NET PHOTOSYNTHETIC AND TRANSPIRATION RATES IN A CHLOROPHYLL-DEFICIENTISOLINE OF SOYBEAN UNDER WELL-WATERED AND DROUGHT CONDITIONS

The gas exchange traits of wild type soybeans (cv. Clark) and a near-i sogenic, chlorophyll-deficient line homozygous for the recessive allel e y9 (y9y9) were compared under either well-watered or water-stress co nditions, Mature leaves of y9 had a 65 % lower chlorophyll content tha n wild type. However, the net photosynthetic rate (P-N) Of y9 leaves w as only 20 % lower than in the wild type, irrespective of water availa bility. Transpiration rates (E) were significantly higher in leaves of y9, compared to the wild type, either under well-watered or stress co nditions. The higher E of y9 correlated with increased stomatal conduc tance, particularly in the abaxial epidermis, where more than 70 % of the stomata were located. The combination of lower P-N and increased E resulted in a significant decrease of water use efficiency in y9, at both water availability levels, The relative water content decreased i n stressed leaves, much more in y9 than in wild type leaves, probably because of the higher E of the mutant line.