Heat stress alters carbon gain but not biomass production
Heat stress impairs carbon assimilation mainly by lowering Rubisco activity, which corresponds to the declining activation state of Rubisco due to the thermal lability of Rubisco activase (Sharwood, 2017). In addition, experimental evidence suggests that increasing thylakoid membrane permeability, thus reducing the amount of ATP and reductant due to electron leakage (Chavan et al., 2019; Law et al., 2001; Law & Crafts-Brandner, 1999). Plants pre-conditioned at warmer temperatures can display advanced thermal stability, so that photosynthesis is less affected during heat stress (Kurek et al., 2007; Law et al., 2001). In the present study, the significant interactions between temperature regime and heatwave provide evidence that the potential impacts of heat stress can be modified by the thermal history of cotton. We found that Asat increased for plants grown under cool temperatures but decreased at warm temperatures when exposed to the heatwave, suggesting that thermal acclimation did not positively affect carbon assimilation during the heatwave. Indeed, the response of Asat to the heatwave cannot be explained by thermal acclimation to growth temperature given that warm-grown plants exhibited higher Asat than cool-grown plants up to 45oC according to the AT response curves. Alternatively, the response pattern of Asat may be partially associated with the variation in gs. Increased gs during short-term heat stress has been observed (Najeeb et al., 2017), which apparently mitigates the stomatal limitation on photosynthesis, and also protects the integrity of the photosynthetic machinery by increasing RuBP regeneration capacity, thus facilitating carbon assimilation (Chavan et al., 2019). Noticeably, the response of gs to the heatwave was temperature regime dependent, which was similar to Hamilton III et al. (2008), who observed decreased gs following a heatwave in warm-grown, but not cool-grown Chenopodium album , indicating a nonlinear relationship between the response of gs to heatwaves and growth temperature.
Rates of Rn exhibited marked increase in response to the heatwave treatment, suggesting a limited thermal acclimation capacity (Atkin & Tjoelker, 2003). This, together with reduced Asat, resulted in decreased leaf carbon gain for leaves exposed to heat stress under warm temperature regimes, while the negative effects of increased Rn on carbon balance was compensated by increased Asat under cool temperature regimes. However, gas exchange variables exhibited fast recovery following the mitigation of heat stress, indicating that the carbon gain of cotton plants was highly resilient to heatwaves and may not affect aboveground biomass production, which did not differ between control and heat stressed plants. However, the heatwave led to the reduction in fruit mass, with the extent of the decrease dependent on growth temperature, illustrating the negative effects of heat stress on cotton yield.