Root hydraulics: The forgotten side of roots in drought adaptation

Roots have long been proposed as a major avenue of research to improve crop adaptation to water limitations. The simple assumption is that deeper and more profuse root systems could tap extra water from the soil profile and alleviate drought effects. However, after decades of research, success in breeding cultivars with improved root systems is lagging behind. Here, we attempt to analyze the possible reasons for this, and re-focus on what root traits might provide the most promising avenues for drought adaptation. We approach the root system from the angle of water extraction, using data from a lysimetric system that allows monitoring and comparing plant water use over the entire crop life cycle and yield, and analyze whether and how differences in water extraction lead to improved yield across different crops. The main message from that analysis is that water extraction during reproduction and grain filling is critical and comes from a number of traits that influence the rate at which plant use the available water before and during stress. Roots may have an effect on this, not from the traditionally thought density or depth, but rather from their hydraulic characteristics. Plants can indeed control water use by controlling leaf area development and this is a “long term” control. Plants also control water losses by controlling stomata opening under high vapor pressure deficit (VPD) conditions, in a transient manner. Both processes (leaf development and stomata opening) are mostly controlled by hydraulic processes. The role of roots in drought adaptation could be there, along with the soil, in setting an hydraulic environment that allow plants to use water in a way that allow maximizing water use for these critical stages

Phosphorus coating on pearl millet seed in low P Alfisol improves plant establishment and increases stover more than seed yield

Phosphorus (P) is one of the main limiting nutrients in the semi-arid regions where pearl millet is grown; its deficiency leads to poor seedling establishment and eventually poor crop yield. Experiments were carried out in pots and field-like conditions to evaluate the effect of seed priming and seed coating with P on the shoot biomass at two and four weeks after sowing (WAS), and on the panicle and stover yield at maturity of three hybrid varieties of pearl millet in low P Alfisol. Overall, seed priming did not increase shoot biomass at two and four WAS. In pots, seed coating at a rate of approximately 400 g P ha−1 increased vegetative biomass over 400% at early stages, and panicle yield by about 50%, over the non-coated treatment, with genotypic variation in the magnitude of the response. In field-like conditions, seed coating restored stover biomass to 85–100% of that in the non-limiting P treatment, whereas the panicle yield remained 25–35% lower than in the non-limiting P treatment, but still 45–65% higher than the non-coated treatment. P deficiency delayed the time to flowering by 20–24 days compared to the non-limiting P treatment, but plants in the seed coating treatment reduced that gap by 10–14 days. Seed P coating treatment appears a valid option to promote pearl millet seedling establishment and then to boost yield under low soil P conditions. The maintenance of an effect of seed coating on panicle yield in pots suggests a positive early effect on developmental processes before panicle initiation

Coping with drought: Resilience versus risk. Targeting the most suitable G*E*M options by crop simulation modeling*

Crop production is axiomatically related to water consumption of transpiring leaves. Crop adaptation to water limitation then becomes an exercise of matching water supply and demand in away that the crop has enough water to complete its cropping cycle. Weather conditions vary greatly across years within environments while both weather and soil conditions vary across locations, which means that drought scenario are extremely variable and these need to be properly characterized as a pre-requisite to undertake drought research. Once the weather scenarios are defined, traits contributing to the crop adaptation to any of these scenarios need to be identified.We believe that much of these traits revolve around the need to optimize plant water use and make it efficient, together with the need to maximize water capture from the soil.Optimization of plant water use consist in identifying traits that will ensure maximum crop growth while keeping sufficient water for the grain filling period, and it deals with controlling water losses, and minimizing leaf canopy development. While tapping more water is surely important, the timing of water extraction to critical crop stages, e.g. the grain filling stage, is even more critical. It depends in great part on the way water has been managed by the plant at earlier stages, in particular to the capacity to develop a smaller crop canopy, or the capacity to restrict plant transpiration, especially under high evaporative demand. Clearly, the development of cultivars capable of better performance under water limited conditions is the result of many possible characteristics that interact with one another andwith the environment, and it is difficult to experimentally determinewhich among these traits has a predominant effect on yield in a given situation. Crop simulation modeling comes in to help to navigate biological complexity by allowing to test the effect of traits on yield acrossmany years of weather andmany locations. It also helps combining both agronomic and genetic options to maximize crop production at the plot level

The future of dryland cereals and legumes for the smallholder farmer in the semi-arid tropics

Besides a major financial crisis since 2008, a major inflation and volatility of the price of the most important staple foods such as rice or wheat is threatening global food security, especially for the millions of poor in developing countries. Food production has returned to the top of the political agenda worldwide. Today, we are at a crossroads. About 850 million people still go to bed hungry. Food production needs to be increased by about 70% in the next 40 years to feed over 9 billion people by 2050 and food systems have to be more inclusive of the poorest population. To achieve this increase and food access for the most vulnerable, many challenges need to be addressed, including a changing climate and shrinking natural resources. Innovations such as high yield wheat and rice cultivars and fertilizers sparked a green revolution in South Asia in the 1960s but they are now showing certain limits in terms of sustainability and profitability

Groundnut (Arachis hypogaea) genotypes tolerant to intermittent drought maintain a high harvest index and have small leaf canopy under stress

Intermittent drought, which varies in intensity, severely limits groundnut (Arachis hypogaea L.) yields. Experiments were conducted to assess root development, water uptake, transpiration efficiency, yield components and their relationships, in 20 groundnut genotypes under well watered (WW), and mild (DS-1), medium (DS-2) and severe (DS-3) intermittent stress. Pod yield decreased 70%, 55% and 35% under severe, medium and mild stress, respectively. Pod yield varied among genotypes, and showed significant genotype-by-treatment effects. Root length density (RLD) varied among genotypes before and after stress, although RLD did not discriminate tolerant from sensitive lines. Total water uptake and RLD under water stress had a weakly significant relationship. Water extraction from the soil profile was highest under severe stress. Water uptake varied among genotypes in all water regimes, but correlated with pod yield under WW conditions. The relative harvest index (HI) (i.e. the ratio of the HI under stress to HI under WW conditions) was closely related to the pod yield in all three intermittent stresses (R2 = 0.68 in DS-1; R2 = 0.65 in DS-2; R2 = 0.86 in DS-3) and was used as an index of stress tolerance. Under medium and severe stresses, the relative HI was negatively related to plant leaf weight (R2 = 0.79 in DS-2; R2 = 0.53 in DS-3), but less so under mild stress (R2 = 0.31). The results suggest that under intermittent stress, genotypes with a lower leaf area may use water more sparingly during the drying cycle with less damaging consequences for reproduction and pod

S’appuyer sur les multiples benefices des legumineuses a graines pour une agriculture plus productive et nutritive dans les tropiques semi-arides (Relying on the numerous advantages of grain legumes for more productive and nutritive agriculture in the semi-arid tropics)

Nitrogen and phosphorus are two of the main nutrients for agriculture but their growing scarcity makes them increasingly less available and affordable for farmers in the semi-arid tropics. Legumes have the comparative advantage over other plant species of being able to independently fix nitrogen, but also to absorb natural phosphorus that is in non-soluble form in the soil. In addition to soil fertility, grain legumes bring other important benefits for farmers in different farming systems. It is a quality fodder source for livestock. Rich in protein, energy and sometimes in lipids, grain legumes are nutritious food for humans. They are ideal as rotation crops with cereals and have also become a cash crop in some regions. Legumes should play an important role in future crop systems in semi-arid tropics. This article discuss the importance legumes, in particular grain legumes, could have as fertilizing and nutritious crops for farmers in semi-arid tropics if the current constraints for large-scale cultivation can be addressed

Understanding crop physiological processes for climate resilience

As everybody knows, the climate is changing and over the next decade will be putting an increasing strain on agriculture production. This paper aims at putting some focus on what can really be addressed (the change in temperature) from what really cannot be predicted and dealt with (rainfall). But even the effect of one factor like temperature triggers a complex myriad of effects and the paper structures what needs to be done in relation to temperature, and focuses on recently discovered mechanism to adapt to a change in temperature. The paper then briefly reviews its biological basis, the mean to phenotype for it at a high rate and precision, and how the use of crop simulation can help us predict the effect of this trait on yield..

Phenotypic Evaluation of Groundnut Germplasm under Drought and Heat Stress

A groundnut germplasm (268 genotypes) was evaluated in four trials over a period of two years, under intermittent drought and fully irrigated conditions. Two trials were exposed to moderate temperature during the rainy season while the two others were subjected to high temperature during summer. The objectives were to segregate the components of the genetic variance and their interactions with water treatment, year and environment (temperature) for agronomic characteristics so as to select high yielding genotypes under hot conditions and to identify traits putatively related to heat and/or intermittent drought tolerance. Under high temperature conditions, drought stress reduced pod yield up to 72% compared to 55% at moderate temperature. The haulm yield decrease due to drought was 34% at high temperature and 42% under moderate temperature. Haulm yield tended to increase under high temperature. For the three traits, genotype by environment interaction (GxE) was significant under well-watered (WW) and water stressed (WS) treatments. The genotype and genotype by environment (GGE) biplots analyses revealed several mega environments under WW and WS treatments indicating that high yielding genotypes under moderate temperature were different from those at high temperature. The GGE biplots analyses also revealed several genotypes with high performance and stability across year and temperature environments under both WW and WS conditions. Regression analyses indicated that among several traits measured during plant growth, only the partition rate was significantly correlated to pod yield suggesting that this trait was contributing to heat and drought tolerance and could be a reliable selection criterion for groundnut breeding program for this stress

Evaluation of Groundnut Germplasm under Drought and Heat Stress in Sahelian Zone

Severe drought and temperature increase are predicted to be the major consequences of climate change. Groundnut is a major crop cultivated in the Sahel zone where water and high temperature stress are serious constraints for its production. Investigating drought and heat effects on physiological traits, yield and its attributes could significantly contribute for improving groundnut productivity and consequently the incomes of farmers. A groundnut germplasm (268 genotypes) was evaluated in four trials during two years under intermittent drought and fully irrigated conditions. Drought stress reduced pod yield up to 72 % compared to 55 % at moderate temperature. The haulm yield decrease due to drought was 34 % at high temperature and 42 % under moderate temperature. Haulm yield tended to increase under high temperature. Genotype by environment interaction (GxE) was significant under well-watered (WW) and water stress (WS) treatments. The genotype and genotype by environment (GGE) biplots analyses revealed several mega environments under WW and WS treatments. The GGE biplots analyses revealed also several genotypes with high performance and stability across year and temperature environments under both WW and WS conditions. The regression analyses indicated that among several traits, only the partition rate was significantly correlated to pod yield

Water extraction under terminal drought explains the genotypic differences in yield, not the anti-oxidants changes in leaves of pearl millet (Pennisetum glaucum (L.) R. Br.)

Pearl millet (Pennisetum glaucum (L.) R. Br.) is a resilient crop suiting the harshest conditions of the semi-arid tropics, in which we assessed possible relationships between crop tolerance, anti-oxidative enzyme activity, and plant / soil water status. Biochemical acclimation and cell homeostasis traits have indeed been proposed as critical for the drought tolerance of crops, but their limited practical application in breeding so far suggests that the role of biochemical acclimation for drought tolerance is still unclear. A possible limitation of previous research may be in not having approached biochemical acclimation from the angle of plant water relations. Four pearl millet genotypes, contrasting for terminal drought tolerance, were evaluated (sensitive H77/833-2, tolerant PRLT2/89-33, and two near isogenic lines carrying a terminal drought tolerance quantitative trait locus) under water stress (WS) and well-watered (WW) conditions in a lysimetric system that simulates field-like conditions. We assessed the genotypic variation and relationship between photosynthetic pigments (chlorophylls a, b, carotenoids), antioxidative isoenzymatic spectrum (superoxide dismutase, ascorbate peroxidase, catalase), physiological traits (soil moisture available, normalized transpiration, stay-green, water extraction), biomass and yield. Investigated biochemical traits were tightly related among each other under WS conditions but not under WW conditions. Two major ascorbate peroxidase isoforms (APX6&7), whose variation in both water regimes reflected the presence/absence of the drought tolerance quantitative trait locus, were identified, but these did not relate to yield. Both, yield and biochemical traits under terminal drought stress were closely related to the traits linked to soil-plant water status (soil moisture available, normalized transpiration, stay-green, water extraction), while yield and the biochemical indicators were not correlated, except for one. It is concluded that there is no direct effect of biochemical traits on yield parameters since both are consequences of soil-plant water status and their putative relation appear to be secondary â through soil-plant water status