Effect of sea water substitution on growth, physiological and biochemical processes of coconut (Cocos nucifera L.) seedlings-A hydroponic study

Coconut is grown along the coasts and islands that are vulnerable to climate change-induced sea level rise. Though coconut is considered moderately salt tolerant, our understanding on the growth and physiological response to sea water, either inundation or subsurface water contamination, is very limited. This understanding will enable to effectively manage coconut in coastal systems under future climatic scenarios. In this study, ten month old hydroponically grown coconut seedlings were subjected to 0, 10, 25, 50, 75 and 100 % of sea water substitution (SWS), equivalent to 2.17, 8.32, 16.32, 30.03, 42.14 and 53.69 dS m(-1) EC, respectively. Substituting Hoagland solution in hydroponic system by sea water of increasing concentration (>50 % SWS) significantly changed physiological processes; Fv/Fm decreased and r s increased as early as 7 and 18 days after treatment imposition (DAT), respectively which led to significant decline in leaf area and root length expansion as early as 24 DAT. At 25 % SWS, root system (root length and root biomass) was stable but the aerial part biomass was declined by 47 %. On the other hand plant height, leaf area, collar girth and biomass accumulation of seedlings under 10 % SWS (8.32 EC) was on par with the control plants suggesting coconut seedlings could tolerate 10 % SWS. Though, P-N declined by 19 % and 43 % at 10 % and 25 % SWS, respectively and a similar decline in g, without a concomitant change in leaf water potential suggested that root-generated signals regulated the stomatal movement in coconut under salinity. Still the biomass accumulation at 10 % SWS was not affected by decline in PN. Under increasing sea water treatments, most of the Na+ absorbed was compartmentalized in mot and shoot, while leaf had more accumulation of K+, that ensured high K+/Na+ ratio in the leaves which is an important salinity tolerant mechanism observed in coconut. The leaf Cl - content also had strong negative correlation with [P-N] (r=-0.873) and biomass (r=-0.833), therefore in addition to K+ and Na+ homeostasis, the level of tolerance to the increased Cl- content in the leaves may also play an important role in salinity tolerance of coconut. This understanding will help in making appropriate strategies for managing coconut grown at coastal systems in the face of sea level rise under climate change.