Drought: Prolonged Exposure to Stress and Respiration

QTL have been reported for chlorophyll fluorescence in wheat stressed by drought or heat(Table 1), but studies in other species suggest that responses to combined drought and heat stress are unique compared to any, individual stress (Mittler, 2006). on the ecosystem level, drought can reduce and increases in plant respiration due to decrease in carbon substrates availability (Schaubergeret al. 2017). on the other hand, if stored carbohydrates are used for respiration and are less available for remobilization after heat stress, drought may exacerbate the effect of increases in respiration induced by heat stress. The grain filling rate of stem stocks increases with increasing temperature, which reduces the duration of grain filling (Blum et al. 1994).

Tolerance to drought and heat stress will then depend on both the initial concentration of remobilizable carbohydrates and the use of these reserves for respiration. Genetic variation for stem water soluble carbohydrates’ content has been explored with known QTL in drought or heat stress and in combined drought and heat stress (Table 1). Yang et al. (2007) genotype x environment (G x E) interactions for QTL were also investigated for the water soluble carbohydrates content of the stem and the efficiency of mobilization under water stress in wheat and significant interactions were found for all traits. They showed they did not move to the grain after water stress and suggested that losses due to respiration could be significant.Zhang et al. (2014) the QTL of water soluble carbohydrates investigated clearlydrought, heat and combined drought and heat stress were able to recognize additive effects and combinations of alleles for equally content and remobilization, suggesting that the underlying genetic mechanisms will not depend on the accumulation of stored carbohydrates. QTLs for respiration are being studied in wheat for the first time under the International Wheat Yield Partnership umbrella.

Under prolonged exposure of stress, photosynthetic activity is more inhibited by extreme accumulation of reactive oxygen species (ROS), causing damage to the membranes, proteins and chlorophyll molecules of the photosynthetic machinery (Price and Hendry, 1991; Jiang and Huang, 2001; Allakhverdievet al. 2008; Silva et al. 2010; Redondo-Gómez, 2013; Awasthiet al. 2014; Das et al. 2016).Plants use a compound antioxidants system to regulate ROS levels and avoid toxicity, but changes in redox status are also perceived by plants as a signature of a specific stress that will result in a corresponding acclimation response (Foyer and Noctor, 2005; Choudhury et al. 2017). The balance between accumulation of ROS in response to stress and their signalling role under stress has not yet been defined. The elimination of ROS is usually induced in drought conditins and heat stress, and a greater antioxidant capacity is generally correlated with stress (Koussevitzky et al. 2008; Suzuki et al. 2014; Wang X et al. 2014).In various wheat genotypes, tolerance to drought or heat stress was associated with a greater antioxidant capacity and a reduction of oxidative damage (Sairam and Saxena, 2000; Sairamet al. 2000; Lascanoet al. 2001; Almeselmaniet al. 2006; Sečenjiet al. 2010; Lu etal. 2017; Zanget al. 2017; Zhang et al. 2017). The effects of drought and heat combined in the ROS system of wheat are unknown, but recent studies highlight the importance of modulation of ROS uptake scavenging, since some pathways are specifically induced by combined stress (Rizhskyet al. 2002; Koussevitzky et al. 2008; Demirevskaet al. 2010; Zandalinaset al. 2017a). The alleles that regulate photorespiration, stability of the membrane and antioxidant capacity during drought and heat have not yet been discovered in wheat.