Direct and indirect influences of water deficit on salt uptake, ion accumulation and root-shoot interactions of grapevines

Abstract

The area affected by salinity in Australian grape production regions is increasing, predominantly due to reliance in some regions on poorer quality water for irrigation and to changes in rainfall patterns resulting in reduced leaching of soil borne salts. Combined with an increased requirement to improve water use efficiency the implementation of deficit irrigation techniques has become common practice. The aim of this research was to assess the effect of saline irrigation water and deficit irrigation techniques on the performance of own-rooted grapevines as well as test the hypothesis that PRD reduces the salt transport to the shoot. A field experiment was established in Padthaway on own-rooted Shiraz vines in seasons 2009-2011. Three irrigation treatments were applied using moderately saline irrigation water (2.3dS/m): control (1.0- 2.3ML/ha), reduced control (RC) and partial rootzone drying (PRD ) (both approximately 50% of control).This study found that grape juice Cl⁻ and Na⁺ concentrations were not affected significantly by irrigation treatment. Seasonal variation in rainfall and total irrigation applied had a greater effect on altering grape juice Cl⁻ and Na⁺ concentrations than the application of irrigation water with the same moderate salinity but with the different irrigation treatments. A pot trial was established to replicate the treatments mentioned above in conjunction with slightly increased saline irrigation water (2.46dS/m) from the field trial for the 2011-2012. At the end of the second year the vines were destructively harvested and growth and ion concentrations for different vine organs assessed. PRD was found to have higher concentrations of Cl⁻, Na⁺, K⁺ and Ca⁺⁺ present on a whole vine basis. Although Cl concentration was elevated in leaves for PRD, it was partitioned away from the leaves on a total content basis relative to both control and RC. This research highlighted that ion partitioning within grapevines will depend on the type of deficit applied and that the higher total root dry weights observed in the PRD treatment could possibly be responsible for the higher whole plant concentrations of Cl⁻, Na⁺, K⁺ and Ca⁺⁺ that were observed. To gain a better understanding of the role Abscisic acid (ABA) plays in modulating the effect of salinity a glasshouse study was undertaken in 2012-2014. The aim was to evaluate the effect of exogenously applied ABA to grapevine root systems, with or without saline irrigation water, on water relations, ion allocation, root hydraulic conductance normalized to root dry weight (Lₒ) and aquaporin expression. Exogenously applied ABA was found to increase Lₒ and decrease water use in ABA-only treatments, while in the presence of excess Cl⁻ salts, it also reduced Cl⁻ transport to the shoot. This reduction could not be accounted for by reduced transpiration. Strong positive correlations were observed between Lₒ and E and Lₒ and gs [s subscript] with a slope of the relationships increasing with both ABA and salt treatments. Aquaporin gene expression was not significantly different between treatments an interesting finding that warrants further investigation. However in a linear combination with leaf water potential, the expression of one aquaporin gene VvPIP2;3, could explain more than 50% of the variation in Lₒ independent of the salt and ABA treatments. The expression of the tonoplast aquaporin VvTIP1;1 was also correlated to the expression of Vv PIP2;1. This study has led to a greater understanding of the implications for growers when irrigating with moderately saline irrigation water in conjunction with some form of deficit irrigation technique. Although the initial hypothesis was negated in both the field and pot trial with Cl⁻ concentrations in the shoot remaining similar to the control, the glasshouse study proved that ABA has the ability to reduce salt transport to the shoots independently of its effects on stomatal conductance and root water transport. Further research to probe the mechanism of the effect of ABA on Cl⁻ transport will require the membrane transporters responsible for Cl⁻ transport to be identified and their possible transcriptional and post-translational control by ABA determined.