Salinity alleviates zinc toxicity in the saltmarsh zinc-accumulator Juncus acutus

The potential importance of Juncus acutus for remediation of Zn-contaminated lands has been recognized, because of its Zn tolerance and capacity to accumulate Zn. Since it is also a halophyte, the extent to which salinity influences its Zn tolerance requires investigation. A factorial greenhouse experiment was designed to assess the effect of NaCl supply (0 and 85 mM NaCl) on the growth, photosynthetic physiology and tissue ions concentrations of plants exposed to 0, 30 and 100 mM Zn. Our results indicated that NaCl supplementation alleviated the effects of Zn toxicity on growth, as Zn at 100 mM reduced relative growth rate (RGR) by 60% in the absence of NaCl but by only 34% in plants treated also with NaCl. This effect was linked to a reduction in Zn tissue concentrations, as well as to overall protective effects on various stages in the photosynthetic pathway. Thus, at 85 mM NaCl plants were able to maintain higher net photosynthesis (A_N) than in the absence of added NaCl, although there were no differences in stomatal conductance (g_s). This contributed to preserving the trade-off between CO₂ acquisition and water loss, as indicated by higher intrinsic water use efficiency (_iWUE). Hence, A_N differences were ascribed to limitation in the RuBisCO carboxylation, manifested as higher intercellular CO₂ concentration (C_i), together with dysfunction of PSII photochemistry (in term of light harvest and energy excess dissipation), as indicated by higher chronic photoinhibition percentages and variations in the photosynthetic pigment profiles in presence of Zn under non-saline conditions.