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Recommend Documents. Headquarters Marcel Dekker, Inc. It was first used Your name. Close Send. The role of photosynthesis to increase the yield of water-stressed crops also involves an important element of scale. In drying soil, tissue expansion is affected earlier and more severely than photosynthesis per unit leaf area, hence the surplus of reduced carbon often observed in water-limited plants; effectively, water deficit uncouples growth from photosynthesis Muller et al.
The evidence supporting this statement is strong, and the superficial corollary is that improvement of photosynthesis is irrelevant for the yield of crops under water stress. Here is where explicit consideration of time scale is important: the decoupling of growth and photosynthesis occurs during periods of stress, but on a seasonal basis the contribution of growth during times of stress is small compared with cumulative growth when soil moisture is adequate Fischer and Turner, ; Loomis, The uncoupling of growth and photosynthesis during water limitations, therefore, does not preclude yield gains from enhanced photosynthesis; the impact of higher photosynthesis on cumulative growth and final yield depend on the seasonal dynamics of water availability Fischer and Turner, ; Schwinning and Ehleringer, The traits are therefore classified into three broad groups: those that generally scale up e.
Dealing with traits at a level of organization below the crop that scale up or do not is trivial: the former is used and the latter is discarded in crop improvement. Traits for which scaling is contingent to higher-level interactions are interesting and challenging. Machine translation is a useful analogy for these traits Crystal, The early attempts to use machines to translate texts between two languages—developed in the s under the influence of post-war cryptography—used a word-for-word approach. This approach produced nonsensical translations because it only considered the semantic aspect of language; grammar was ignored.
There are close biological and agronomic links between the economies of carbon, water, and nitrogen of crops; however, the efficiencies in the use of water and nitrogen can be unrelated, related positively synergy or negatively trade-off depending on the environmental and genetic sources of variation, the level of organization, and time scale at which efficiencies are defined Cabrera-Bosquet et al.
At the crop level, there is a robust nitrogen-driven trade-off between yield per unit evapotranspiration and yield per unit nitrogen uptake that is supported empirically Belder et al. The experiment of Cabrera-Bosquet et al. For a factorial combination of three water regimes, two nitrogen regimes, and four genotypes, shoot biomass per unit evapotranspiration was negatively related to photosynthetic nitrogen-use efficiency light-saturated photosynthesis per unit nitrogen in flag leaf , and positively related to both nitrogen-use efficiency biomass per unit N available and N uptake efficiency nitrogen in shoot biomass per unit nitrogen supply.
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The relationship between biomass per unit evapotranspiration and nitrogen utilization efficiency biomass per unit shoot nitrogen was nitrogen-dependent: these traits were unrelated at high nitrogen supply and negatively related at low nitrogen supply. A genotype-driven link between nitrogen nutrition and yield under water stress has been demonstrated for wheat in Australia. Selection for yield and agronomic adaptation in the water- and nitrogen-scarce environments Fischer, has consistently improved wheat yield since the early 20th century to the present day. Changes in traits related to the carbon, water, and nitrogen economy of the crop help to understand trade-offs and synergies in the capture and efficiency in the use of these resources; phenotypic changes have been characterized in two series of studies focusing on early breeding until the mids Siddique et al.
In parallel with sustained yield gains, crop water use remained largely unchanged, hence a linear increase in yield per unit transpiration Fig. Early breeding adjusted phenology to local conditions and harvest index increased consistently during the whole period. Despite the sustained increase in harvest index, this trait can contribute further to wheat yield improvement in Australia Sadras and Lawson, in contrast to European varieties that are closer to their biophysical limit Foulkes et al.
The limits to further improvement mediated by enhanced harvest index highlight the importance of improved biomass production; selection for yield in the UK has already increased radiation-use efficiency, biomass, and nitrogen uptake of wheat in recent decades Shearman et al. Figure 2B shows the rate of change in shoot biomass and underlying traits for Australian wheats released over the last five decades. There was an increase in biomass at maturity which was fully accounted for by the increase in biomass at anthesis.
Increased biomass at anthesis was associated with enhanced radiation-use efficiency between stem elongation and anthesis. It is noteworthy that the enhancement of crop photosynthetic capacity was unrelated to leaf-level photosynthesis and respiration but was related to improved crop nitrogen nutrition and shifts in the distribution of light and nitrogen in the canopy Fig.
Selection for yield and maybe greenness?
Whereas all leaves of modern wheats are greener than those of their older counterparts, the largest differences are found at the bottom of the canopy Fig. This is partially related to enhanced light penetration but for a given light intensity, modern varieties have a higher concentration of foliar nitrogen Fig. The premium for higher quality wheats, particularly in the last two decades, may also have indirectly contributed to changed N dynamics in more recent varieties Sadras and Lawson, This transformation of the wheat phenotype is striking in view of the complexities involved in direct manipulation of crop photosynthesis Sinclair et al.
A genotype-driven link between nitrogen nutrition and drought adaptation has also been demonstrated for soybean in the USA Sinclair et al. Owing to soybean's dependence on symbiotic nitrogen fixation, the similar association between nitrogen nutrition and adaptation to drought in soybean and wheat is only superficial. For soybean, the proposition was that maintenance of nitrogen fixation favours yield under water stress Sinclair et al. Different screening approaches were combined, including coarser methods to screen large numbers of lines selection for low ureide concentration in petioles, putatively reflecting sensitivity to drought and more refined methods for fewer lines acetylene reduction activity.
Selected lines were compared with high-yielding commercial cultivars under broad environmental conditions. Two lines were identified that outperformed commercial checks under water deficit, but trade-offs were apparent under high-yielding conditions. Sustained yield improvement of wheat in low-rainfall, low-nitrogen environments of Australia highlight the achievement of direct selection for yield for well over a century.
In the last 50 years, breeding for wheat yield has increased crop photosynthesis and shoot biomass in both the stressful environments of Australia and in the high-yielding European environments. Thus, whereas selection for yield in favourable environments may or may not improve stress adaptation depending on the combination of genotypes and environments, traits such as radiation-use efficiency can be adaptive across wide environmental ranges.
Dissecting radiation-use efficiency showed an impressive transformation of the phenotype including improved nitrogen uptake and changes in the distribution of light and nitrogen in the canopy. Further progress in drought adaptation could benefit from a dual perspective on nitrogen and water. Direct selection for yield continues to be streamlined and improved particularly with the help of superior statistical tools that have improved logistics and the design of field trials and data interpretation; costs for field trials, however, remain high as tens of thousands and often hundreds of thousands of plots are required.
Owing to larger interaction between genotype and environment, more plots are required in drier than in more favourable conditions. Thus, indirect methods will continue to attract attention as more cost-effective methods of selection are required. However, the failure of indirect methods to contribute to yield in many cases is not because yield is complex, it is rather that methods and perspectives do not account for the levels of organization and time scales where traits are relevant or otherwise.
Opinion papers like ours often advocate for collaborations across disciplines. Progress is being made in this direction, as illustrated by the International Wheat Consortium Reynolds et al. However, epistemological barriers remain Andersen and Wagenknecht, that partially relate to levels of organization, and their implications for spatial and time scales, system structure and dynamics, theories and methods. Whereas crop improvement is a common interest across levels, theoretical and methodological gaps are real obstacles for effective collaborations.
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One of the main contributions of molecular science has been a deeper focus on evolution as the fundamental source of biological diversity Buss, whereas evolutionary insights are central to crop science Donald, ; Evans, ; Denison, , hence the opportunity to expand the trading zone by asking questions about crop adaptation to drought in an evolutionary framework.
Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Sign In. Advanced Search. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents. Direct selection for yield as a method and a benchmark to improve stress adaptation. Indirect selection to improve stress adaptation.
Limitations of indirect approaches in the context of levels of organization. Trade-offs and synergies between improved nitrogen nutrition and yield in water-limited environments. Concluding remarks. Improvement of crop yield in dry environments: benchmarks, levels of organisation and the role of nitrogen V.
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