Na+ induces the tolerance to water stress in white clover associated with osmotic adjustment and aquaporins-mediated water transport and balance in root and leaf

Aquapotins (AQPs) could be involved in Na+-regulated tolerance to water stress in white clover. White clovers were subjected to water stress (-0.3 MPa) induced by polyethylene glycol 8000 for 16 days in the presence or in absence of NaCl (30 mM) pretreatment. Na+ pretreatment significantly enhanced the tolerance to water stress in white clovers based on growth and physiological analyses. Na+-induced increases in endogenous Na+ content and transcript levels of NHX6, SOS1, and SKOR associated with ions compartmentalization could be responsible for higher osmotic adjustment (OA) as opposed to the accumulation of more organic osmolytes (proline and water soluble carbohydrates) in leaf and root. Na+ pretreatment not only further elevated endogenous ABA content caused by water deficit, but also enhanced the accumulation of AQPs and the expression of genes encoding AQPs under water stress. The increase in ABA content induced by Na+ could contribute to the enhancement of ABA-activated stress defense in relation to transcription factors and AQPs in root and leaf. Na+-treated white clover exhibited significantly greater root growth and OA in root and leaf, but the significant decline in transpiration rate in leaf under water stress, which provided a beneficial foundation for the AQPs-regulated water transport leading to a favourable water status in white clover under water stress. These results indicated that Na+ could play a positive role in activating ABA-regulated AQPs in response to water deficit in mesophytic plants.