Drought stress is one of the major threats to rice production. The weakening of leaf photosynthesis due to drought is the main reason for the reduction of grain yield, but its mechanism is still obscure. The objectives of this study were to reveal the physiological mechanism of drought stress affecting photosynthetic capacity and grain yield. Pot experiments were conducted with drought-tolerant cultivars Hanyou113 (HY113) and Zhonghan3 (ZH3) and drought-sensitive cultivar Huanghuazhan (HHZ) under four water management treatments (traditional flooding (CK), mild drought stress (LD), moderate drought stress (MD) and severe drought stress (HD)) at heading stage in 2013 and 2014. Compared with CK, grain yield was significantly reduced by 14.9%, 30.8%, and 12.8% in HY113, HHZ, and ZH3 under LD, 32.9%, 33.7%, and 22.9% in HY113, HHZ and ZH3 under MD and 53.6%, 45.6%, and 30.7% in HY113, HHZ, and ZH3 under HD, respectively. The photosynthetic rate (P_n) decreased by 49.0% from 20.0 to 10.2 µmol m⁻² s⁻¹ in HY113, and 67.6% from 23.4 to 7.58 µmol m⁻² s⁻¹ in HHZ, and 39.3% from 23.4 to 14.2 µmol m⁻² s⁻¹ in ZH3 under HD. The P_n of HHZ was similar to that of ZH3 under CK conditions. During the drought periods from LD to HD at heading stage, the leaf water potential (LWP) reduced 31.9%, 54.8%, and 15.7% in HY113, HHZ, and ZH3, respectively. The non-photochemical quenching (NPQ) of HY113, HHZ, and ZH3 flag leaves increased by 150%, 97.6%, and 218%, respectively. The effective quantum yield of PSII photochemistry (Φ_PSII) of flag leaves reduced by 20.3%, 11.9%, and 22.1% in HY113, HHZ, and ZH3, respectively. The enzymatic activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) increased by 11.4%, 18.0%, and 21.8% in HY113, and 13.2%, 14.3%, and 30.9% in HHZ, and 13.4% 21.7%, and 17.6% in ZH3 under MD on average across two seasons. The yield reduction of drought-resistant cultivars (HY113, ZH3) was smaller than that of conventional cultivars (HHZ). Maintaining leaf water potential (LWP), P_n, photosystem II (PSII) original light energy conversion efficiency, non–photochemical quenching coefficient (NPQ), and increasing in the ratio of photochemical reaction energy in fluorescence and antioxidant enzyme activity is the physiological basis to achieve a relatively high photosynthesis. These traits could be the target for breeder to develop drought-tolerant varieties.

干旱胁迫是水稻生产的主要威胁之一。干旱引起的叶片光合作用减弱是粮食减产的主要原因，但其机制尚不清楚。本研究的目的是揭示干旱胁迫影响光合能力和粮食产量的生理机制。用耐旱品种汉油113（HY113）和中含3（ZH3）和干旱敏感品种黄花沾（HHZ）在四种水分管理处理（传统洪水（CK）、轻度干旱胁迫（LD）、中度干旱胁迫（ MD) 和严重干旱胁迫 (HD)) 在 2013 和 2014 年抽穗期。与 CK 相比，HY113、HHZ 和 ZH3 在 LD、32.9%、 33.7% 和 22.9% 在 HY113，MD 下的 HHZ 和 ZH3 以及 HD 下 HY113、HHZ 和 ZH3 分别为 53.6%、45.6% 和 30.7%。光合速率（P_Ñ）下降49.0％20.0至10.2微摩尔米^-2 小号^-1在HY113，并从23.4到7.58微摩尔米67.6％^-2 小号^-1从23.4在HHZ，和39.3％至14.2微摩尔米^-2 小号^{- 1}在 ZH3 下高清。在CK条件下，HHZ的P _n与ZH3相似。在抽穗期从LD到HD的干旱期间，HY113、HHZ和ZH3的叶水势(LWP)分别降低了31.9%、54.8%和15.7%。HY113、HHZ和ZH3旗叶的非光化学淬灭（NPQ）分别提高了150%、97.6%和218%。PSII 光化学的有效量子产率 (Φ _PSII) 的旗叶在 HY113、HHZ 和 ZH3 中分别减少了 20.3%、11.9% 和 22.1%。超氧化物歧化酶（SOD）、过氧化物酶（POD）和过氧化氢酶（CAT）的酶活性在 HY113 中分别增加了 11.4%、18.0% 和 21.8%，在 HHZ 中分别增加了 13.2%、14.3% 和 30.9%，以及 13.4% 21.7% 和 17.6% 在 ZH3 下 MD 下两个赛季平均。抗旱品种（HY113、ZH3）的减产幅度小于常规品种（HHZ）。维持叶水势 (LWP)，P _n、光系统II（PSII）原始光能转换效率、非光化学猝灭系数（NPQ）以及荧光中光化学反应能与抗氧化酶活性之比的增加是实现较高光合作用的生理基础。这些性状可能是育种者开发耐旱品种的目标。