LEAF-AREA EXPANSION AND ASSIMILATE PRODUCTION IN SUNFLOWER (HELIANTHUS-ANNUUS L.) GROWING UNDER LOW PHOSPHORUS CONDITIONS

Reductions in leaf area and plant growth as a consequence of phosphoru s (P) limitations have been attributed both to direct effects of P sho rtage on leaf expansion rate and to a reduced production of assimilate s required for growth. Canopy assimilation and leaf area expansion are closely interrelated processes. In this work we used experimental and simulation techniques to identify and study their importance in deter mining leaf area on sunflower (Helianthus annuus L.) growing under P-d eficient conditions. Experiment 1 was done outdoors, in Buenos Aires, Argentina, and Experiment 2 in a glasshouse in Wageningen, The Netherl ands. In both experiments we studied the effects of soil P addition on leaf appearance, leaf expansion, dry matter accumulation, and leaf ph otosynthesis of non-water stressed plants grown in pots containing a P -deficient soil. Before sowing the equivalent amounts of 0-600 kg of s uper phosphate ha(-1) were added to the pots. Phosphorus deficiency de layed leaf appearance increasing the value of the phyllochron (PHY) up to 76%, the rate of leaf area expansion during the quasi-linear phase of leaf expansion (LER) was reduced by up to 74%, with respect to hig h P plants. Phosphorus deficiency reduced by up to 50% the rate of lig ht saturated photosynthesis per unit of leaf area (AMAX) in recently e xpanded leaves, while at low levels of leaf insertion in the canopy, A MAX was reduced by up to 85%, when compared to that in high P plants. Phosphorus deficiency also reduced the duration of the quasi-linear ph ase of leaf expansion by up to eight days. The values of LER were rela ted (r = 0.56, P < 0.05) to the mean concentration of P in all the lea ves (Leaves P%) and not to the concentration of P in the individual le af where LER was determined (r = 0.22, P < 0.4) suggesting that under P deficiency individual leaf expansion was not likely to be regulated by the total P concentration at leaf level. The values of AMAX of indi vidual leaves were related (r = 0.79, P < 0.01) to the concentration o f total P in the corresponding leaf (Leaf P%). LER showed a hyperbolic relationship with Leaves P% (R-2 = 0.94, P < 0.01, n = 13) that satur ate at 0.14%. AMAX showed a hyperbolic relationship with Leaf P% (R-2 = 0.73, P < 0.01, n = 53) that saturated with values of Leaf P% higher than 0.22. A morphogenetic model of leaf area development and growth was developed to quantify the effect of assimilate supply at canopy le vel on total leaf area expansion, and to study the effects of model pa rameters on the growth of sunflower plants under P-deficient condition s. With this model we identified the existence of direct effects of P deficiency on individual leaf area expansion. However, we calculated t hat under mild P stress conditions up to 83% of the reduction in the o bserved leaf area was explained by the particular effects of P% on the rate of leaf appearance, on the duration of the linear period of leaf expansion, and on the value of AMAX. We also calculated that the effe cts of P deficiency on the value of AMAX alone, explained up to 41% of the observed reductions in total leaf area between the highest and th e intermediate P level in Experiment 2. Possible mechanisms of action of the direct effects of P on individual leaf expansion are discussed in this paper.