17.2.1  Annual plants

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Figure 17.3 Growth of four contrasting species over a range of saline conditions. Saltbush is Atriplex amnicola, a halophyte of Western Australia (data from Aslam et al. 1986). Kallar grass (Diplachne fusca) is a halophytic grass widespread in many continents including Australia (data from Myers and West 1990). Barley is one of the most salt tolerant crops (data from Greenway 1962; Munns 1995) and lupins are one of the most sensitive (data from Jeschke et al. 1986 and B.J. Atwell, unpublished). Further comparisons with halophytes are shown in Figure 17.19

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Table 17.4

Salinity tolerance is generally assessed by measuring growth over several weeks in a range of NaCl concentrations. Dramatic differences then become evident between halophytes and crop species (Table 17.4; Figure 17.3). Halophytes such as Kallar grass (Diplachne spp.) and saltbush (Atriplex spp.) are used to revegetate salt-affected land, and provide fodder for low-intensity grazing. Atriplex amnicola, like most saltbush species, grows best in soils containing low to moderate amounts of NaCl, and can even grow at salinities higher than seawater. Diplachne, like most halophytic grasses, is not as salt tolerant as saltbush and many other dicotyledonous halophytes, but can grow and set seed at salinities above 250 mM NaCl, which is already toxic to most cultivated species. Barley is one of the most salt tolerant crop species, and many cultivars endure salinity up to 125 mM NaCl, still growing at 50% of unsalinised controls. By contrast, lupins are particularly salt sensitive. Even the more tolerant variants suffer a 50% reduction in growth at 50 mM NaCl and a breakdown in exclusion mechanisms by about 125 mM NaCl (Figure 17.6).

(a)  Salt traffic into plants

Relationships between salt and water movement into plants determine salt tolerance and are explored here in some detail.

Salinity and water upake

Water flow (Jwater) from soil to root xylem depends upon root anatomy and driving forces for water movement. Resistance to flow varies according to anatomy (r in Equation 17.3) while the water potential gradient between the soil and xylem (Ysoil – Yxylem in Equation 17.3) constitutes a driving force. Water will flow from soil to plant as long as Yxylem is lower than Ysoil (see Section 3.2).

 

Soil salinity impedes water uptake by roots because a lower Ysoil reduces the gradient for water movement into a plant. Reduced water uptake may lead to internal water deficits, which trigger changes in hormone production. These changes will cause stomata to close (partially) and so restrict photo-synthesis, as well as slowing leaf growth and development. Plants growing on highly saline soils must therefore exclude soil solutes and maintain a positive turgor. Solute exclusion by roots alleviates salt stress; osmotic adjustment in shoots contributes to turgor maintenance.

Salt exclusion by roots

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Figure 17.4 Leaf salt accumulates over time, so that od leaves usually carry higher loads of solutes than younger leaves. Moreover, a longer vascular network servicing young (distal) leaves provides additional opportunity for salt compartmentation. Net outcomes are evident in these Na+ concentrations for roots and individual leaf blades of wheat (Triticum aestivum cv. Chinese Spring) grown at 200 mM NaCl

Water entering plants carries some soil solutes that are eventually deposited in leaves following transpiration so that salt gradually builds up with time. Consequently, salt concentrations are generally higher in older leaves than in younger leaves (Figure 17.4) leading to necrosis or even premature death.

Depending on prevailing weather, plants transpire from 30 to 70 times more water than they retain. Consequently, any soil solutes not excluded by roots will end up in leaves at a concentration 30 to 70 times that of the soil solution. Ideally, a plant should admit only a small fraction of the original soil salt. For example, if a plant is transpiring 40 times more water than it retains, it should admit only 1/40 or 2.5% of the soil salt, and exclude the other 97.5%. If achieved, leaf salt con-centration would stay comparable to soil salt concentration, and that plant would survive indefinitely provided salts remain compartmentalised.

Water use efficiency (WUE) bears on this outcome because WUE can range from 2 to 9 g dry mass produced per kilogram water transpired depending upon evaporative conditions. In winter months, WUE is highest and salts will not concentrate in leaves to the same extent as they would during summer when WUE is lowest.

The proportion of water retained in leaves relative to that transpired can be calculated directly from WUE if the ratio of water to dry mass (DM) within shoot tissues is also known (i.e. H2O/DM in Equation 17.4):

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Figure 17.5 Shoot salt concentration is largely determined by root exclusion efficiency. This hypothetical case takes a saline soil equivalent to 100 mM NaCl, and assumes a plant transpires 40 times more water than it retains (i.e. only 2.5% of a transpirational stream is retained). If salt- exclusion efficiencies were 90, 95 and 97.5%, shoot salt concentration would reach an equilibrium value of 400, 200 and 100 (mM salt on a tissue water basis). If salt-exclusion efficiency was 0%, shoot salt concentration would reach 4000 mM. (Unpublished calculations courtesy Rana Munns)

Taking a WUE of 5 g DM per kg H2O, and an H2O/DM ratio of 5:1, then 25 g of water stays in the shoot for every 1 kg of water transpired. That is, a plant retains about 2.5% of water absorbed; 97.5% is transpired. Put another way, this plant transpires about 40 times more water than it retains. Salt exclusion is thus crucial, and practical consequences of different degrees of exclusion are easily simulated (Figure 17.5). Soil salinity in this case is taken as equivalent to 100 mM NaCl.

If salt was not excluded at all, shoot concentration would soon be 40 times the external concentration, that is, 4000 mM. If 90% was excluded, that is, 10% of the salt was admitted, then shoot concentration would reach four times the external solution, that is, 400 mM. This is still too high for most species. (An H2O/DM ratio of 5:1 is taken as typical for cer-eal leaves; an evergreen perennial such as citrus would be about 4:1, while soft leaves on plants such as lupins would be more like 8:1. Salt concentrations in leaves expressed on a tissue moisture basis will vary accordingly.)

Where roots act as highly effective ‘filters’, as much as 95% of soil salt can be excluded, and cell sap concentrations in leaves would then be about two times the external solution. This degree of exclusion occurs in most halophytes, and in highly salt tolerant crop species.

Equation 17.4 implies two mechanisms for adaptation to a saline soil: (1) an increased WUE and (2) an increased succulence (water content) of leaves. An increase in these characters would increase the fraction of transpired water that is retained in leaves, and for a given degree of salt exclusion would result in a lower concentration in leaves.

Succulence is a character displayed by many dicotyledonous species in response to salinity, especially halophytes, but it also occurs in salt-sensitive crop species such as lupins and beans, as well as more salt tolerant ones such as cotton. Water content per unit leaf area can increase by 50% in response to soil Na+. Leaves become thicker and more succulent or fleshy.

Increased WUE would also increase the fraction of absorbed water retained in leaves. As there is only subtle variation in WUE between species (Section 15.3), cultivar differences in salt tolerance will not be based on differences in WUE, but salt injury can at least be diminished in winter crops by adjusting time of planting and thus improving WUE. If such a crop can complete most growth during winter and early spring when the temperature is low and the relative humidity high, then WUE will be high and less salt is likely to accumulate in leaves. In crops such as rice that require high temperatures for optimum growth, high evaporative demand is unavoidable and results in low WUE, often as low as 2 g dry mass per kilogram of water transpired. Significantly, rice is also one of the most salt sensitive crop species known, and the subject of intense selection for salt tolerance (Section 17.2.3).

Species differences in salt exclusion

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Figure 17.6 Na+ in xylem fluid collected from barley and lupin grown at a range of external NaCl concentrations. Sap was collected from the base of each plant to show the ability of the root system to exclude NaCl. Barley is regarded as tolerant, whereas lupin is sensitive to saline soil (see Table 17.3 and Figure 17.2). (Based on Munns 1988, 1995)

All plants control NaCl uptake to some extent, but certain species excel and maintain a remarkably low concentration of Na+ or Cl in the xylem sap flowing from roots to shoots. For example, a salt-tolerant species such as barley (Figure 17.6) is a strong excluder, and can maintain a relatively low con-centration in xylem sap against a very wide range of soil concentrations. When sap was collected from the shoot base of barley plants grown in salinities of 25 to 200 mM NaCl, Na+ ion concentration in xylem sap stayed at about 5 mM over the whole range of salinities. In contrast, lupin (a weak excluder and thus salt sensitive) had a poor ability to regulate xylem salt concentration once the soil solution exceeded about 125 mM. A similar pattern was found for Cl ion exclusion.

Pathways of water and solute movement across roots

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Figure 17.7 A schematic root in cross-section showing basic tissues with symplasmic and apoplasmic routes for radial ingress of water and ions from soil solution to xylem. Bypass flow results from apoplasmic flow of water and ions. (Original drawing courtesy Rana Munns)

Water and ions move concurrently across roots from epidermis to xylem via two parallel pathways: apoplasmic and symplasmic. The apoplasmic pathway lies outside the plasma membranes of cells constituting a tissue system and comprises a network of cell walls, whereas the symplasm is a collective term for pathways through successive plasma membranes of adjacent cells that are interconnected by plasmodesmata. Apoplasmic flow (or ‘bypass flow’) bypasses such plasma membranes (Figure 17.7).

A common feature of roots is an endodermis (Figure 17.7) with suberised (Casparian) strips in radial cell walls that block continuity in the apoplasmic pathway between cortex and stele. Solutes then cross the endodermis via passage cells within this layer and so traverse a plasma membrane. Plant membranes have a low permeability to Na+ and Cl ions so that an endodermis with Casparian strips probably restricts inward flow of Na+ and Cl ions.

Roots of many other plants also have Casparian strips in their hypodermis (e.g. citrus; Section 17.2.2) which form a further apoplasmic barrier to radial flow of water and solutes. These hypodermal Casparian bands usually mature about
20 mm further back from the root tip than those in the endodermis, which are well developed at 10–20 mm behind the root apex. Apoplasmic barriers in the hypodermis and endodermis work in tandem to restrict Na+ and Cl flow to the xylem of roots in saline soils.

Apportionment of transpirational water flow between apoplasmic and symplasmic pathways remains a contentious issue. Similarly the proportion of salt that reaches xylem conduits via an apoplasmic versus symplasmic pathway is also ill defined. As described above, a salt-tolerant plant must exclude at least 95% of the salt in the soil solution. Accordingly, about 95% of transpirational water must move via a symplasmic pathway, and less than 5% via an apoplasmic route. Most water entering roots would certainly meet an apoplasmic barrier in the hypodermis or endodermis, and would then enter the symplasm where solute-exclusion mechanisms operate. However, when a root is mechanically damaged or invaded by pathogens, or when lateral roots are breaking through the normal barriers of the endodermis and hypodermis, apoplasmic flow would increase. Until repair mechanisms are completed, apoplasmic flow will be substantial and foliar solutes will increase. In rice seedlings, root damage caused by transplanting was repaired within 6 h, or at least rendered sufficiently functional to control uptake of NaCl within that time frame (Yeo et al. 1987). Apoplasmic uptake of Na+ was inferred from the rate of accumulation of a fluorescent dye (an apoplasmic tracer) that moved with the transpiration stream but did not adhere to cell walls or membranes. De-position of this dye in transpiring leaves indicated that an apoplasmic pathway (or bypass) accounted for up to 5% of total trans-pirational flow (Garcia et al. 1997, and discussed further in Section 17.2.3).

K+/Na+ selectivity

K+ is an essential nutrient, and under saline conditions plants must restrict Na+ and Cl entry while maintaining K+ uptake. However, the ratio of K+ to Na+ in saline soils is often extremely low, so that Na+ ions can inhibit uptake of K+ ions. If K+ uptake is not maintained, tissue Na+ concentrations become too high, an unfavourable cytoplasmic K+ to Na+ ratio results, and enzyme functions are inhibited due to ion imbalance. Fortunately, K+ transporter proteins in plasma membranes of plant cells have highly specific mechanisms for uptake of K+ and so forestall ion imbalance under mild salin-ity. Accordingly, salt-tolerant species maintain K+ uptake more effectively than salt-sensitive species (Schachtman et al. 1989; Colmer et al. 1995).

Roots of salt-tolerant species thus maintain K+ uptake despite competitive inhibition by Na+ due to selectivity of K+ uptake over Na+. Traditionally, this K+ to Na+ selectivity (SK,Na) has been described as:

Net SK,Na from Equation 17.5 embodies net selectivity of several transport processes, all of which contribute to tissue concentration of K+ and Na+ ions. Net SK,Na for root tissue would be the combined result of processes involved in uptake of both K+ and Na+, transport of these ions into xylem conduits, and possibly export of Na+ back to the soil solution.

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Figure 17.8 Salt-exclusion mechanisms in roots are enhanced by Ca2+ ions, with improved selectivity for K+ over Na+. In cotton, this Ca2+ effect is demonstrated by an absolute difference as well as a change in spatial distribution of SK, Na (the ratio of K+/Na+ in root tissue to K+/Na+ in the external solution). Cotton was grown in 150 mM NaCl at two levels of Ca2+: 1 mM (lower curve) and 10 mM (upper curve) (Based on Zhong and Läuchli 1994)

Na+ ion exclusion is enhanced by elevated Ca2+ supply (Figure 17.8) and this helps to maintain K+ concentration in root tissues, resulting in an improved net SK,Na. An especially high SK,Na in root tips (Figure 17.8) reflects their much higher K+ requirement compared with older tissues. Ca2+ is required for regulation of K+ and Na+ transport processes across plasma membranes of plant root cells and is needed in higher con-centrations on sodic (Na+-rich) soil (Section 17.1).

Saline soils are commonly high in Ca2+ as well as in Na+ so that vascular plants have evolved with mechanisms for Na+ ion exclusion that work in a Ca2+ environment. When experi-ments are conducted on salt tolerance, extra Ca2+ must be supplied along with Na+ addition to satisfy the Ca2+ require--ment of this exclusion mechanism and avoid abnornally high ingress of Na+ ions. As a useful rule of thumb the Ca2+ to Na+ ratio in bathing solutions of laboratory experiments should be at least 1:15. This is about the maximum ratio needed to suppress abnormal uptake of Na+, and mimics the minimum ratio present in most Australian soils of marine origin, or those subjected to cyclic salinisation from on-shore winds. Soils derived from parent material rich in Ca2+ and Mg2+ salts such as those in the Colorado River valley present a qualitatively different form of salinity, and a ratio of Ca2+ to Mg2+ to Na+ of 1:1:2 (as Cl salts) is employed to mimic that situation.

(b)  Salt traffic within plants

Removing xylem salt

Salt traffic within a plant depends on the exclusion capacity of root cells plus exchange between other tissues. For example, in many species including soybean and maize (Greenway and Munns 1980), Na+ can be removed from xylem conduits in upper (proximal) parts of roots and moved into adjacent living cells by exchanging with K+. By the time xylem sap reaches transpiring leaf blades, Na+ concentration is much lower than Cl, so that Na+ entry into leaf cells is reduced. Such Na+/K+ exchange during xylem extraction is not universal, and leaf Na+ in most vascular plants is similar to Cl ion concentration, indicating little K+/Na+ exchange has taken place.

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Figure 17.9 Concentrations of Na+, Cl- and K+ in xylem sap entering leaves of different ages (collected from the base of the leaf where it joined the stem) of six-week-old barley growing at 100 mM NaCl. K+ varies little while Na+ (like Cl-) is removed from the xylem as the transpiration stream ascends the stem. By implication, the mechanism for Na+ compartmentation does not necessitate comprehensive exchange with K+ ions . (Based on Wolf et al. 1991)

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Figure 17.10 Increases in Na+ content over a 7 d period in different parts of a barley plant grown in 100 mM NaCl. The numbers are µmol Na+. The total amount of Na+ taken up by the plant over 7 d was 286 µmol, the net amount deposited in the roots was 124 µmol (43%), and the remaining amount deposited in the stem and leaves as shown. (Based on Wolf et al. 1991)

Stem tissues also represent a substantial buffer for ions in the transpiration stream, and when xylem sap is collected from different stem segments of a barley seedling (Figure 17.9) concentrations of Na+ and Cl can decrease markedly from base to tip. Consequently, much less Na+ enters younger leaves compared with older leaves. In this case, the xylem sap con-cen-tration of K+ did not change substantially, implying little K+/Na+ exchange by stem tissues.

Removal of salt from xylem conduits thus reduces salt flux into young leaves and reproductive organs, and leads to substantial deposition of Na+ in stem tissues. Precise measure-ments of Na+ deposited in different parts of a barley plant over a two-week period (Figure 17.10) show that Na+ deposited in each internode was about half that deposited in the leaves issuing from it. Failing such retention by stem tissues, the amount of Na+ in leaves would have been 50% higher. Most Na+ taken was deposited in recently matured leaves and internodes. Less Na+ was deposited in older leaves; they were senescing and transpiring little during this experiment. Na+ deposition in younger leaves was constrained by a low Na+ concentration in the xylem sap supplying them (Figure 17.9). Removal of xylem salt therefore reduced Na+ movement to uppermost and rapidly growing leaves, and most importantly to reproductive organs.

Import by apices and leaves

Growing points such as root tips, shoot apices and rapidly expanding leaves, together with reproductive organs and especially fertilised ovaries, all import photoassimilate. Their resources are supplied via phloem connections, with only a minor contribution via xylem elements. Importantly, phloem K+ concentration is not affected by salinity, so that the K+ to Na+ ratio of these developing tissues can be maintained at a desirably high level. In salt-tolerant plants such as barley, exclusion of Na+ and Cl from phloem sap is particularly effective.

In addition, xylem water supplied to such growing points is also low in Na+ and Cl, but high in K+. This was evident in barley plants with an elongating stem, from the lower Na+ and Cl concentrations in the xylem sap supplying upper younger leaves compared with xylem sap supplying older lower leaves (Figure 17.9). Apical tissues are thus well protected from salt. Both vegetative and reproductive apices contain little Na+ or Cl+, around 20 mM, even when plants are growing in a highly saline soil (e.g. Lazof and Läuchli 1991).

In contrast to rapidly developing tissues, mature leaves present a totally different circumstance where xylem sap furnishes water for transpiration, and xylem solutes accumulate at sites of evaporation. Given only meagre removal of leaf salt via phloem retranslocation, salt accumulation in leaves can be calculated from exclusion efficiency and transpiration rate. Taking a saline soil at 100 mM NaCl, and an exclusion efficiency of 99%, xylem salt concentration will be about
1 mM. Assuming a transpiration rate of 2 mmol m–2 s–1, salt will build up in transpiring leaves at 8 µmol g–1 water per leaf per day, or 8 mM d–1. At this rate, salt-sensitive crop plants will start to show serious dysfunction after 15 d once leaf tissue concentration has reached about 125 mM NaCl. By com-parison, barley can tolerate leaf salts up to a concentration of around 500 mM maximum, and such leaves could live for about 60 d.

At 90% exclusion, and a salt concentration in xylem sap of 10 mM, leaf salt would increase at 80 mM d–1, and even a barley leaf would live for only 6 d.

In nature, on moderately saline soil, both Na+ and Cl ion concentrations in leaf blades of salt-tolerant plants will increase at about 10–20 mM d–1. These leaves live about 30 d. Given such observations, there must be about a 98% exclusion of salt from the transpiration stream entering those leaf blades. Such exclusion is attributable to a combination of root restriction on ingress plus retention of salt within stems and leaf bases. Acting in concert, these processes restrict salt distribution to leaves.

Leaf salt

Despite exclusion by roots, some solutes reach leaves and are compartmented. Solutes are taken up from the apoplasm to avoid desiccation of protoplasm and sequestered in vacuoles to avoid toxic concentrations in cytoplasmic compartments, such as chloroplasts and mitochondria. Salt arriving in leaves is deposited initially in cell walls adjacent to vascular conduits. Here salt would lead to dehydration of adjacent protoplasm if not relocated to vacuoles and balanced by accumulation of physiologically compatible solutes in cytoplasmic compart-ments. Put simply, this accumulation of osmotically active material represents an osmotic adjustment in response to a decrease in leaf water potential due to salt stress. In functional terms, osmotic adjustment enables cells to continue attracting water (via osmosis) and maintain turgor despite a reduction in bulk tissue water potential. Turgor-dependent processes such as cell expansion are thus sustained (discussed further in Section 17.3).

Cells undergoing division in shoot and root apices or other meristematic tissues such as rapidly expanding leaves have only small vacuoles. As a consequence, exclusion of Na+ and Cl from these cells is more crucial than in cells with large vacuoles, but mechanisms of long-distance transport, where ions are excluded from phloem conduits, protect them from salt impact.

In general, cytoplasmic NaCl concentrations over 100 mM interfere with enzyme activity, whereas Na+ and Cl ions sequestered in vacuoles are removed from major metabolic processes. However, Na+ and Cl accumulation in vacuoles necessitates a counterbalancing osmoticum in cytoplasmic compartments. Nitrogen-based compatible solutes including amino acids such as proline and quaternary ammonium com-pounds such as glycinebetaine reach substantial levels, especially in halophytes, and help protect cytoplasmic function (Section 17.3).

Consistent with vascular anatomy and salt distribution in transpiring leaves, early symptoms of salt toxicity include ‘salt burn’ at hydathodes in leaf margins. Further increase in leaf salt is lethal and results in formation of a distinct margin between living and dead tissue (i.e. between turgid and collapsed cells in Figure 17.11).

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Figure 17.11 Leaf rolling and expanding necrotic (‘burnt’) patches along leaf edges (macrophotograph in a) are typical visible symptoms of salt injury in white lupin (Lupinus albus).A progressive collapse of cells surrounding these patches due to solute excess precedes expansion of ‘burnt’ areas on leaves (scanning electron micrograph in b). These plants were held in 100 mM salt solution for only 2 d under mild conditions (slow transpiration). Scale bar in (a) = 1 cm; scale bar in (b) = 100 µm (Material supplied by Rana Munns; macrophotograph and micrograph courtesy Stuart Craig and Celia Miller)

Threshold concentration for such leaf tissue collapse differs between species, but as a general guide leaf cells in salt-tolerant vascular plants (non-halophytes such as quandong) can endure nearly 500 mM NaCl in cell vacuoles. Salt-sensitive plants such as lupins might tolerate up to 250 mM NaCl (Table 17.4). Maximum tolerable thresholds are reached more quickly under strong evaporative conditions such as those that commonly prevail during growing seasons in Australasia and result in leaf burn (Figure 17.16). In contrast, leaves of halophytes are remarkably adapted to tissue salt (Sections 17.3, 17.4) and may tolerate up to 1000 mM NaCl.

In all cases leaves soon die when salt arriving in the trans-piration stream can no longer be sequestered in vacuoles. This happens when transport systems in vacuolar membranes can no longer pump salts from cytoplasmic compartments into vacuoles against a strengthening concentration gradient. A rapid accumulation of salt in cell walls and cytoplasm follows; rapid because these compartments represent such a small volume. Protoplast dehydration from cell wall salt and toxic impact from cytoplasmic salt soon result in cell death.

Salt stress thus results in an accelerated loss of older leaves. Greater cumulative transpiration by these older leaves understandably results in higher Na+ and Cl concentrations within their tissues, and they die prematurely. This rate of loss becomes a crucial issue determining plant survival. If expansion of new leaves exceeds loss of old leaves, and a positive carbon balance is maintained at a whole-plant level, those individuals will at least complete a cycle of growth and reproductive development. However, if leaf death exceeds rate of replacement, carbon balance goes negative. Successful completion of growth and reproduction in such salt-stressed plants then becomes a race against time.

In this race, new leaves are produced at a rate that is not dependent on the health of older leaves. Rather, production of new leaves depends upon the water potential of the soil solution, and is analogous to a drought-stressed plant. A low soil water potential leads to internal water deficits and changes in hormonal balance that reduce initiation and expansion of new leaves. Moreover, during this race against time, salts do not build up in growing points to concentrations that inhibit growth. As explained above, meristematic zones depend upon phloem transport, so that salt is largely excluded. Ironically, salt accumulated by a plant does not inhibit growth of new leaves directly, hence production of new leaves depends more on soil salt than on plant salt.

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Figure 17.12 Two lines of wheat, one salt-tolerant (solid line) and one salt-sensitive (broken line) showed identical growth rates as non-salinised controls. Genotypic differences in salt tolerance became apparent within two weeks of salinisation (150 mM NaCl). In this ‘race against time’, growth curves did not begin to diverge until the proportion of dead leaves on the sensitive line exceeded about 10% of total leaf area (indicated by a vertical arrow). (Based on Munns et al. 1995)

Nevertheless, loss of old leaves is a direct consequence of salt accumulation, so that genotypes with only weak exclusion of soil salt or an inability to sequester salt in cell vacuoles will have a greater rate of senescence, and will be disadvantaged in carbon gain despite generation of new foliage. Compare, for example, two closely related cultivars that differ in salt tolerance but not in growth rate under non-salinised conditions (Figure 17.12). After two weeks in a saline soil (150 mM NaCl), both cultivars showed a strong reduction in growth (dry matter formation), but a difference between the cultivars was not clearly evident until four weeks. The more sensitive cultivar was, however, carrying many more dead leaves, and once the proportion of dead leaves reached more than about 10% of total leaf area plant growth slowed and many individuals started to die. An important principle emerged. Initial growth reduction was due to an osmotic impact of root-zone salt, whereas genetic differences in shoot sensitivity were attribu-table to differences in exclusion of root-zone salt from transpiring leaves.

 

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