CORRELATION BETWEEN THE INHIBITION OF PHOTOSYNTHESIS AND THE DECREASEIN AREA OF DETACHED LEAF DISCS OR VOLUME ABSORBANCE OF PROTOPLASTS UNDER OSMOTIC-STRESS IN PEA (PISUM-SATIVUM)/

Exposure to osmotic stress reduces leaf area and protoplast volume whi le decreasing photosynthesis. But the measurement of protoplast volume is tedious, while rapid determinations of leaf area in the field are difficult. We evaluated the quantitative relationship between the exte nt of decrease in area of detached leaf discs or the volume of protopl ast of pea (Pisum sativum) and reduction in their photosynthetic capac ity under osmotic stress. Osmotic stress was induced by increasing sor bitol concentration in the surrounding medium of the leaf discs from z ero to 1.0 M (-3.1 MPa), and in case of protoplasts from 0.4 M (-1.3 M Pa, isotonicity) to 1.0 M (-3.1 MPa, hypertonicity). There was a high degree of positive correlation between the extent of reduction in the area of detached leaf discs or the volume of protoplasts (indicated by diameter or absorption at 440 nm) and the decrease in photosynthesis. The correlation coefficients between inhibition of photosynthesis and the decrease in leaf disc area or protoplast volume were 0.96 and 0.9 9, respectively. We therefore suggest that the decrease in absorbance at 440 nm (corrected for turbidity at 750 nm) can be used as a simple measure to predict the inhibition due to osmotic stress of photosynthe sis in mesophyll protoplasts. Similarly, the reduction in area of deta ched leaf discs could also be a very simple and useful criterion to as sess osmotic tolerance of photosynthesis.