Drought stress is a factor affecting plant growth in the arid regions of Tunisia. The experiment was performed on two species: Hedysarum coronarium (H. coronarium; Sulla coronaria) and Hedysarum carnosum (H. carnosum; Sulla carnosa) which were subjected to two water regimes (60 and 100% of field capacity). The aim was to study the photochemical stability of PSII (using the fluorescence transient OJIP in light- and dark-adapted states) and gas exchange under drought stress. Results revealed that drought stress affected the majority of the studied physiological parameters: a sharp decrease in relative water content, total chlorophyll content, and gas exchange in both species during the first month of treatment followed by a more or less constant phase (2 months) and a decrease to the lowest levels of the studied parameters. Nevertheless, H. coronarium plants maintained relatively higher stomatal conductance, photosynthesis activity, transpiration rate, and total chlorophyll content under drought stress conditions. In addition, a change in fluorescence intensity as a function of time was noted in both species. The effect of drought stress was also invested on the different fluorescence intensities of the transitions J, I, and P and consequently on the different oxidation–reduction states of PSII electron acceptors. Drought stress had no effect on the fluorescence level O, whereas the effect which was observed at the J transition level was much higher at the I and P levels. In addition, H. coronarium was more able to protect its PSII from water-deficit stress as consequently was more drought-tolerant compared to the endemic species of H. carnosum in arid climate. Overall, our study contributed to the understanding of the mechanisms of adaptation and stability of PSII under drought stress in H. carnosum and H. coronarium.

干旱胁迫是影响突尼斯干旱地区植物生长的一个因素。实验在两个物种上进行：冠花柴( H. coronarium ; Sulla coronaria ) 和红花柴( H. carnosum ; Sulla carnosa) 受两种水情（田间持水量的 60% 和 100%）的影响。目的是研究 PSII 的光化学稳定性（在光和暗适应状态下使用荧光瞬态 OJIP）和干旱胁迫下的气体交换。结果表明，干旱胁迫影响了大部分研究的生理参数：在处理的第一个月期间，两个物种的相对含水量、总叶绿素含量和气体交换急剧下降，随后是或多或少的恒定阶段（2 个月）并降低到研究参数的最低水平。尽管如此，H. coronarium植物在干旱胁迫条件下保持相对较高的气孔导度、光合作用活性、蒸腾速率和总叶绿素含量。此外，在两个物种中都注意到荧光强度随时间的变化。干旱胁迫的影响还体现在跃迁 J、I 和 P 的不同荧光强度上，从而影响 PSII 电子受体的不同氧化还原状态。干旱胁迫对荧光水平 O 没有影响，而在 J 跃迁水平观察到的影响在 I 和 P 水平要高得多。此外，H. coronarium更能够保护其 PSII 免受缺水胁迫，因此与地方性物种相比更耐旱干旱气候下的H. carnosum 。总的来说，我们的研究有助于理解 PSII 在H. carnosum和H. coronarium干旱胁迫下的适应和稳定性机制。