Introduction
Global cotton farming is increasingly challenged by rapid changes in climate (Reddy, Hodges, & McKinion, 1997; Williams et al., 2015). Cotton plants (Gossypium hirsutum L.) are known to tolerate a variety of abiotic stresses, yet climate anomalies and extremes can place cotton at greater risk of yield loss (Schlenker & Roberts, 2009; Snider, Oosterhuis, Skulman, & Kawakami, 2009; Ullah, Sun, Yang, & Zhang, 2017). Projections of global climate issued by IPCC indicate a 2oC to 4oC rise in global average temperature by 2050 across different CO2 emission scenarios (Pachauri et al., 2014), accompanied by increased intensity and frequency of drought stress and heatwaves in many arid regions worldwide (Dai, 2013; Perkins, Alexander, & Nairn, 2012). These climate scenarios will most likely generate environmental conditions beyond the optimal range for cotton growth, potentially resulting in more severe yield reduction in the near future unless management strategies are developed to improve crop adaptation. A better understanding of the potential impacts of rising temperatures and drought on cotton growth will provide valuable information guiding agronomic management required to maintain stable fibre production in the future.
Water availability is one of the most limiting factors constraining cotton productivity, especially in arid and semi-arid regions where water demand often exceeds irrigation capacity and significant land areas are often grown under rain-fed conditions (Bange, Carberry, Marshall, & Milroy, 2005; Ullah et al., 2017). During periods of water deficit stress, stomata typically close to minimize transpiration, which comes at the expense of carbon gain, given that water and CO2 exchange share the same pathway at the leaf level (Flexas, Bota, Loreto, Cornic, & Sharkey, 2004). Protracted drought stress can also affect photosynthetic electron transport, which may cause cell damage and lead to chronic down-regulation of photosynthesis (Impa, Nadaradjan, & Jagadish, 2012; Kitao & Lei, 2007; Sekmen, Ozgur, Uzilday, & Turkan, 2014). The response of cotton photosynthesis to water deficit has been examined extensively (Broughton et al., 2017; Chastain et al., 2014; Snider et al., 2015; Yi et al., 2016). It is known that leaf stomata of cotton are highly sensitive to water deficit, attaining complete closure during the early phase of drought stress (Li, Smith, Choat, & Tissue, 2019), which enables cotton to cope with short-term, mild drought without strongly affecting biomass production. However, prolonged drought can greatly compromise carbon gain, with cascading negative consequences on growth and yield (Broughton et al., 2017; Wang et al., 2016).
Temperature plays a major role in regulating plant performance due to high thermal sensitivity of many enzymatic reactions involved in carbon gain and growth regulation (Long & Ort 2010). Temperature per sehas been demonstrated to be a chief regulator of cotton growth (Pettigrew, 2008; Reddy, Hodges, & Reddy, 1992; Reddy, Baker, & Hodges, 1991). Moreover, elevated temperature can increase the level of atmospheric drought (i.e. high VPD), which will aggravate the negative effects of drought (Broughton et al., 2017). Early findings indicate that increased average daily temperatures at the beginning and the end of the growth season can promote biomass accumulation, yet long-term exposure to sub-optimal growth temperatures will result in substantial yield loss (Bange, 2007; Pettigrew, 2008; Reddy, Baker, et al., 1991; Reddy, Reddy, & Baker, 1991). It has been shown that the growth of cotton is maximized within the temperature range of 20~30oC (Reddy, Hodges, et al., 1992; Reddy, Baker, et al., 1991), while the optimum thermal range for enzymatic activity, germination, flowering and lint production is 28 ± 3oC (Burke & Wanjura, 2010); when growth temperature exceeds 35oC, rates of photosynthesis decline due to declining Rubisco activity as a result of deactivation and increased respiration (Loka & Oosterhuis, 2010; Sharwood, 2017). The thermal operating range of plants is dependent on physiological acclimation. For example, the temperature dependence of photosynthesis can exhibit phenotypic plasticity in response to growth temperature, such that warm-grown plants typically have higher optimum temperature of photosynthesis (Topt), thus enabling plants to maintain positive carbon gain under new thermal regimes (Way & Yamori, 2014; Yamori, Hikosaka, & Way, 2014). Rapid thermal adjustment in photosynthesis is of fundamental importance to cotton growth and yield, given that elevation in temperatures is prevalent in regions where cotton is often planted (Broughton et al., 2020; Singh, Prasad, Sunita, Giri, & Reddy, 2007).
Projections of climate change suggest a larger increase in night-time temperature (Pachauri et al., 2014), which can have distinct effects on crop physiology and productivity compared to elevated daytime temperature (Izquierdo, Aguirrezábal, Andrade, & Pereyra, 2002; Mohammed & Tarpley, 2009; Prasad, Pisipati, Ristic, Bukovnik, & Fritz, 2008; Prasad & Djanaguiraman, 2011; Wolfe-Bellin, He, & Bazzaz, 2006). Respiration is thermally sensitive and dominates the carbon flux in darkness. Nocturnal warming may promote carbon loss, leading to decreased carbohydrate availability, which are key determinants of fruit yield and quality (Loka & Oosterhuis, 2010; Pettigrew, 2001, 2008). The carbohydrate shortage related plant growth anomalies can be further intensified by nocturnal warming related down-regulation of photosynthesis (Reddy, Baker, et al., 1991; Sinsawat, Leipner, Stamp, & Fracheboud, 2004). Moreover, elevated night-time temperature can cause early abscission of reproductive structures, further decreasing reproductive dry matter (Soliz, Oosterhuis, Coker, & Brown, 2008). Reduced yield caused by elevated night temperature has been observed in many crops, especially for species characterized by higher respiratory thermal sensitivity, including cotton (Gipson & Joham, 1968; Mohammed & Tarpley, 2009; Prasad et al., 2008; Soliz et al., 2008).
From a physiological perspective, short-term pulses of high temperature can push plants beyond their thermal thresholds, resulting in the sudden collapse of many metabolic processes (Zhu et al., 2018). High temperature impairs the photosynthetic apparatus by disrupting photosynthetic pigments, inhibiting activity of photosystem II and deactivating various enzymes involved in photosynthetic carbon reactions (Chavan, Duursma, Tausz, & Ghannoum, 2019; Law, Crafts-Brandner, & Salvucci, 2001). Furthermore, heatwaves can increase water loss without improving photosynthesis, which will exacerbate the impairments of drought stress on carbon gain (Najeeb, Sarwar, Atwell, Bange, & Tan, 2017). For cotton plants, reduced photosynthesis, growth, fruit production and fibre quality have been observed in plants subjected to short-term increased temperature (Carmo-Silva et al., 2012; Snider et al., 2009). It is proposed that the response to heatwaves can be modified by plant thermal history, such that warm grown plants might be less affected by heat stress compared to cool-grown counterparts (Haldimann & Feller, 2005; Kurek et al., 2007; Larkindale & Vierling, 2008; Salvucci & Crafts-Brandner, 2004). Yet, it is unclear if the negative impacts of heatwaves on cotton can be buffered by warm growth temperature.
The impacts of global climate change factors on cotton physiology and growth have been documented (Broughton et al., 2020; Broughton et al., 2017; Echer, Oosterhuis, Loka, & Rosolem, 2014; Loka & Oosterhuis, 2010; Ullah et al., 2017; Williams et al., 2015), but uncertainty still remains regarding the response of cotton to multiple interactive stress conditions. Here, we investigated the effects of day-time and night-time growth temperature, water deficit and heatwaves on carbon assimilation and growth of cotton. Plants were raised under four temperature treatments under well-watered conditions until the development of flower buds, and then plants from each treatment were subjected to water deficit stress and subsequently to a five-day heatwave. Leaf gas-exchange characteristics were measured shortly following the heatwave treatment, and during recovery from the heatwave; dry mass production was measured at the end of the experiment. We hypothesised that: (1) warmer daytime growth temperature will increase net carbon assimilation and rates of development, but this beneficial effect is dependent on water availability; (2) nocturnal warming will negatively affect plant biomass production by decreasing net carbon gain; (3) the response to heat and water deficit stress can be modified by growth temperature, therefore warm-grown cotton will be less affected by the heatwave; and (4) cotton is highly resilient to heatwaves, so carbon gain will undergo fast recovery following the mitigation of heat stress.