Evaluation of salt tolerance and contributing ionic mechanism in nine Hami melon landraces in Xinjiang, China

Melon (Cucumis melo L.) is an important horticultural crop in the world. The objective of this work was to evaluate salinity stress tolerance and understand the ionic mechanism conferring this trait in nine Hami melon landracesfrom Xinjiang, China. Plants were grown in a potting mix (peat: vermiculite: perlite = 1:1:1, v/v) under glasshouse conditions and irrigated with Hoagland's nutrient solution containing either 0 (control) or 100 mM NaCl. Plant dry weight, single fruit weight, Na⁺ and K⁺ content of all leaves, stem and roots of the whole plant were measured at day 70 after commencing the treatment. Also, Na⁺ and K⁺ content of the second fully expanded leaf from the top were measured at day 20, 40 and 60 after stress onset. Net fluxes of K⁺ and Na⁺ were measured from plant roots in laboratory experiments using a noninvasive MIFE (microelectrode ion flux estimation) technique, and expression levels of the leaf tonoplast Na⁺/H⁺ antiporters (NHX) were investigated. Salt injury index and leaf chlorophyll content were also measured. The results showed that, according to the relative (NaCl/Control) plant dry weight and a single fruit weight, the salt tolerance of the nine landraces can be classified into three types: ‘Huangdanzi’, ‘Zajiaojiashigua’, ‘Kalakesai’ and ‘Paotaihong’ are salt tolerant; ‘Laotiepi’, ‘Kaeryunxi’ and 'sairekekekouqi’ are intermediate in salt tolerance; and ‘Akekekouqi’ and ‘Paodangua’ are salt sensitive. This conclusion was also supported by the results of measuring of the salt injury index and leaf chlorophyll content. The salt tolerance of landraces was positively correlated with the relative K⁺ content of all leaves, but correlated negatively with the relative Na⁺ content of the second fully expanded leaf, and relative K⁺ content of the root. Compared with salt sensitive landraces, salt tolerant genotypes had significantly lower relative Na⁺ content of the second fully expanded leaf, lower relative root K⁺ content (except Paotaihong), and higher relative K⁺ content of all leaves. They also displayed lower root net K⁺ efflux and higher Na⁺ efflux (except Zajiaojiashigua) capacity. The intermediate landrace ‘Laotiepi’ had very high Na⁺ content and high level of NHX2/3/6 expression in leaves but still possessed high chlorophyll content, suggesting that this landrace had an efficient vacuolar Na⁺ sequestration mechanism in the leaf mesophyll. Taken together, our results showed that Hami melon landraces in Xinjiang showing significant difference in salt tolerance, and that K⁺ retention in the shoot and Na⁺ exclusion from young leaves are the main mechanisms conferring salinity stress tolerance of Hami melon landraces in Xinjiang.