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.