LEAF DEVELOPMENT IN LOLIUM-TEMULENTUM L - PHOTOSYNTHESIS IN RELATION TO GROWTH AND SENESCENCE

Length, area, weight and concentrations of pigment were measured from just after emergence until senescence of the fourth leaf of Lolium tem ulentum L. seedlings grown in nutrient solution in a controlled enviro nment. At the same time the progress of photosynthesis, respiration an d carbon contents of the leaf were recorded. A new approach to calcula ting plastochron indices for grasses was developed, based on logistic curves fitted to leaf lengths. Dry weight and photosynthetic rates per unit area were greatest in young leaves. Subsequently photosynthetic capacity at light saturation declined, as did photosynthetic rates at the light incident on the leaf, although the decrease was less pronoun ced in the latter case. Dry weight per leaf remained fairly constant o nce full expansion was reached, with perhaps a slight rise in the olde st leaves. The photosynthesis and carbon content data were combined to calculate the carbon balance of leaf 4, with respect both to the atmo sphere and to the rest of the plant. Net gas exchange became positive about 7 d before full leaf expansion. Leaf weight continued to increas e so that the carbon balance with the rest of the plant did not become positive until just after final leaf size had been attained. These re sults are discussed in relation to the likely contribution of the vari ous components of the balance to plant productivity.