Water and nitrogen are the most limiting factors of crop production worldwide. In the plant, both are closely associated with photosynthetic CO2 assimilation (A). Water deficit decreases A due to reduced stomatal conductance, limiting the diffusion of CO2, and also with the progression of stress, as a result of reduced capacity of the photosynthetic apparatus (Apot) resulting from metabolic changes in the leaf. This metabolic inhibition is more detrimental for the recovery of plants upon rehydration and its development depends on factors such as levels of irradiance and the intensity and duration of drought. The metabolism of leaf N is greatly affected by water deficit and probably plays a key role in the development of the metabolic inhibition of A. However, there is little information on the metabolic inhibition of A in C4 plants, especially with regard to the metabolism and nutritional status of leaf N. This lack of information is particularly striking in the case of sugar cane. The objective of this study is to examine the role of changes in the metabolism and nutritional status of N in reducing Apot in recently expanded leaves of sugar cane plants submitted to drought. To this end, three experiments will be conducted. First, sugar cane genotypes will be examined regarding their photosynthetic N use efficiency (PNUE, the relation A under saturating light / total leaf N). In a second experiment with two selected genotypes, the temporal relationships, during the progression of drought, between different variables of N metabolism with those related to photosynthesis, C metabolism, oxidative stress, and redox and energy status of the cell will be examined. This data will be used to analyze the coordination between the metabolism of C and N. In a third experiment, the influence of N supply and source (NO3-or NH4+) upon the drought-induced development of metabolic inhibition of A will be examined.
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