An obligate-halophytic mangrove, Rhizophora mucronate, does not require Na+ for the uptake of nutrient ions in their roots

Abstract

Abstract We previously reported that the seedlings of a mangrove, Rhizophora mucronate (Rhizophoraceae; formerly R. stylosa), exhibited very low survivability when cultured in the absence of NaCl. This suggests that R. mucronate is an obligate halophyte (plant that absolutely requires salt for survival), but the possible mechanism for the apparent salt requirement in R. mucronate remains unclear. In the present study, we examined the possibility that R. mucronate utilizes Na+ as a co-transport ion for the uptake of nutrient ions, as is known for the seagrass Zostera marina. To minimize the influence of individual differences in ion uptake activities among sample plants, we adopted a modified split-root method. In this method, the root system of an individual R. mucronate seedling was split into two halves by making a longitudinal cutting into the lower halves of their hypocotyl. Then, each half was respectively immersed in a culture medium that did or did not contain 240 mM NaCl, and the rates of nutrient (phosphate, nitrate, and ammonium ions) uptake in the presence or absence of Na+ were measured in a single seedling and compared. The results demonstrated that ion uptake in R. mucronate was, in contrast to the case with Z. marina, not hindered in the absence of Na+, suggesting that nutrient uptake in R. mucronate is not dependent on Na+. Based on this and other results, possible alternative mechanism(s) for the apparent Na+-requirement observed in R. mucronate seedlings are discussed.