PLANT LEAF-AREA EXPANSION AND ASSIMILATE PRODUCTION IN WHEAT (TRITICUM-AESTIVUM L.) GROWING UNDER LOW PHOSPHORUS CONDITIONS

Under phosphorus deficiency reductions in plant leaf area have been at tributed to both direct effects of P on the individual leaf expansion rate and to a reduced availability of assimilates for leaf growth. In this work we use experimental and simulation techniques to identify an d quantify these processes in wheat plants growing under P-deficient c onditions. In a glasshouse experiment we studied the effects of soil P addition (0-138 kg P2O5 ha(-1)) on tillering, leaf emergence, leaf ex pansion, plant growth, and leaf photosynthesis of wheat plants (cv. IN TA Oasis) that were not water stressed. Plants were grown in pots cont aining a P-deficient (3 mg P g(-1) soil) sandy soil. Sowing and pots w ere arranged to simulate a crop stand of 173 plants m(-2). Experimenta l results were integrated in a simulation model to study the relative importance of each process in determining the plant leaf area during v egetative stages of wheat. Phosphorus deficiency significantly reduced plant leaf area and dry weight production. Under P-deficient conditio ns the phyllochron (PHY) was increased up to a 32%, compared to that o f high-P plants. In low-P plants the rate of individual leaf area expa nsion during the quasi-linear phase of leaf expansion (LER) was signif icantly reduced. The effect of P deficiency on LER was the main determ inant of the final size of the individual leaves. In recently expanded leaves phosphorus deficiency reduced the photosynthesis rate per unit leaf area at high radiation (AMAX), up to 57%. Relative values of AMA X showed an hyperbolic relationship with leaf P% saturating at 0.27%. Relative values of the tillering rate showed an hyperbolic relationshi p with the shoot P% saturating at values above 0.38%. The value of LER was not related to the concentration of P in leaves or shoots. A morp hogenetic model of leaf area development and growth was developed to q uantify the effect of assimilate supply at canopy level on total leaf area expansion, and to study the sensitivity of different model variab les to changes in model parameters. Simulation results indicated that under mild P stress conditions up to 80% of the observed reduction in plant leaf area was due to the effects of P deficiency on leaf emergen ce and tillering. Under extreme P-deficient conditions the simulation model failed to explain the experimental results indicating that other factors not taken into account by the model, i.e. direct effects of P on leaf expansion, must have been active. Possible mechanisms of acti on of the direct effects of P on individual leaf expansion are discuss ed in this work.