INFLUENCE OF PHOSPHORUS DEFICIENCY ON PHOTOSYNTHESIS IN SUNFLOWER ANDSOYBEAN PLANTS

The effects of phosphorus deficiency on photosynthesis were examined i n Helianthus annuus L. cv. Stromboli and Glycine max (L.) Merr. cv. Ho dgson plants grown in hydroculture. The most striking effect of Pi (ph osphate) deficiency on growth parameters was the 75% reduction in tota l leaf area in both species. Root growth was not significantly affecte d by the phosphate deficiency; consequently the shoot/root ratio under went a marked decline in both species. This can be regarded as a respo nse on the part of the plant, to ensuring maximum possible phosphorus uptake by the roots. Phosphorus deficiency affected the CO2 fixation p rocess, but in a different manner and to a different extent in the two species. In sunflower the mechanism of photosynthesis was markedly af fected by phosphate deficiency as demonstrated by a significant reduct ion in the light saturated CO2 assimilation rate, the decline in quant um yield for CO2 uptake and the decline in all fluorescence parameters . Soybeans were less severly affected by phosphate deficiency: maximum CO2 fixation rate decreased less than in sunflower, quantum yield for CO2 uptake was not significantly affected, and fluorescence parameter s declined only slightly.