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.