Changes in cell size and tissue hydration (‘succulence’) cause curvilinear growth responses to salinity and watering treatments in euhalophytes
Introduction
Euhalophytes from the family Amaranthaceae are salt accumulating plants that have a growth optimum in environments containing more than 0.5 percent sodium chloride (Chapman, 1942). Euhalophytes typically have a ‘curvilinear’ growth response to external salinity, with peak growth at intermediate salinities (50–350 mM NaCl; Flowers and Colmer, 2008; Rozema and Schat, 2012; Song and Wang, 2015). For example with 56 day old Suaeda maritima, when nutrient solutions were without NaCl, shoot dry mass (DM) was ∼0.3 g, if they contained 170 mM NaCl shoot DM was ∼67% higher, and with 680 mM NaCl the shoot DM was ∼75% lower than the optimum (Yeo and Flowers, 1980). Similar growth responses to salinity are known to occur for Salicornia rubra, Salicornia bigelovii, Salicornia dolichostachya, Sarcocornia natalensis, Suaeda aegyptiaca, Halosarcia pergranulata and Disphyma australe (Tiku, 1976; Ayala and O’Leary, 1995; Katschnig et al., 2013; Naidoo and Rughunanan, 1990; Eshel, 1985; Short and Colmer, 1999; Neales and Sharkey, 1981).
Another feature of the growth of euhalophytes under saline conditions is the development of ‘succulence’. Succulence is indicated by increases in water content per cell (Jennings, 1968; Zhao et al., 2013), increases in shoot tissue hydration (e.g. the ratio of tissue water to DM) (Storey and Jones, 1979; Zotz and Winter, 1994; Inan et al., 2004; Qi et al., 2009; Han et al., 2013) and increases in leaf thickness (Black, 1958; Aslam et al., 1986). It has been suggested that succulence is an adaptive trait in the stems and leaves of halophytes to dilute ions (e.g. Jennings, 1968), however we argue that the phenomenon could simply be caused by differences in cell size associated with the changes in growth due to salinity. In general, plant cellular expansion occurs because cells behave as simple osmometers; their expansion is caused by increases in turgor within the cell and the cell wall behaves as a ‘linear viscoelastic polymer’, whose thickness is maintained constant by the deposition of new materials (Lockhart, 1965). If this is true, and if the cell walls constitute the bulk of the cellular organic DM, then tissue DM and the ratio of tissue water/DM (an indicator of succulence) would increase as cell size increased. Indeed the ratio of tissue water/DM can be thought of as an index of cell size, albeit at the whole tissue scale.
Why should there be changes in shoot DM growth and tissue hydration with changes in external salinity? Let us consider two alternative causes for the growth responses described above to increasing salinity. At one extreme, the changes might be because plants vary the number of cells in their tissues, increasing cell number as the external salinity increases from sub-optimal to optimal levels, and decreasing their number as salinities increase further from optimal to supra-optimal concentrations (Scenario 1). If this is so, then increases in growth would be accompanied by no change in directly measured cell size (where such measurements are possible to make), and with no change in the ratio of tissue water/tissue DM. At the other extreme, the changes in growth by euhalophytes with increasing salinity might be because plants vary in cell size, increasing as the external salinity increases from sub-optimal to optimal levels, and decreasing as salinities increase further from optimal to supra-optimal concentrations (Scenario 2). If this is so, then increases in growth would be accompanied by increases in directly measured cell size (where such measures are possible to make) and by increases in the ratio of tissue water/tissue DM.
Is there evidence for either of these scenarios occurring in euhalophytes? Scenario 2 (changes in cell size) provided a better explanation for the increase in growth with Suaeda maritima. In this species there was a 74% increase in FM as the external salinity increased from 0 to 340 mM (applied when plants were 21 days old for a further 56 days) and the sum of cations in extracted leaf sap was 78% higher in plants grown under saline compared with non-saline conditions (Yeo and Flowers, 1980). At 340 mM NaCl, the surface area of epidermal cells was more than twice that of plants grown under non-saline conditions (Yeo and Flowers, 1980), the proportion of total shoot tissue volume composed of cell walls was 40% lower under saline than non-saline conditions (Hajibagheri and Flowers, 1989) and the ratio of tissue water to tissue DM was 29% higher under saline than non-saline conditions (Yeo and Flowers, 1980). That the increase in growth was accompanied by increased ion concentrations in the tissues was consistent with the view that better tissue osmotic adjustment increased cell size.
The cause of the decrease in tissue growth with further salinity (optimal to supraoptimal concentrations) is still subject to conjecture (Flowers and Colmer, 2008). One suggestion is that at supra-optimal salinities there is an accumulation of ions in the apoplast which decreases turgor (c.f. Oertli, 1968; Flowers et al., 1991). Some support for this view comes from studies with the euhalophyte Suaeda maritima: when grown at 200 mM NaCl, concentrations of Na+, K+ and Cl− in the apoplasm of root cortical cells (determined by X-ray spectrometry) were ∼80-120 mM (Hajibagheri and Flowers, 1989), and turgor pressures measured in the leaves of plants grown at 200–400 mM NaCl were quite low (< 0.07 MPa) once those leaves had expanded (Clipson et al., 1985). Also, with the euhalophyte Sarcobatus vermiculatus grown in soil at 100–450 mM NaCl, direct measurements showed Na+ and K+ concentrations in the apoplasm of ∼170-225 and ∼50 mM respectively in leaves (James et al., 2006). Again, there could be two consequences of these changes. At one extreme, the decrease in tissue growth might be because the tissues produce fewer cells of similar size under supraoptimal conditions compared with optimal conditions (Scenario 1). If this is so, then decreases in growth would be accompanied by increases in tissue Na+ and/or Cl-, but these would not be accompanied by a decrease in directly measured cell size or in the ratio of tissue water/DM. The alternative is that the decrease in tissue growth might be because the plants produced similar numbers of cells as under optimal conditions, but these were smaller than under optimal conditions (Scenario 2). If this was so, then decreases in growth would be accompanied by increases in tissue Na+ and/or Cl-, and by a decrease in directly measured cell size and in the ratio of tissue water/DM.
Euhalophytes often grow in situations where soils are saline due to the presence of shallow groundwater (Barrett-Lennard et al., 2013). Groundwater is therefore one of the major sources of water for halophytes in arid and semi-arid landscapes, which they access from the capillary fringe above the water-table (c.f. Barrett-Lennard and Malcolm, 1999; Alharby et al., 2018).
Halophytes are widely found in landscapes affected by soil salinity and water deficit (Flowers, 1985; Flowers et al., 1986; Glenn et al., 2013). To some degree these two factors can be different manifestations of the same stress, because the salinity of the soil solution (the major factor that affects their growth on saltland) is the ratio of salt concentration to water content in the soil. The salinity of the soil solution therefore becomes more adverse for growth as the soil becomes more saline and also as the soil becomes drier. The interaction between salinity and drought on euhalophyte growth has rarely been studied.
We conducted an experiment in pots of soil with two euhalophytic species [Salicornia europaea L. and Suaeda maritima subsp. salsa (L.) Soó (syn. Suaeda salsa Pall.] which were watered with saline water from the base of the pot (mimicking the situation of groundwater in the field) to two percentages of field capacity. We hypothesised (H1) that if the increase in growth that occurs between sub-optimal and optimal salinity was due primarily to increases in cell size (Scenario 2), then as the external salinity (manipulated by combinations of soil water and salt) increased from non-saline to optimal concentrations we should be able to observe, simultaneously: (a) increases in shoot growth, (b) increases in internal Na+ and Cl− concentrations in the shoots, (c) increases in epidermal cell size, and (d) an increase in the shoots of the ratio of tissue water/DM. Furthermore (H2), if the decrease in growth as external salinities increase from optimal to supra-optimal levels is due to decreased osmotic adjustment (due to increased accumulation of Na+ and Cl− in the cell walls) and decreased cell size (also Scenario 2), then we should be able to observe simultaneously: (a) decreases in shoot growth, (b) further increases in internal Na+ and Cl− concentrations, (c) decreases in epidermal cell size, and (e) a decrease in the shoots of the ratio of tissue water/DM.
Section snippets
Plant material
Seeds of Salicornia europaea and Suaeda maritima subsp. salsa were collected from the halophyte botanic garden of the Fukang Desert Ecosystem Observation and Experiment Station (44.29 °N, 87.93 °E) in Xinjiang Province, in north-western China. They were stored in a refrigerator at ∼4 °C before use in experiments.
Plant culture and experimental procedure
Seeds of the two species were sown into drained plastic pots (33 cm high, 30 cm upper diameter, 19 cm lower diameter) filled with 12 kg of water-rinsed dry sandy soil. There were 20
Results
The use of soil in this trial, the introduction of NaCl through the base of the pot and the watering of the plants to two percentages of field capacity created great heterogeneity within the pot. We therefore describe: firstly, the effect of these conditions on the stratification of salt and water in the pot, secondly the effects of this on the solute potential gradients between the soil and plants, thirdly the effects of the variation in solute potential on plant growth, cell size and tissue
Discussion
This experiment was conducted to test two hypotheses (H1, H2). In accord with H1, we confirmed earlier observations with Suaeda maritima (Yeo and Flowers, 1980; Hajibagheri et al., 1984) that the increase in growth between suboptimal and optimal salinity was associated with increases in cell size (Scenario 2, Fig. 1). In both species in the present study there were simultaneous increases in shoot growth, increases in the size of the readily observable cells of the epidermis, and an increase in
Author statement
The authors of this paper are: Ma Fenglan, Edward G. Barrett-Lennard and Chang Yan Tian. The research was conceptualised by Tian and Fenglan. The data were collected by Fenglan under the supervision of Tian, and were analysed by Fenglan with support from Barrett-Lennard.
Acknowledgements
We are grateful to Professor Timothy D. Colmer who assisted with the placement of Fenglan at The University of Western Australia and for helpful comments during the writing of this paper. We thank two anonymous referees for their comments on the manuscript. We also thank Professor Baoshan Wang (Shandong Normal University in China) for constructive suggestions on the experiment. Dr Wenxuan Mai assisted in the culture of the plants. Fenglan received support from “Study on key technologies of
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