Title: Turgor maintenance by osmotic adjustment, an adaptive mechanism for coping with plant water deficits
Abstract: Plant, Cell & EnvironmentVolume 40, Issue 1 p. 1-3 CommentaryFree Access Turgor maintenance by osmotic adjustment, an adaptive mechanism for coping with plant water deficits Neil C. Turner, Corresponding Author Neil C. Turner [email protected] The UWA Institute of Agriculture, The University of Western Australia, M082, Locked Bag 5005, Perth, WA, 6001 AustraliaSearch for more papers by this author Neil C. Turner, Corresponding Author Neil C. Turner [email protected] The UWA Institute of Agriculture, The University of Western Australia, M082, Locked Bag 5005, Perth, WA, 6001 AustraliaSearch for more papers by this author First published: 10 October 2016 https://doi.org/10.1111/pce.12839Citations: 37AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Commentary on Blum ‘Osmotic Adjustment is a Prime Drought Stress Adaptive Engine in Support of Plant Production’ Plants of arid and semiarid regions usually have to withstand periods of water shortage on an annual basis. They have developed a range of mechanisms of adaptation to survive periods without access to water. Many survive as dry seeds awaiting the next rainfall season, while others develop mechanisms to reduce and withstand water loss during periods of water shortage. While many of these mechanisms enable the plant to survive the lack of precipitation, productivity is low, as curtailing water loss by transpiration also reduces carbon gain by photosynthesis, and survival mechanisms are therefore of limited utility for crop and pasture plants in which high productivity is generally a requirement. Crop and pasture breeders seek to identify and select traits or mechanisms that enable high biological or reproductive yields to be achieved under water-limited conditions. Osmotic adjustment, or osmoregulation (Morgan 2000), has been recognized as one such mechanism of drought resistance and dehydration avoidance and in this issue of Plant, Cell and Environment, Blum (2016) reviews the evidence for the role of osmotic adjustment on the yields of 12 crop species when subjected to water shortage or drought stress. Osmotic adjustment is the active accumulation of solutes in the plant in response to decreasing water potential, thereby maintaining turgor (Turner & Jones 1980). Figure 1 shows the changes in turgor potential, water potential and osmotic potential of a cell or tissue as water is removed and the cell volume is reduced. With no solute accumulation, the osmotic potential decreases with the concentration of the solutes in the cell or tissue as water is withdrawn (line A, Fig. 1). The magnitude of this decrease in osmotic potential depends on the elasticity of the cell wall and size of the cell (not shown in Fig. 1). Osmotic adjustment is not this passive decrease in the osmotic potential as water is withdrawn from the cells but is the active process of solute accumulation in addition to solute concentration (line C, Fig. 1). For full turgor maintenance, the osmotic potential decreases as the water potential decreases, while partial turgor maintenance is due to limited solute accumulation and solute concentration (line B, Fig. 1). A range of solutes have been shown to accumulate with osmotic adjustment; in crop plants, soluble sugars, carboxylic acids, potassium, calcium, chloride, nitrate and amino acids accounted for 50–100% of the osmotic adjustment in wheat, sorghum and sunflower (Turner & Jones 1980; Turner 1986). Figure 1Open in figure viewerPowerPoint Modified Höfler diagram of the idealized relationships between relative cell volume, water potential, osmotic potential and turgor potential in plants with no osmotic adjustment (a), partial turgor maintenance by partial osmotic adjustment (b), and full turgor maintenance by full osmotic adjustment (c). The water potential is common for all three scenarios. In (a), the osmotic potential decreases due to the concentration of the solutes, while in (b) and (c) solutes accumulate as the water potential falls, decreasing the osmotic potential to a greater degree than that from solute concentration. Morgan (1980) observed full turgor maintenance in some wheat species and cultivars in response to soil drying, but the majority of observations have been of partial turgor maintenance (Morgan 1980; Turner & Jones 1980), while some species and/or genotypes show no osmotic adjustment in response to soil drying (Morgan 1980; Turner et al. 1987). The degree of osmotic adjustment varies not only with species, but the rate of drying of the soil or plant and the environment surrounding the leaf or plant (Turner & Jones 1980; Blum 2016). While osmotic adjustment maintains cell turgor of roots and leaves as the soil dries, full turgor maintenance is finite and partial turgor maintenance ultimately results in loss of turgor, albeit at lower leaf water potentials (Morgan 1980; Turner & Jones 1980). Thus, while osmotic adjustment has been shown to enable growth and photosynthesis to continue at lower soil and leaf water potentials, both processes are ultimately reduced with soil drying. This has led several scientists to question the benefits of osmotic adjustment and whether these benefits result in increased yields or are important only for survival (e.g. Serraj & Sinclair 2002, and others cited by Blum 2016). Since Morgan (1980) identified genotypic differences among wheat species/cultivars in the degree of osmotic adjustment, numerous studies have shown variation among species and cultivars in osmotic adjustment induced by water shortage. Although not all studies have demonstrated a yield advantage (e.g., Turner et al. 2007), Morgan went on to demonstrate that cultivars and accessions with a high degree of osmotic adjustment had higher yields than those with a low degree of osmotic adjustment under drought stress, primarily as a result of greater water extraction (Morgan & Condon 1986). Morgan's research resulted in the release of a new wheat cultivar, Mulgara, derived from lines that showed up to a doubling of yields from osmotic adjustment at dryland sites in north-east Australia (Morgan 2000; Richards 2006). Blum (2016) uses these and other studies to provide clear evidence that osmotic adjustment is associated with increased yields under drought stress in 24 out of 26 studies on barley, canola, castor bean, chickpea, field pea, maize, mustard, pigeon pea, sorghum, soybean, sunflower and wheat. While this high proportion of results showing increased yields under water stress as a result of osmotic adjustment may be because scientists tend not to publish negative results (Blum 2016), the fact that 24 studies in 12 crop species showed increased productivity when osmotic adjustment was observed is compelling evidence that it is an adaptive trait for sustaining crop yields under drought stress. While Blum (2016) provides data on the degree of osmotic adjustment observed in the studies, he does not indicate the magnitude of the yield increase, only that there is a positive association between the degree of osmotic adjustment and yield. In part this is because Blum (2016) recognizes that other traits may influence yield under drought (see Turner 1986 for a discussion of these), but this did not prevent Richards (2006) from estimating the yield benefits of several physiological adaptive mechanisms in wheat, including osmotic adjustment. Further, Blum (2016) does not indicate the stress environments of the studies showing advantages from osmotic adjustment. Elsewhere, Blum (2015) argues that osmotic adjustment is triggered by a dehydration signal and enables physiological functions, such as photosynthesis, leaf growth and root growth, to continue as the soil dries, making this trait suited to environments with deep soils and intermittent water shortages, rather than environments, such as harsh Mediterranean environments, in which soils are shallow, terminal drought is prevalent, and osmotic adjustment was shown not to benefit the yield of chickpea (Turner et al. 2007). Finally, Blum (2016) was careful to select studies in which osmotic adjustment was measured by one or more of four recognized ways of measuring the degree of osmotic adjustment. However, none are amenable to rapid phenotyping of a large range of germplasm, particularly of plants growing in soil. Recently, Mart et al. (2016) have described an easily measurable technique for estimating osmotic adjustment by measuring the water potential when plants are visibly and permanently wilted (turgor loss point). The degree of osmotic adjustment measured as the water potential at the turgor loss point was correlated with the degree of osmotic adjustment measured by traditional methods and varied among wheat cultivars. If this method is shown to be reliable in other cereals, legumes and oilseed crops, it will provide a method of rapidly evaluating a range of genotypes or accessions for osmotic adjustment and enable a wider evaluation of the benefit of osmotic adjustment to yield of genotypes under water-limited conditions. References Blum A. (2015) Towards a conceptual ABA ideotype in plant breeding for water limited environments. Functional Plant Biology 42, 502– 513. Blum A. (2016) Osmotic adjustment is a prime drought stress adaptive engine in support of plant production. Plant, Cell & Environment doi: 10.1111/pce.12800. Mart K.B., Veneklaas E.J. & Bramley H. (2016) Osmotic potential at full turgor: an easily measurable trait to help breeders select for drought tolerance in wheat. Plant Breeding 135, 279– 285. Morgan J.M. (1980) Osmotic adjustment in the spikelets and leaves of wheat. Journal of Experimental Botany 31, 655– 665. Morgan J.M. (2000) Increases in grain yield of wheat by breeding for an osmoregulation gene: relationship to water supply and evaporative demand. Australian Journal of Agricultural Research 51, 971– 978. Morgan J.M. & Condon A.G. (1986) Water use, grain yield and osmoregulation in wheat. Australian Journal of Plant Physiology 13, 523– 532. Richards R.A. (2006) Physiological traits used in breeding of new cultivars for water-scarce environments. Agricultural Water Management 80, 197– 211. Serraj R. & Sinclair T.R. (2002) Osmolyte accumulation: can it really help increase crop yield under drought conditions? Plant, Cell & Environment 25, 333– 341. Turner N.C. (1986) Crop water deficits: a decade of progress. Advances in Agronomy 39, 1– 51. Turner N.C. & Jones M.M. (1980) Turgor maintenance by osmotic adjustment: a review and evaluation. In Adaptation of Plants to Water and High Temperature Stress (eds N.C. Turner & P.J. Kramer), pp. 87– 103. John Wiley & Sons, New York. Turner N.C., Stern W.R. & Evans P. (1987) Water relations and osmotic adjustment of leaves and roots of lupins in response to water deficits. Crop Science 27, 977– 983. Turner N.C., Abbo S., Berger J.D., Chaturvedi S.K., French R.J., Ludwig C., … Yadava H.S. (2007) Osmotic adjustment in chickpea (Cicer arietinum L.) results in no yield benefit under terminal drought. Journal of Experimental Botany 58, 187– 194. Citing Literature Volume40, Issue1January 2017Pages 1-3 FiguresReferencesRelatedInformation