Soil water deficit and warming independently affect leaf carbon gain and biomass production
We show that elevated growth temperature increased carbon assimilation. This is consistent with the observation of Osanai et al. (2017) on cotton plants with similar temperature treatments, and is also in accordance with studies showing up-regulated carbon assimilation rate under warmer growth temperature within the optimum thermal range (Downton & Slatyer, 1972; Reddy, Baker, et al., 1991; Reddy, Reddy, & Hodges, 1995). Early studies revealed that cotton growth was thermally sensitive, with carbon assimilation and dry mass production greatly decreased when growing under unfavourable temperature regimes (Reddy, Baker, et al., 1991). Downton and Slatyer (1972) reported that the carbon assimilation of cotton was maximized at 25/20oC. However, modern varieties are commonly more heat tolerant, such that stability of photosynthesis can be retained at higher temperatures up to 36oC (Zhao, Reddy, Kakani, Koti, & Gao, 2005). The upregulation in Asat at higher growth temperatures indicates that the elevated temperature treatment was still within the thermal optimum range of this cotton variety. Increased Asat can be partially attributed to higher gs under warmer growth temperatures, which alleviated some of the stomatal limitation on photosynthesis, as evidenced by increased Ci/Ca under these temperatures. An increase in Asat under warmer growth temperatures may also be facilitated by thermal acclimation of photosynthesis, as signified by the higher Aopt at warmer temperature regimes. This occurred despite the relatively unchanged Topt across temperature treatments (Way & Yamori, 2014), and might be underpinned by adjustments in biochemical components of photosynthesis such as Rubisco carboxylation and electron transport, as well as antioxidative capacity (Kurek et al., 2007; Law et al., 2001; Law & Crafts-Brandner, 1999). On the other hand, decreased soil water availability had relatively minor effects on Asat, probably because the water deficit stress was moderate and did not generate substantial physiological effects; e.g. the lowest gs in the water deficit treatment (0.38 mol m-2 s-1) was only slightly lower than the gs threshold defining the initiation of drought-induced down-regulation of photosynthesis (i.e. 0.4 mol m-2 s-1) (Medrano, Escalona, Bota, GulĂ­as, & Flexas, 2002).
The decreased H50 for plants in warm temperature regimes demonstrated the stimulatory effect of high temperature on developmental rate, which is consistent with many previous studies (Gipson, 1986; Reddy, Reddy, & Hodges, 1992; Reddy, Davidonis, Johnson, & Vinyard, 1999). However, aboveground dry mass was reduced by elevated growth temperatures despite increased leaf level carbon assimilation. The higher vegetative dry mass under cool temperature regimes was similar to Reddy et al. (1992), who suggested that vegetative growth was favored by lower temperatures. Meanwhile, plants grown under cool temperature regimes also exhibited higher fruit dry mass. Similarly, Pettigrew (2008) reported that lint yield was slightly decreased in response to +1oC warming for field grown cotton. Together, these results suggest that warm growth temperature will compromise cotton growth and yield despite the positive effect on leaf carbon assimilation.