Natural variation in photosynthetic capacity, growth, and yield in 64 field-grown wheat genotypes.

Increasing photosynthesis in wheat has been identified as an approach to enhance crop yield, with manipulation of key genes involved in electron transport and the Calvin cycle as one avenue currently being explored. However, natural variation in photosynthetic capacity is a currently unexploited genetic resource for potential crop improvement. Using gas-exchange analysis and protein analysis, the existing natural variation in photosynthetic capacity in a diverse panel of 64 elite wheat cultivars grown in the field was examined relative to growth traits, including biomass and harvest index. Significant variations in photosynthetic capacity, biomass, and yield were observed, although no consistent correlation was found between photosynthetic capacity of the flag leaf and grain yield when all cultivars were compared. The majority of the variation in photosynthesis could be explained by components related to maximum capacity and operational rates of CO2 assimilation, and to CO2 diffusion. Cluster analysis revealed that cultivars may have been bred unintentionally for desirable traits at the expense of photosynthetic capacity. These findings suggest that there is significant underutilized photosynthetic capacity among existing wheat varieties. Our observations are discussed in the context of exploiting existing natural variation in physiological processes for the improvement of photosynthesis in wheat.

Genetic and management approaches to boost UK wheat yields by ameliorating water deficits.

Faced with the challenge of increasing global food production, there is the need to exploit all approaches to increasing crop yields. A major obstacle to boosting yields of wheat (an important staple in many parts of the world) is the availability and efficient use of water, since there is increasing stress on water resources used for agriculture globally, and also in parts of the UK. Improved soil and crop management and the development of new genotypes may increase wheat yields when water is limiting. Technical and scientific issues concerning management options such as irrigation and the use of growth-promoting rhizobacteria are explored, since these may allow the more efficient use of irrigation. Fundamental understanding of how crops sense and respond to multiple abiotic stresses can help improve the effective use of irrigation water. Experiments are needed to test the hypothesis that modifying wheat root system architecture (by increasing root proliferation deep in the soil profile) will allow greater soil water extraction thereby benefiting productivity and yield stability. Furthermore, better knowledge of plant and soil interactions and how below-ground and above-ground processes communicate within the plant can help identify traits and ultimately genes (or alleles) that will define genotypes that yield better under dry conditions. Developing new genotypes will take time and, therefore, these challenges need to be addressed now.

Adaptation of photosynthesis in marama bean Tylosema esculentum (Burchell A. Schreiber) to a high temperature, high radiation, drought-prone environment.

Marama bean, Tylosema esculentum, is a tuberous legume native to the Kalahari region of Southern Africa where it grows under high temperatures (typical daily max 37 degrees C during growing season) and radiation (frequently in excess of 2000 micromol m(-2) s(-1)) in sandy soils with low rainfall. These conditions might be expected to select for increased water-use efficiency of photosynthesis. However, marama was found to give similar leaf photosynthetic rates to other C3 plants for a given internal leaf CO2 concentration and Rubisco content. Under conditions of increasing drought, no increase in water-use efficiency of photosynthesis was observed, but stomata closed early and preceded any change in leaf water potential. The possibility of subtle adaptations of photosynthetic characteristics to its natural environment were investigated at the level of Rubisco kinetics. The specificity factor of marama Rubisco was slightly lower than that of wheat, but the apparent Km for CO2 in air (Km') was about 20% lower than that of wheat. This is consistent with better adaptation for efficient photosynthesis at high temperatures in marama compared to wheat, although the net benefit is predicted to be very small (<0.5% at 35 degrees C). The sequence of marama rbcL gene shows 27 deduced amino acid residue differences from that for wheat, and the possibility that one or more of these cause the difference in Rubisco Km' is discussed.

Genetic and agronomic approaches to decreasing acrylamide precursors in crop plants.

Progress in developing genetic and agronomic approaches for reducing the levels of the principal precursors of acrylamide, asparagine and sugars in crop plants is reviewed. The factors that affect asparagine and sugar accumulation, particularly in cereal seeds and potato tubers, are described. Asparagine levels appear to be the key parameter in determining acrylamide formation in processed wheat flour and agronomic strategies for reducing asparagine accumulation in wheat grain are reviewed. Sulphur availability has been shown to be particularly important, with sulphur deprivation causing a dramatic increase in grain asparagine levels and acrylamide risk. Nitrogen availability is also a factor, with increasing nitrogen availability causing grain asparagine levels and acrylamide risk to rise. In potato, attention has been focused on sugars, and there has been some success in reducing sugar accumulation in stored potatoes by genetic modification, with a resultant reduction in acrylamide formation. However, the wisdom or otherwise of this dogma is discussed. Other possible genetic targets for manipulation or development as genetic markers in breeding programmes are reviewed. Plant breeders and farmers are encouraged to exploit the varietal differences in acrylamide risk that have already been identified and to develop good agronomic practice to reduce the levels of acrylamide precursors in cereals and potato.

Plant responses to water stress.

This Special Issue comprises a series of papers that develops the theme of plant responses to water stress, encompassing recent developments at the molecular level, through responses of photosynthesis and metabolism, to their application in crop selection and yield. The consideration of water deficits is particularly timely, given the huge developments in this area in the past decade. This issue specifically sets out to place molecular and physiological processes and their agronomic applications in an environmental context.

Manipulation of Rubisco: the amount, activity, function and regulation.

Genetic modification to increase the specificity of Rubisco for CO(2) relative to O(2) and to increase the catalytic rate of Rubisco in crop plants would have great agronomic importance. The availability of three-dimensional structures of Rubisco at atomic resolution and the characterization of site-directed mutants have greatly enhanced the understanding of the catalytic mechanism of Rubisco. Considerable progress has been made in identifying natural variation in the catalytic properties of Rubisco from different species and in developing the tools for introducing both novel and foreign Rubisco genes into plants. The additional complexities of assembling copies of the two distinct polypeptide subunits of Rubisco into a functional holoenzyme in vivo (requiring sufficient expression, post-translational modification, interaction with chaperonins, and interaction with Rubisco activase) remain a major challenge. The consequences of changing the amount of Rubisco present in leaves have been investigated by the use of antisense constructs. The manipulation of genes encoding Rubisco activase has provided a means to investigate the regulation of Rubisco activity.