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).