selection of host-plant genotype: the next step to increase grain legume N2 fixation activity.

Symbiotic N2 fixation research thus far has been primarily focused on selection of bacteria. However, little progress in impacting crop yields has resulted from this approach. Bacteria introduced in field soils rarely compete well with indigenous bacteria, including mutated lines selected for high nitrogen fixation capacity. Consequently, introduction of 'elite' bacteria in fields commonly does not result in crop yield increase. This review highlights that the primary regulation of N2 fixation is a result of response of integrated physiological activity at the plant level. Nitrogen feedback from the host plant plays an important role in regulating the N2 fixation rate. Rapid sequestration of fixed nitrogen by the plant is especially important for high N2 fixation activity. In addition, water cycling in the plant between the shoot and nodules plays a key role in sustaining high N2 fixation activity. Therefore, attention in selecting the host-plant genotype is suggested to be the next step to increasing N2 fixation activity of grain legumes.

Is nitrogen accumulation in grain legumes responsive to growth or ontogeny?

Nitrogen (N) accumulation in legumes is one of the main determinants of crop yield. Although N accumulation from symbiotic nitrogen fixation or N absorption from the soil has been widely investigated, there is no clear consensus on timing of the beginning of N accumulation and the termination of N accumulation and the physiological events that may be associated with these two events. The analyses conducted in this study aimed at identifying the determinant of N accumulation in two grain legume species. Nitrogen accumulation dynamics and mass accumulation and development stages were recorded in the field for several genotypes of common bean (Phaseolus vulgaris) and faba bean (Vicia faba) under different growing conditions. This study showed that during the vegetative stages, N accumulation rate was correlated with mass accumulation rate. However, the maximum accumulation of N did not correspond to the time of the maximum mass accumulation. In fact, for both species, N accumulation was found to persist in seed growth. This challenges a common hypothesis that seed growth causes a decrease in N accumulation because of a shift of the photosynthate supply to support the seed growth. Even more surprising was the shift of the active accumulation of N in faba bean to late in the growing season as compared with common bean. N accumulation by faba bean only was initiated at high rates very late in vegetative growth and persisted at high rates well into seed fill.

Limited-transpiration response to high vapor pressure deficit in crop species.

Water deficit under nearly all field conditions is the major constraint on plant yields. Other than empirical observations, very little progress has been made in developing crop plants in which specific physiological traits for drought are expressed. As a consequence, there was little known about under what conditions and to what extent drought impacts crop yield. However, there has been rapid progress in recent years in understanding and developing a limited-transpiration trait under elevated atmospheric vapor pressure deficit to increase plant growth and yield under water-deficit conditions. This review paper examines the physiological basis for the limited-transpiration trait as result of low plant hydraulic conductivity, which appears to be related to aquaporin activity. Methodology was developed based on aquaporin involvement to identify candidate genotypes for drought tolerance of several major crop species. Cultivars of maize and soybean are now being marketed specifically for arid conditions. Understanding the mechanism of the limited-transpiration trait has allowed a geospatial analyses to define the environments in which increased yield responses can be expected. This review highlights the challenges and approaches to finally develop physiological traits contributing directly to plant improvement for water-limited environments.

Mapping Water Stress Incidence and Intensity, Optimal Plant Populations, and Cultivar Duration for African Groundnut Productivity Enhancement.

Groundnut production is limited in Sub-Saharan Africa and water deficit or "drought," is often considered as the main yield-limiting factor. However, no comprehensive study has assessed the extent and intensity of "drought"-related yield decreases, nor has it explored avenues to enhance productivity. Hence, crop simulation modeling with SSM (Simple Simulation Modeling) was used to address these issues. To palliate the lack of reliable weather data as input to the model, the validity of weather data generated by Marksim, a weather generator, was tested. Marksim provided good weather representation across a large gradient of rainfall, representative of the region, and although rainfall generated by Marksim was above observations, run-off from Marksim data was also higher, and consequently simulations using observed or Marksim weather agreed closely across this gradient of weather conditions (root mean square of error = 99 g m-2; R2 = 0.81 for pod yield). More importantly, simulation of yield changes upon agronomic or genetic alterations in the model were equally predicted with Marksim weather. A 1° × 1° grid of weather data was generated. "Drought"-related yield reduction were limited to latitudes above 12-13° North in West Central Africa (WCA) and to the Eastern fringes of Tanzania and Mozambique in East South Africa (ESA). Simulation and experimental trials also showed that doubling the sowing density of Spanish cultivars from 20 to 40 plants m-2 would increase yield dramatically in both WCA and ESA. However, increasing density would require growers to invest in more seeds and likely additional labor. If these trade-offs cannot be alleviated, genetic improvement would then need to re-focus on a plant type that is adapted to the current low sowing density, like a runner rather than a bush plant type, which currently receives most of the genetic attention. Genetic improvement targeting "drought" adaptation should also be restricted to areas where water is indeed an issue, i.e., above 12-13°N latitude in WCA and the Eastern fringes of Tanzania and Mozambique.

Leaf emergence (phyllochron index) and leaf expansion response to soil drying in cowpea genotypes.

Drought can result in severely decreased leaf area development, which impacts plant growth and yield. However, rarely is leaf emergence or leaf expansion separated to resolve the relative sensitivity to water-deficit of these two processes. Experiments were undertaken to impose drought over approximately 2 weeks for eight cowpea (Vigna unguiculata) genotypes grown in pots under controlled environmental conditions. Daily measures of phyllochron index (PI, leaf emergence) and leaf area increase (leaf expansion) were obtained. Each of these measures was referenced against volumetric soil water content, i.e. fraction transpirable soil water. Although there was no clear difference between leaf emergence and leaf expansion in sensitivity to drying soil, both processes were more sensitive to soil drying than plant transpiration rate. Genotypic differences in the soil water content at the initiation of the decline in PI were identified. However, no consistent difference in sensitivity to water-deficit in leaf expansion was found. The difference in leaf emergence among genotypes in sensitivity to soil drying can now be exploited to provide guidance for plant improvement and crop yield increase.

Pot binding as a variable confounding plant phenotype: theoretical derivation and experimental observations.

MAIN CONCLUSION: Theoretical derivation predicted growth retardation due to pot water limitations, i.e., pot binding. Experimental observations were consistent with these limitations. Combined, these results indicate a need for caution in high-throughput screening and phenotyping. Pot experiments are a mainstay in many plant studies, including the current emphasis on developing high-throughput, phenotyping systems. Pot studies can be vulnerable to decreased physiological activity of the plants particularly when pot volume is small, i.e., "pot binding". It is necessary to understand the conditions under which pot binding may exist to avoid the confounding influence of pot binding in interpreting experimental results. In this paper, a derivation is offered that gives well-defined conditions for the occurrence of pot binding based on restricted water availability. These results showed that not only are pot volume and plant size important variables, but the potting media is critical. Artificial potting mixtures used in many studies, including many high-throughput phenotyping systems, are particularly susceptible to the confounding influences of pot binding. Experimental studies for several crop species are presented that clearly show the existence of thresholds of plant leaf area at which various pot sizes and potting media result in the induction of pot binding even though there may be no immediate, visual plant symptoms. The derivation and experimental results showed that pot binding can readily occur in plant experiments if care is not given to have sufficiently large pots, suitable potting media, and maintenance of pot water status. Clear guidelines are provided for avoiding the confounding effects of water-limited pot binding in studying plant phenotype.

Determination of coefficient defining leaf area development in different genotypes, plant types and planting densities in peanut (Arachis hypogeae L.).

Rapid leaf area development may be attractive under a number of cropping conditions to enhance the vigor of crop establishment and allow rapid canopy closure for maximizing light interception and shading of weed competitors. This study was undertaken to determine (1) if parameters describing leaf area development varied among ten peanut (Arachis hypogeae L.) genotypes grown in field and pot experiments, (2) if these parameters were affected by the planting density, and (3) if these parameters varied between Spanish and Virginia genotypes. Leaf area development was described by two steps: prediction of main stem number of nodes based on phyllochron development and plant leaf area dependent based on main stem node number. There was no genetic variation in the phyllochron measured in the field. However, the phyllochron was much longer for plants grown in pots as compared to the field-grown plants. These results indicated a negative aspect of growing peanut plants in the pots used in this experiment. In contrast to phyllochron, there was no difference in the relationship between plant leaf area and main stem node number between the pot and field experiments. However, there was genetic variation in both the pot and field experiments in the exponential coefficient (PLAPOW) of the power function used to describe leaf area development from node number. This genetic variation was confirmed in another experiment with a larger number of genotypes, although possible G × E interaction for the PLAPOW was found. Sowing density did not affect the power function relating leaf area to main stem node number. There was also no difference in the power function coefficient between Spanish and Virginia genotypes. SSM (Simple Simulation model) reliably predicted leaf canopy development in groundnut. Indeed the leaf area showed a close agreement between predicted and observed values up to 60000 cm2 m-2. The slightly higher prediction in India and slightly lower prediction in Niger reflected GxE interactions. Until more understanding is obtained on the possible GxE interaction effects on the canopy development, a generic PLAPOW value of 2.71, no correction for sowing density, and a phyllochron on 53 °C could be used to model canopy development in peanut.