Performance of Ziziphus jujuba trees correlates with tissue mineral content under salinity conditions

The tolerance of perennial fruit crops to salinity is usually determined by analyzing tree performance in relation to EC or NaCl concentration in the soil and/or irrigation water or in relation to the period during which the plants were grown under salinity conditions. In response to these conditions, toxic ions accumulate in the plant over time, but very few studies have sought to correlate ion concentration directly to tree vegetative and/or reproductive performances. We investigated the utility of a direct short-term approach that involved "loading" Ziziphus jujuba trees with different levels of NaCl and determining the correlation between the accumulation of specific toxic ions in the leaves, flowers and roots (Na⁺ and Cl^-) and the change in tree vegetative and reproductive performance. Z. jujuba cultivar 'Ben-Li' trees planted in pots were irrigated with four salinity levels: 13.2, 31.7 and 61.9mM NaCl and tap water (1.6mM Na⁺ and 1.1mM Cl^-) as control. The salinity threshold for normal growth and flowering was found to be 0.40% Na⁺ and 2.68% Cl^- in leaves (% of dry weight), 0.27% Na⁺ in roots, and 0.10% Na⁺ and 0.52% Cl^- in flowers. Na⁺ accumulation was correlated with impairment in six of the seven growth and flowering traits tested, while Cl^- was found to be correlated to only three of these seven traits, suggesting that for the salinity range tested Na⁺ is the more toxic ion for Z. jujuba trees. The present study shows that the rise in toxic ion concentrations in the plant tissues correlates with changes in flowering and tree growth, thereby making it possible in the short term both to determine the salinity threshold above which there is a reduction in yield and to predict in the longer term changes in tree performance as a function of ion accumulation.