How plants respond to increases in DGW and its impact on their growth rates
Osmotic adjustment is an important drought-tolerance mechanism that offsets turgor loss through the accumulation and maintenance of soluble substances in cells (Cushman, 2001). Leaf or stem succulence facilitates osmotic adjustment by regulating internal ion concentrations in many xerophytic shrubs (Ogburn & Edwards, 2010). Variation in osmotic solute levels under different groundwater depths (Si et al., 2015), and thus, the corresponding osmotic regulation, have been reported in a few case studies (Pan et al., 2016; Zolfaghar, Villalobos-Vega, Cleverly, et al., 2015). Here, osmotic adjustments were evident by decreases inΨ atπ 100 and Ψ TLP as DGW increased (Fig. 6). Na+has been reported as an important inorganic osmoregulatory substance used by H. ammodendron to cope with summer drought (Kang, Duan, Wang, Zhao, & Yang, 2013). Higher Na+ concentration during LGS compared to EGS may facilitate this xeric species’s resistance to summer drought (Table 1). However, Na+ concentration did not correlate to increasing DGW during either the EGS or LGS, which may indicate that the accumulation of Na+ was not a key factor in sustaining negative π 100 as DGW increased.
The levels of organic compounds, such as soluble sugar and proline, increase under water-stress conditions and are, therefore, potentially important contributors to osmotic adjustment (Hong, Lakkineni, Zhang, & Verma, 2000; Si et al., 2015). In our current study, the proline concentration was lower during LGS than EGS, and thus, it did not increase the plant’s tolerance to summer droughts. Previous studies have proposed that proline does not play an important role in osmoregulation forH. ammodendron (Song et al., 2006). Soluble sugar concentration was higher at the deepest DGW site compared to the shallowest DGW site during both EGS and LGS (Table 1), indicating that sugar concentration increased along with DGW. Additionally, sugar concentration correlated with π 100 as DGW increased during EGS. Thus, we ascertained that soluble sugars act as important osmotic substances for decreasing π 100(Fig. 9a). The use of these substances, in turn, decreases the growth rate as DGW increases (Fig. 8a and Fig. 9c). Decreasingπ 100 limited the expansion of plant cell walls during leaf cellular growth (Passioura & Fry, 1992). Limited studies have investigated such an apparent correlation between leaf turgor and leaf growth rates (Kroeger et al., 2011), and fewer have investigated the effects of physiological processes, including those involving osmotic solutes, on decreasing π 100, which then constrains growth. Here, we are the first to display such a correlation.
The plasticity of Huber values is important for plants to respond to variations in groundwater depth and to sustain the homeostasis of leaf water use and water budget (Carter & White, 2009). Here, Huber values increased along with DGW during the EGS (Fig. 8c). Huber values were lower for trees over shallow groundwater (8.19 × 10−4) than for trees over deep groundwater (13.14 × 10−4; Fig. 8c) during the EGS, which resulted in an increased capacity of stems to transport water to leaves (Carter & White, 2009). These results are consistent with those of other studies on the same or closely related species across climatic gradients (Addington et al., 2006; Canham et al., 2009; Magnani, Grace, & Borghetti, 2002). Furthermore, decreased growth rate was marginally significant to the increase in Huber values with increasing DGW during EGS (Fig. 9e) as similarly reported for Prosopis tamarugo (Garrido et al., 2020). Therefore, as an important strategy to hydrological drought induced by a greater DGW, osmotic adjustment may constrain growth but also facilitate the plasticity of Huber values and help xeric trees buffer suboptimal water-supply conditions. However, these adjustments cannot fully compensate for the effects of the hydrological drought, as shown by decreased growth rates, even becoming negative during the LGS, at almost all sites.