Flowering
Water plays crucial role in maintaining physiological responses in plants, thus decreasing water potential affects floral morphology and also impairs interaction with pollinators (Ullah et al., 2022). Intensity of the impact of stress varies at various stages of flower setting. For example, exposure of wheat plants to drought during post-anthesis stage, it either repressed grain maturation or left the grain completely unfilled (Dhanda & Sethi, 2002; Thakur et al., 2010) while when exposed during pre-anthesis stage cultivars showed increased drought tolerance at seed filling stage (Wang et al., 2014, 2015). The drought stress during panicle initiation and flowering stages in rice leads to drastic reduction in spikelet and grain number per panicle (Ghoneim 2018, 2020; Gaballah et al., 2022; Ishimaru et al., 2022). Some plants employ drought escape strategies by favoring early flowering, increased growth rate, higher photosynthesis capacity and preservation of reserves for seed development (Gaballah et al., 2022). Flowering, vegetative and seed maturation stages of maize are most vulnerable to water scarcity, causing yield loss up to 50, 25 and 21% respectively (Sah et al., 2020). Under drought stress, abscisic acid (ABA) accumulation occurs and suppress the source-sink relationship and hence disturbing the supply of sugars towards anthers, disrupting pollen development and inducing sterility (Ji et al., 2011). For example, previous studies have shown that accumulation of ABA at young microspore stage of pollen development inhibits expression of cell wall invertase enzyme in tapetal layers of anthers in drought stressed wheat plants (Onyemaobi et al., 2021).
Seed maturation
Seed filling is an essential stage of growth for all plants which involves assimilation of nutrients and transport of various constituents for synthesis of lipids, proteins, and carbohydrates for seed development (Barnabás et al., 2008; Awasthi et al., 2014). Grain maturation process is extremely susceptible to changes in environment affecting quantitative and qualitative traits of yield (J. Yang & Zhang, 2006). Synthesis and storage of reserves for development of seeds is highly dependent on the moisture content of storage cell and decrease in water potential during drought interferes with seed filling (Ochatt, 2015).
Source-sink relationship plays a crucial role in transfer of various constituents for development of seed and it is highly affected by drought as the stress declines the rate of photosynthesis thereby reducing source strength. Water scarcity also leads to loss of turgor pressure in phloem cell, making sucrose more viscous and resisting its flow from conducting cells to the sink (seeds) (Sevanto, 2014). Grain filling depends on transport of assimilates from vegetative sources to seeds directly either at pre- or post-anthesis stage (J.Yang and Zhang, 2006). For example, in rice and wheat assimilates from stems and sheaths provide about 10-40% of total seed weight during pre-anthesis stage (Gebbing & Schnyder, 1999). During drought and heat stress, supply of assimilates from stem increases up to 40% in seeds (Bidinger et al., 1977; Gebbing & Schnyder, 1999). Sucrose transporters (SUTs) genes play an important role in the transport of sucrose from leaves to sink. It has been reported in wheat, barley, maize, and soybean that drought alters the expression of SUT genes (Xu et al., 2018). Expression of SUT varies in different stress conditions, for example, inArabidopsis , expression of AtSUT2 is upregulated under drought stress while it is downregulated under heat stress conditions (increase of about 15°C) (Xu et al., 2018). In contrast, in poplar plants, expression of PtaSUT4, a symplastic loader shows upregulation under heat stress while it is downregulated under water stress resulting in decreased transport of sucrose from source to sink (Xue et al., 2016). In lupin seeds, drought reduced the activity of cytosolic acid invertase to about five-folds thereby decreasing availability of sucrose in seeds (J.-Y. Kim et al., 2000).
Drought stress at grain maturation stage exhibited increased leaf senescence and reduced grain filling phase owing to increased ABA levels in rice and wheat (J. C. Yang et al., 2003; J. Yang & Zhang, 2006). Drought-resistant plants with stay-green trait display delayed leaf senescence (Jordan et al., 2012). At the initial stage of grain filling, drought stress reduced the germination percentage (approximately 9%) of the successive generations in soybean (Smiciklas et al., 1992). Similarly, in Arachis hypogaea, drought reduced germination without affecting seedling vigor (Ketring, 1991). Drought disturbs gamete development and activates ABA signaling which interferes with carbohydrates assimilation thereby affecting seed size and seed maturation (Sehgal et al., 2018).