Understanding growth limitation in wheat and sunflower under low phosphorus conditions.

The study described in this thesis focuses on the understanding of growth of leaf area and biomass in wheat and sunflower under low phosphorus conditions.Chapters 2 and 3 address the question whether P-deficiency limits leaf area expansion directly by inhibiting the individual leaf expansion, or through its effects on the availability of assimilates for growth. Experimental and simulation techniques were used to find an answer to this question. It was concluded that in sunflower the effects of P on sink size and assimilate supply were important for determining plant leaf area expansion (Chapter 2). Instead, wheat responded to a low P supply mainly by reducing sink size (Chapter 3). A sensitivity analysis of the effects of different model parameters at different levels of P supply, on the simulated total leaf area, showed that the value of the phyllochron, in wheat and sunflower, and tillering in wheat were crucial in determining the total leaf area.Chapter 4 focuses on the effects of P on the generation of leaves and tillers in wheat. Here, it was shown that the emergence of main stem leaves in wheat plants grown under P-deficient conditions was delayed (higher phyllochron) as both, the rate of leaf primordia initiation in the apex, and the rate of individual leaf expansion were reduced.In Chapter 5, the effects of P and assimilate supply on tillering were studied in a field experiment. P deficiencies directly altered the normal pattern of tiller emergence by slowing down the emergence of leaves on the main stem, and by reducing the maximum rate of tiller emergence. In general terms, assimilate supply seemed to play a minor role determining tillering. Including the effects of P on the phyllochron and on the maximum tillering rate in a morphogenetic model of the generation of leaves and tillers, it was possible to explain the dynamics of tiller emergence of a field experiment.In Chapter 6, a comparative study of the response to P in wheat and sunflower is presented. The different responses of wheat and sunflower to P were explained in terms of their capacity for P uptake, root to shoot ratio, and the response of photosynthesis to P.In Chapter 7, the results obtained in this thesis are discussed with respect to our present understanding of the processes involved in crops grown under P-deficient conditions.Finally, needs of research in P-limiting growth are identified and new avenues for future work discussed