Stephen R. Grattan

Salinity–mineral nutrient relations in horticultural crops

The relations between salinity and mineral nutrition of horticultural crops are extremely complex and a complete understanding of the intricate interactions involved would require the input from a multidisciplinary team of scientists. This review addresses the nutrient elements individually and we emphasise research directed towards the organ, whole-plant and field level. We have attempted to synthesise the literature and reconcile results from experiments conducted in a variety of conditions such as soil and solution cultures, those using mixed and single-salt (only NaCl) compositions, and those conducted over short (days) and long periods (months) of time. Crop performance may be adversely affected by salinity-induced nutritional disorders. These disorders may result from the effect of salinity on nutrient availability, competitive uptake, transport or partitioning within the plant. For example, salinity reduces phosphate uptake and accumulation in crops grown in soils primarily by reducing phosphate availability but in solution cultures ion imbalances may primarily result from competitive interactions. Salinity dominated by Na+ salts not only reduces Ca2+ availability but reduces Ca2+ transport and mobility to growing regions of the plant, which affects the quality of both vegetative and reproductive organs. Salinity can directly affect nutrient uptake, such as Na+ reducing K+ uptake or by Cl- reducing NO3/- uptake. Salinity can also cause a combination of complex interactions that affect plant metabolism, susceptibility to injury or internal nutrient requirement. Despite a large number of studies that demonstrate that salinity reduces nutrient uptake and accumulation or affects nutrient partitioning within the plant, little evidence exists that adding nutrients at levels above those considered optimal in non-saline environments, improves crop yield. Nutrient additions, on the other hand, have been more successful in improving crop quality such as the correction of Na-induced Ca2+ deficiencies by supplemental calcium. Nutrient additions may also reduce the incidences of injury as has been observed in the reduction of Cl-toxicity symptoms in certain tree crops by nitrate applications. It is reasonable to believe that numerous salinity-nutrient interactions occur simultaneously but whether they ultimately affect crop yield or quality depends upon the salinity level and composition of salts, the crop species, the nutrient in question and a number of environmental factors.

PLANT GROWTH AND DEVELOPMENT UNDER SALINITY STRESS

Plant growth and development are adversely affected by salinity – a major environmental stress that limits agricultural production. This chapter provides an overview of the physiological mechanisms by which growth and development of crop plants are affected by salinity. The initial phase of growth reduction is due to an osmotic effect, is similar to the initial response to water stress and shows little genotypic differences. The second, slower effect is the result of salt toxicity in leaves. In the second phase a salt sensitive species or genotype differs from a more salt tolerant one by its inability to prevent salt accumulation in leaves to toxic levels. Most crop plants are salt tolerant at germination but salt sensitive during emergence and vegetative development. Root and shoot growth is inhibited by salinity; however, supplemental Ca partly alleviates the growth inhibition. The Ca effect appears related to the maintenance of plasma membrane selectivity for K over Na. Reproductive development is considered less sensitive to salt stress than vegetative growth, although in wheat salt stress can hasten reproductive growth, inhibit spike development and decrease the yield potential, whereas in the more salt sensitive rice, low yield is primarily associated with reduction in tillers, and by sterile spikelets in some cultivars.

Drainage water reuse

Saline-sodic (4 < EC[dS/m] < 30; 10 <SAR < 40) drainage water can be used toirrigate crops that are moderatelysensitive, moderately tolerant and tolerantto salinity. However, in order to besustainable, particular attention isrequired towards crop selection, control ofsoil salination, and crop and soilmanagement to maintain soil permeability towater and air. Potential negative impactsof B, Mo and Se on crop yields, foragequality and wildlife also must be takeninto account and, if necessary, mitigated.The focus of this paper is the Californiaexperience, along the Westside San JoaquinValley (WSJV), related to these concerns.

Biomass accumulation and potential nutritive value of some forages irrigated with saline-sodic drainage water

A controlled study using a sand-tank system was conducted to evaluate 10 forage species (bermudagrass, ‘Salado’ and ‘SW 9720’ alfalfa, ‘Duncan’ and ‘Polo’ Paspalum, ‘big’ and ‘narrow leaf’ trefoil, kikuyugrass, Jose tall wheatgrass, and alkali sacaton). Forages were irrigated with sodium-sulfate dominated synthetic drainage waters with an electrical conductivity of either 15 or 25 dS/m. Forage yield was significantly reduced by the higher (25 dS/m) salinity level of irrigation water compared to the lower (15 dS/m) level. There was wide variation in the sensitivity of forage species to levels of salinity in irrigation water as reflected by biomass accumulation. With the exception of bermudagrass, which increased accumulation at the higher level of salinity, and big trefoil, which failed to establish at the higher level of salinity, ranking of forages according to the percent reduction in biomass accumulation due to the higher level of salinity of irrigation water was: Salado alfalfa (54%) = SW 9720 alfalfa (52%) > Duncan Paspalum (41%) > narrow leaf trefoil (30%) > alkali sacaton (24%) > Polo Paspalum (16%) > Jose tall wheatgrass (11%) = kikuyugrass (11%). Bermudagrass and Duncan Paspalum were judged to be the best species in terms of forage yield and nutritive quality. Kikuyugrass, which had the third highest biomass accumulation, was judged to be unacceptable due to its poor nutritional quality. Although narrow leaf trefoil had a relatively high nutritional quality, its biomass accumulation potential was judged to be unacceptably low. Alfalfa cultivar’s biomass accumulations were the most sensitive to the higher level of salinity, among forages that survived at the higher salinity level, although actual accumulations at the higher salinity were high relative to other forages. Increased salinity influenced several forage quality parameters, including organic matter (OM), crude protein (CP), neutral detergent fibre (NDF), and in vitro gas production, generally leading to higher nutritional quality at the higher salinity level, although their significance varied amongst species and cuttings.

Feasibility of Irrigating Pickleweed ( Salicornia bigelovii Torr) with Hyper-saline Drainage Water

Reuse of drainage water (DW) for irrigation reduces the volume of DW requiring treatment or disposal. We conducted a greenhouse study to evaluate the performance of the halophyte Salicornia bigelovii Torr. when irrigated with hyper-saline DW and seawater (SW) treatments, ranging from 1/3 strength to full strength (18-49 dS m(-1)), in a sand-culture system. Results indicate that Salicornia grows well over the entire range of iso-osmotic SW and DW salinity treatments. Moreover, when boron (B) was added to SW treatments to concentrations equivalent to that of corresponding 1/3- and 2/3-strength DW treatments (i.e., 9 and 17 mg L(-1)), growth was not affected, and tissue B concentrations were <150 mg kg(-1) dry wt. However, when plants were irrigated with synthetic DW where B was reduced to solution culture levels (0.5-1.0 mg L(-1)), plants generally performed worse than when irrigated with actual DW high in B at the same salinity level. Evapotranspiration (ET) rates exceeded that lost from an evaporation pan from 1.5 to 2.5 times. Using a method accounting for changes in the isotopic signature of water in the reservoir due to evaporation, we estimated that high ET rates were due primarily to high transpiration rates (>78% of ET). The salt content in the tissue was very high (ash content 43-52%), but ionic composition in the shoot tissue reflected that of the treatment water used to irrigate the plants. These data indicate that hyper-saline DW, characteristic of Californias San Joaquin Valley, can be used to irrigate Salicornia and substantially reduce drainage volumes.

Characterization of leaf boron injury in salt-stressed Eucalyptus by image analysis

Symptoms of boron toxicity (i.e., necrosis of leaf tips and margins) have been observed on eucalyptus trees in the San Joaquin Valley of California where the trees are being tested for their effectiveness at reducing the volume of agricultural drainage effluents. In a controlled, outdoor sand-tank study, Eucalyptus camaldulensis Dehn., Clone 4544 trees were grown and irrigated with combinations of salinity and B to determine their influence on tree growth and water use. Irrigation water quality treatments were prepared to simulate the Na-sulfate salinity, high B nature of these drainage effluents. Electrical conductivities (ECiw) of the waters ranged from 2 to 28 dS m-1 and B concentrations ranging from 1 to 30 mg L-1. As an integral component of this study , we developed a method to quantify and correlate foliar damage with leaf B concentrations. By scanning both injured and uninjured leaves into computer files and processing with image analysis, we were able to simultaneously correlate salinity stress with its overall effect on leaf area as well as to quantify the relative fraction of leaf area affected by specific-ion (i.e., B) injury. Leaf area was unaffected by B stress but was reduced by salinity only in the younger leaves. Boron injury was correlated with increasing irrigation water B only in older leaves. The relative injured area (RIA) of the older leaves was related to the B concentrations of leaves from trees grown at various salinities . A regression equation was developed from injury data obtained from trees grown under boron and salinity stress for 223 days (r2=0.90). From this relationship, we were able to estimate leaf boron concentrations from injury symptoms in leaves selected at random from main trunk branches of trees grown for 333 days under the same stress conditions. The results suggest that this method may have potential as an effective tool for monitoring the response to toxic levels of boron in eucalyptus, once B toxicity has been established by analytical means. The RIA appears to be mitigated by increased salinity of the irrigation water and is consistent with the general reduction in leaf B by salinity. The interactive effects of boron and salinity on foliar injury depends on the physiological age of the leaf.

Irrigation with Saline Water

Crop production in arid and semi-arid regions of the world is dependent upon an adequate supply of suitable-quality water. A supply of water is considered adequate when sufficient quantities are readily available for irrigation throughout the season to meet crop-water needs. This depends not only on the absolute quantity of water available, but on the scale of irrigated agriculture imposed on a region. In areas where irrigated agriculture frequently encounters irrigation water shortages, emphasis is usually placed on methods of increasing water quantity (e.g., utilizing groundwater; Howitt and M’Marete 1991), rather than considering whether water demands placed on such arid regions have been too high to ensure a dependable long-term supply. In some extremely arid countries, however, development of new irrigation water supplies is necessary to maintain a stable food supply. In order to expand its water resource base, the country or region must be able to utilize poorer quality water. The quality of water is considered suitable for crop production providing it can be used alone or in conjunction with other water sources and can sustain economic yields over the long term. The actual quality of water that is suitable for irrigation, as outlined in Chapter 2, depends upon crop salt tolerance, site conditions, and management practices.

Drainage Water Reuse: Concepts, Practices and Potential Crops

Reuse of drainage water for irrigation is recognized as a viable means of reducing the amount of saline-sodic spent water that will ultimately require treatment or disposal in the western San Joaquin Valley (SJVDP 1990). This practice is not a long-term solution in itself, but rather an integral component to drainage water management in the San Joaquin Valley. For reuse to be successful, soil salinity and boron (B) cannot accumulate to levels damaging to crop growth; soil physical conditions conducive to water infiltration must be sustained; and trace element accumulation in crops and forages must remain low enough not to threaten the health of humans or livestock (Oster and Grattan 2002).