Soybean (Glycine max (L.) Merr. \u27Amsoy 71\u27) and sunflower (Helianthus annuus L. \u27S-1888\u27) were grown in the greenhouse and growth chamber. The effects of water stress, endogenous and exogenous abscisic acid (ABA) on photosynthesis and related parameters were investigated. Water stress was imposed on soybean and sunflower growing in the same pot in growth chamber by withholding water. In addition, an improved method for ABA analysis by HPLC and GCMS system was developed in this study;Water stress affected on photosynthetic rate and its related parameters. Drastic decreases of leaf water potential, transpiration rate, and photosynthetic rate occurred as water stress developed, and these decreases were accompanied by sharp increases of leaf stomatal resistance and leaf temperature. After rewatering, leaf water potential, transpiration rate and CER increased with decreased stomatal resistance and leaf temperature. CER recovery was slower than that of water potential. Sunflower showed a consistently greater CER than that of soybean both during water stress period and the rewatering period. The difference in CER was apparently due to consistently lower stomatal resistance for sunflower along with lower leaf temperature as well as higher transpiration rate;Leaf endogenous ABA in response to water stress showed a different pattern between soybean and sunflower. ABA increased dramatically as desiccation continued for both species, and was 3.2 fold higher in soybean than that in sunflower under severe water stressed conditions when water potentials were less than -15 bars. The greater ABA content in soybean was associated with a greater stomatal resistance, a lower transpiration rate, and a lower CER. Exogenously applied ABA showed effects similar to that of endogenous ABA. It is clear that the difference in photosynthesis between soybean and sunflower was largely due to the differences in ABA contents, at least under water stressed condition;The procedures developed in this study provided a relatively quick, simple, efficient, and reliable method for extraction, purification and quantification of ABA from highly pigmented green tissues, such as soybean and sunflower leaves
