17.2.2  Perennial plants

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Woody perennials of horticultural significance are commonly grown in Australasia under high-input conditions that frequently lead to soil salinisation. Continued production then depends upon skilful management of irrigation and judicious use of rootstocks that confer salt tolerance on grafted plants. Citrus and grapevines are taken here as case studies, and sub-sequently compared with two natives where environmental selection pressures in their native habitats have led to a remarkable degree of salt tolerance.

(a)  Citriculture

Citrus species include some of the world’s most salt sensitive crops (Table 17.3; Maas 1990), but rootstocks can confer considerable tolerance on commercial scions grafted on to them by restricting uptake of Na+ and Cl ions from saline environments.

With soil solutions of around 50 mM NaCl, salt-sensitive rootstocks of citrus take up large amounts of Cl, and to a lesser extent Na+. Visible symptoms of toxic levels include marginal and interveinal leaf burn, usually followed by leaf fall. At lower intracellular concentrations, even though there are no visible symptoms of salt damage, metabolism may be perturbed due to accumulation of either Na+ or Cl in the cytosol (a specific ion effect) or indirectly by imposition of an osmotic stress (Section 17.3).

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Table 17.5

During salt stress, seedlings of citrus rootstocks differ greatly in Cl transport to their leaves or the leaves of scion species grafted on to them. Seedling leaves of Rangpur lime (Citrus reticulata var. austera hybrid?) accumulate much less Cl compared to leaves on a salt-sensitive rootstock like Etrog citron (Citrus medica L.)(Table 17.5). Cl ion concentration can be 10 times lower in leaves of Rangpur lime compared to Etrog citron!

Differentiation of rootstocks into strong and weak excluders is based on Cl levels in leaves rather than roots because leaves show stronger contrasts as a function of rootstock (Table 17.5). Root Cl concentrations are generally comparable for strong and weak excluders (expressed as an upward flux of water per unit mass of root). Clearly, rootstocks that are efficient at restricting Cl from foliage are able to restrict ingress of ions from surrounding soil.

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Figure 17.13 Na+ retention by stem segments from seedlings of trifoliate orange (Poncirus trifoliata) (open symbols) and Rangpur lime (Citrus reticulata var. austera hybrid?) (closed symbols) demonstraes strong exclusion of Na+ ions by roots of trifoliate orange with subsequent compartmentation in stem tissues. This compartmentation results in low leaf Na+ in scions grafted to trifoliate rootstocks compared with those grafted to Rangpur lime. (Based on Grieve and Walker 1983)

Overall, citrus rootstocks do not exclude Na+ as effectively they do Cl, but even so some genetic variation in Na+ exclusion is evident. For example, trifoliate orange (Poncirus trifoliata (L.) Raf.) is closely related to Citrus species and restricts foliar accumulation of Na+ (Table 17.5) and partitioning can also occur in shoots independently of rootstock. Compared to Rangpur lime, which is a strong Cl excluder, trifoliate orange selectively partitions Na+ and Cl so that Na+ is concentrated in stem base and woody roots. A profile of Na+ accumulation along a stem of trifoliate orange (Figure 17.13) thus shows a marked decrease in Na+ concentration from stem base to apex which accords with extremely low levels of Na+ in the leaves of trifoliate orange. This profile contrasts with Rangpur lime where less Na+ is retained in stems but much more accumulates in leaves.

Budding different scion varieties to a range of rootstocks differentiates between rootstock and scion effects on Na+ and Cl accumulation in citrus foliage. As a general rule, Cl exclusion tends to be rootstock dependent whereas Na+ exclusion varies according to both rootstock and scion. By implication, roots represent a major site for regulation of Cl traffic, whereas basal roots and stems are major sites for regulation of Na+ traffic via reabsorption of this ion from the
transpiration stream.

Root anatomy

Notionally, fibrous roots of citrus seem to present a large surface area for water and nutrient uptake as well as salt exclusion because root length density is around 10–50 km m–3. However, those values are still an order of magnitude lower than other woody perennials such as grapevines (c. 250 km m–3), which in turn are lower than grasses (c. 500 km m–3; Table 3.1). Salt exclusion by citrus rootstocks is therefore all the more remarkable, and warrants some understanding of their anatomy.

A transverse section of a fibrous root (Figure 17.14) shows (outside to inside) an epidermis, a hypodermis (or specialised outermost layer of cortical cells; see exodermis Section 3.6), a cortex with 10 or more cell layers and a vascular core (stele). Protoxylem strands (arcs) range from two to five in different roots, and are surrounded by an endodermis. Most fibrous roots undergo little secondary thickening, although the epidermis may slough off.

Citrus roots develop a prominent hypodermis (‘h’ in Figure 17.14b) consisting of a moribund layer of thick-walled cells below the epidermis and interspersed with thin-walled passage cells (‘p’). Many epidermal cells degenerate, but pockets of functional cells often remain covering the passage cells; these passage cells form a bridge between epidermis and cortex. Hypodermal cell walls running parallel to root surfaces are lignified whereas the inner and radial walls are suberised. Such a hypodermis is thus an important barrier to movement of salt and water into citrus roots (see also Plate 1 in Storey and Walker 1987).

The root cortex consists of large cells with many inter-cellular airspaces at cell junctions. This creates a tortuous apoplasmic pathway for water and ions moving into xylem conduits. Cell walls of cortical cells are not thickened with suberin and are therefore relatively permeable to water and ions. Citrus root cortex is also an important storage tissue for starch which accumulates within amyloplasts held in the cytosol of cortical cells.

Pathways for Na+ and Cl transport across roots

Assuming the hypodermis is a significant barrier to water uptake, ions will enter the symplasm via either passage cells or epidermal cells. Where roots grow in saline medium (e.g.
50 mM NaCl) these cells are bathed in the external culture solution, but show very high K+ to Na+ ratios (SK,Na selectivities). Passage cells and adjacent epidermal cells are likely entry points into the symplasm and show much higher SK,Na selectivities than either outer or inner cortical cells.

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Table 17.6

In addition to uptake through the symplasm, roots of some citrus species have a transpirational (and thus solute) bypass through their apoplasm. Accumulation of foliar Na+ and Cl varies accordingly, and can be linked to transpiration but rootstock effects are once again apparent (Table 17.6). Foliar
levels of Na+ and Cl increase significantly in the salt-sensitive rootstock Etrog citron under conditions of low humidity (fast transpiration), whereas neither Na+ nor Cl flow is coupled to transpirational flux in the salt-resistant rootstock Rangpur lime. By implication, Rangpur lime maintains little if any bypass flow.

Decoupling water flow from Cl uptake in Rangpur lime is especially noteworthy because transpiration (H2O flux per unit mass of root) is about twice as high in this rootstock as it is in Etrog citron when both rootstocks are grown in 50 mM NaCl.

Notwithstanding apoplasmic flow, a significant part of Na+ and Cl absorption by citrus roots occurs via the symplasm and is mediated by properties of root cell membranes. Taking a root as a simple filter (a single membrane) for Na+ and Cl ions, exclusion efficiency can be measured by the ratio of Na+ or Cl in the xylem to the Na+ or Cl in the external salt solution.

In grasses or cereals where the H2O/DM ratio was taken as 5:1 (discussed above), about 25 g of water stays in the shoots for every kilogram of water transpired. In evergreen perennials where the H2O/DM ratio is taken as 4:1, only 20 g of water is retained for every kilogram of water transpired. Accordingly a filtration process is required to reduce the xylem con-centration of Na+ and Cl by about 50 times to prevent excess build up of ions in leaf tissues (Section 17.2.1; and Munns 1988). Xylem Cl concentrations of Rangpur lime are about 2 mM when roots experience 50 mM NaCl, and are only 3 mM or thereabouts in Etrog citron, the weaker Cl excluder. Filtration of soil water by citrus roots thus lowers [Cl] in xylem conduits by a factor of 25–50, a process which is especially efficient in salt-resistant Rangpur lime (Storey 1995).

(b)  Viticulture

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Figure 17.15 Leaf burn on own-rooted sultana grapevines (syn. Thompson Seedless) at the end of a growing season at NSW Agriculture Dearton and irrigated with salinised (12 mM NaCl) irrigation water. Vines correspond to those describe by Prior et al. (1992) (see colour plate 64) (Photograph courtesy P.E. Kriedemann)

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Table 17.7

Australian vineyards are often subject to salt stress from either airborne particles via on-shore winds (Margaret River area in southwest Western Australia) or more commonly from brackish irrigation water and sodic soils in the lower Murray–Darling basin. Leaf burn (Figure 17.15) is one obvious manifestation of irrigation salt damage, with an attendant reduction in yield and vine vigour that intensifies over successive seasons.

Salinity impact is often insidious, and symptoms are not necessarily expressed until the second or subsequent seasons of irrigation with low-quality water. Cumulative effects then become apparent, and especially with own-rooted vines. Fortunately rootstocks are available to alleviate this problem.

Commercial cultivars of grape (Vitis vinifera) have long been recognised as differing in salt sensitivity, but credit must go to Sauer (1968) and Bernstein et al. (1969) for discovering that vine species used as nematode-resistant rootstocks also act as strong excluders of soil salt, and restrict Cl entry to shoots. Taking sultana (syn. Thompson Seedless) as a representative scion used extensively for irrigated vineyards (Table 17.7), vines grafted to one of three different rootstocks have consistently lower Cl content in petioles, leaf blades and berries compared with sultana vines on their own roots. In Table 17.7, Ramsay refers to a selection of Vitis champini, Schwarzmann is a hybrid of V. rupestris ¥ V. riparia, while Harmony has a more complex pedigree including V. champini, V. longii and a further interspecific hybrid between European and native American species.

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Table 17.8

Commercial cultivars of V. vinifera are irrigated worldwide, and are thus at risk in potentially saline vineyards when established on their own roots rather than as grafted vines. Accumulation of leaf Cl during one growing season by muscat grapes grafted to different rootstocks (Table 17.8) illustrates this principle. Further data from Shiraz, Cabernet Sauvignon and sultana scions (Downton 1977) all demon-strated an unambiguous benefit from rootstocks in lowering Cl ion content of petioles, leaves and berries where
own-rooted vines were consistently and appreciably higher. Na+ exclusion was also enhanced by rootstocks (Table 17.8) although responses were less clear cut compared with Cl ion exclusion, and some cases were reported where berry Na+ was somewhat higher in grafted vines. Different exclusion mechanisms apply to Na+ compared to Cl ions (Section 4.2) so comparative exclusion of these two ion species will not necessarily correlate. However, K+ uptake was enhanced by salt-excluding rootstocks (see data for petiole K+ in Table 17.8) and would have contributed to overall salt tolerance.

(c)  Pistachio and quandong

In citriculture and viticulture, salt-sensitive scions are grafted onto salt-excluding rootstocks, so that salt tolerance relies primarily on exclusion of soil salt. By contrast, horticultural use of pistachio (Pistacia vera) (Walker et al. 1987, 1988) and quandong (Santalum acuminatum) (Walker 1989) also draws on a capacity to endure leaf salt. Both species have adapted to adverse soil conditions and root exclusion of soil solutes has become complemented by leaf tolerance of remarkably high tissue levels of NaCl.

Pistachio suffers reduction in growth after prolonged salinisation (20 weeks at 150 mM Cl) but leaf gas exchange remains unaffected and leaf turgor is maintained for shorter periods (five weeks at 100 or even 175 mM Cl). In that case (Walker et al. 1988) leaf tissue Cl as high as 255 mM (tissue moisture basis) was without effect on either CO2 assimilation or transpiration.

In keeping with reduced shoot extension, and thus a diminished sink for photoassimilate, salt-stressed plants ac-cumu-lated more starch, sucrose and reducing sugars. Increased proline accumulation accompanied these higher concentrations of soluble carbohydrates and would have contributed to an increase in tissue osmotic pressure (an osmotic adjustment) from 2.14 MPa to 2.74 MPa. By implication, both stomatal function and mesophyll activity had been preserved via an accumulation of compatible solutes in cytoplasmic compart-ments that balanced salt accumulation in vacuoles.

Quandong (native peach) is a small indigenous tree with ornamental value that produces attractive red or yellow fruit with horticultural potential. Seedling trees are known to withstand highly saline irrigation water at Quorn (inland South Australia) (Sedgley 1982) and this prompted Walker (1989) to undertake a closer analysis of their salt tolerance.

Quandong is a partial root parasite (Case study 15.4) with a wide host range. Seedling trees were established with strawberry clover as a host plant by Walker (1989) and salinised over a wide range (0.5–500 mM Cl) for up to 14 weeks. Treatment with 50, 100 or 200 mM Cl had little effect on growth at seven weeks, but 200 mM Cl did have an impact by 14 weeks. Remarkably, leaf gas exchange was unaffected by salinity with leaf Cl and Na+ remaining around 273 and 227 mM (tissue moisture basis) respectively. Longer and stronger salinisation reduced both gas exchange and chlorophyll concentration (but without visible damage) so that photosynthesis on a chlorophyll basis was not reduced despite leaf Cl and Na+ concentrations of up to 489 and 404 mM (tissue moisture basis). Contrast this outcome with citrus where leaf tissue Cl above about 150 mM reduces photosynthesis, and Cl levels above about 350 mM lead to leaf damage.

Populations of both pistachio and quandong seedlings as studied here showed individual variation with respect to foliar salt tolerance so that future horticulture can expect to target genotypes with even further improvement in this trait.

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