Impacts of nocturnal warming on plant performance depends on temperature regime
Studies examining the effects of nocturnal warming on plants are ambiguous, with some species exhibiting lower photosynthesis and yield, but an opposite pattern being observed for other species (Echer et al., 2014; Frantz, Cometti, & Bugbee, 2004; Mohammed & Tarpley, 2009; Prasad et al., 2008; Xu, Zhou, & Shimizu, 2009). The underlying mechanisms linking nocturnal warming and rates of photosynthesis are uncertain, but may relate to the source-sink strength for carbohydrates, which has been shown to exert control on photosynthesis in a feedback manner (Paul & Pellny, 2003). In cotton plants, it is proposed that night-time warming may inhibit photosynthesis (Reddy, Reddy, et al., 1991). However, our results showed that leaf Asat was not strongly affected by +4oC night-time temperature regardless of the day-time temperature regime. In addition, Rn was unaffected by elevated night-time temperature, which is consistent with Frantz et al. (2004) and Soliz et al. (2008), probably due to a low thermal sensitivity of respiration. This may explain the unchanged Asat by nocturnal warming, given that the constraint on photosynthesis imposed by carbohydrate sink strength was not ameliorated by respiration, as has also been demonstrated in trees (Turnbull, Murthy, & Griffin, 2002; Turnbull et al., 2004).
We expected that night-time warming would increase respiration and decrease net carbon gain and biomass production in cotton, unless photosynthesis was elevated by warming. However, biomass was unaffected or increased by nocturnal warming in some cases, suggesting that the response to nocturnal warming may be species-specific (Li et al., 2014; Wolfe-Bellin et al., 2006). Although leaf carbon balance was not strongly affected by nocturnal warming in the present study, aboveground biomass production was dependent on both temperature and water treatments. Of note, Echer et al. (2014) reported that increasing nocturnal growth temperature (+5 oC) from 32/24oC to 32/29oC decreased reproductive dry mass. In our study, fruit mass production was stable up to 32/26oC in the warmer nocturnal temperature regime, indicating that cotton yield response to elevated night-time temperature may be a threshold-like function (Schlenker & Roberts, 2009).