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.