Photosynthesis and Partitioning of Dry Matter in Sunflower

Abstract

Environmental and plant factors affecting photosynthesis in sunflower were studied in controlled environments and under field conditions. The response to temperature had a broad optimum around 27°C indicating that temperature is unlikely to limit photosynthesis of the crop under normal summer growing conditions. As found for most crops, carbon dioxide was the main factor limiting photosynthesis under optimum conditions of temperature and irradiance. Photosynthetic rates varied with the age and position of the leaf on the main stem. The maximum rate achieved by any leaf was obtained by fitting an exponential relationship of the form P = A(l -e-BI), which gave an adequate description of the light response curve. The photosynthetic rate of leaves increased to a maximum just prior to full leaf expansion and then declined. An increasing rate of photosynthesis was also characteristic of the leaf profile up the main stem, culminating in highest rates in the upper part of the leaf canopy. These data indicate that the larger recently developed active leaves, which are produced on the upper part of the stem, have the greatest potential photosynthetic capacity and are most effective in supplying carbon assimilates at all stages during the development of the crop. In the field, rates of photosynthesis for individual leaves were of the same order of magnitude and followed the same pattern as those recorded in controlled environments. Also, their diurnal patterns of photosynthesis and stomatal conductance were relatively unaffected by reduced water potentials as low as -2.0 to -2.5 MPa during the middle of the day. The partitioning of dry matter in sunflower into the various above ground parts was characterized and at physiological maturity the seed represented 55% of the dry weight of the inflorescence but only 33% of the dry weight of the total plant. There was a linear increase with time in the oil content of the seed following fertilization, reaching a maximum at about 20-25 days, several weeks before the seed reached physiological maturity. These data can provide the necessary response functions required for the photosynthetic submodel of a more comprehensive model to predict the yield and adaptation of sunflower under varying environmental conditions.