Cotton is often exposed to high temperatures during the reproductive stage, which can negatively affect its productivity. The objectives were to: i) test whether heat stress impacts during the reproductive stage on photosynthesis are due to instant temperature effects or to acclimation produced during the heat stress period, ii) evaluate the role of stomatal and mesophyll conductance on net photosynthetic rate and iii) identify possible interactions between heat stress and different source/sink ratios during the reproductive period. Two field experiments were carried out in 2016 (Exp. 1) and 2017 (Exp. 2). Two heating treatment periods were imposed as follows: pre-flowering between 15 days before flower bud and flowering (H1) and post-flowering between flowering and 15 days later (H2). Each treatment had a control group (C1 and C2, respectively). In Exp. 1, two genotypes with contrasting crop cycles were compared. In Exp. 2, 50% defoliated plants (D-) were compared with intact plants (D+) under the same temperature treatments using one genotype. Average daily maximum temperature of heated treatments for both experiments was 37.9 ± 0.79ºC, 5.8ºC higher than the controls. Independently of the period, thermal stress had a negative impact on photosynthesis in both genotypes through an acclimation response, reducing it up to 35% compared with controls when heath-stressed and control plants were measured at the same temperature. Instant responses to temperature were not observed. This decrease was mainly determined by mesophyll conductance, and no recovery was observed 15 days after the end of treatments. Photosynthesis depletion was conditioned by the source/sink ratio, showing a complete recovery only in defoliated plants. It is concluded that thermal stress had a negative acclimation impact on photosynthesis, without responses to changes in instant temperature, and this acclimation is modulated mainly by mesophyll conductance.

中文翻译：

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叶肉电导调节在田间条件下暴露于热应激的棉花作物的光合速率

棉花在繁殖阶段经常暴露在高温下，这可能对其生产力产生负面影响。目的是：i) 测试生殖阶段的热应激对光合作用的影响是由于瞬时温度效应还是热应激期间产生的适应，ii) 评估气孔和叶肉导度对净光合速率的作用，以及 iii ) 确定生殖期间热应激和不同源/汇比率之间可能存在的相互作用。2016 年（实验 1）和 2017 年（实验 2）进行了两次现场实验。施加如下两个加热处理期：开花前花蕾和开花前15天（H1）和开花后至开花后15天（H2）。每个处理都有一个对照组（分别为 C1 和 C2）。在 Exp。如图 1 所示，比较了具有不同作物周期的两种基因型。在 Exp。2、在相同温度处理下，使用一种基因型将 50% 落叶植物 (D-) 与完整植物 (D+) 进行比较。两个实验的加热处理的平均每日最高温度为 37.9 ± 0.79ºC，比对照组高 5.8ºC。与时期无关，热胁迫通过驯化反应对两种基因型的光合作用产生负面影响，当在相同温度下测量健康胁迫植物和对照植物时，与对照相比，热胁迫降低了 35%。没有观察到对温度的即时反应。这种下降主要由叶肉电导决定，在处理结束后 15 天没有观察到恢复。光合作用耗竭受源/汇比的影响，仅在落叶植物中显示完全恢复。得出的结论是，热胁迫对光合作用产生了负面的驯化影响，而对瞬时温度的变化没有反应，并且这种驯化主要受叶肉电导的调节。