Context and rationale
The research objectives of the ABC team can be placed in the general context of increasing demands for wheat grain yield and quality in a setting of climate change and changing agricultural practices. According to the FAO, cereal production needs to increase by ~50% between now and 2050 if we are to meet global demands, and this will mainly be achieved through an increase in yield rather than in cultivated land areas. At the same time, this increase will occur in a context of climate change which, although it predicts a rise in atmospheric CO2 levels that is favourable to C3 plants, will also involve a shortening of cycles due to a rise in the mean temperature and an increase in the frequency and intensity of high temperatures affecting grain weight, and water deficits unfavourable to grain and protein yields. In addition, this will have to be achieved using limited input resources for the economic, ecological or health reasons that were widely discussed by the French Environment Round Table (Grenelle de l’Environnement) and which concern nitrate fertilisation in particular. Alongside agronomic solutions, linked notably to optimising crop rotations and sequence management, plant breeding must contribute through the selection of wheat varieties that are adapted to these new production conditions. These new varieties must display a high level of tolerance to major abiotic stresses (limited water supply, high temperature, limited nitrogen application) and exploit the rise in CO2 atmospheric concentration. In this context, the main objectives of the ABC team are to obtain the scientific knowledge and develop tools to facilitate this selection.
The development cycle of wheat can be broken down into two phases separated by flowering. The "source" organs (leaves and roots) develop during the pre-flowering period. These organs will enable the assimilation of carbon through photosynthesis and the absorption of mineral elements via the roots. At the end of this period, the number of grains will be set and will constitute the priority "sinks" after flowering. The development of source and sink organs is governed by processes that give rise to marked compensations (e.g. number of organs/size of organs) under the influence of climate variables. At the flowering stage, the resulting structure defines a canopy status in terms of a source/sink ratio that will markedly affect subsequent crop functioning. Just after flowering, there is a rapid and important growth of grain volume, that sets a potential grain size for the accumulation of storage compounds. This early phase of grain development is followed by a grain filling-phase, where mainly carbon (starch ~70%) and nitrogen (~12%) accumulate in grain endosperm. These two phases of grain development are determinant for the carbon and protein yield, and is particularly sensitive to abiotic stresses. High temperatures during the early phase can markedly limit potential grain size, even if different effects on final grain mass have been observed, depending on when these high temperatures occur. We have also shown that the Grain Protein Deviation (GPD) is strongly correlated with the amount of nitrogen absorbed after flowering. In this context, the two high-priority research issues that will be addressed by the ABC team concern: (1) identification of the principal processes involved in grain development that determine its potential size under both optimum and abiotic stress conditions, and (2) determination of the genetic and ecophysiological factors affecting post-flowering nitrogen absorption dynamics.