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.