Stomatal, biochemical and morphological factors limiting photosynthetic gas exchange in the mangrove associate Hibiscus tiliaceus under saline and arid environment

The effect of salinity on leaf area and the relative accumulation of Na⁺ and K⁺ in leaves of the mangrove associate Hibiscus tiliaceus were investigated. Photosynthetic gas exchange characteristics were also examined under arid and non-arid leaf conditions at 0, 10, 20 and 30‰ substrate salinity. At salinities ≥ 40‰, plants showed complete defoliation followed by 100% mortality within 1 week. Salinities ≤ 30‰ were negatively correlated with the total leaf area per plant (r² = 0.94). The reduction in the total plant leaf area is attributed to the reduction in the area of individual leaves (r² = 0.94). Selective uptake of K⁺ over Na⁺ declined sharply with increasing salinity, where K⁺/Na⁺ ratio was reduced from 6.37 to 0.69 in plants treated with 0 and 30‰, respectively. Under non-arid leaf condition, increasing salinity from 0 to 30‰ has significantly reduced the values of the intrinsic components of photosynthesis V_c,max (from 50.4 to 18.4 μmol m^-2 s^-1), J_max (from 118.0 to 33.8 μmol photons m^-2 s^-1), and V_TPU (from 6.90 to 2.30 μmol m^-2 s^-1), while stomatal limitation to gas phase conductance (S_L) increased from 14.6 to 38.4%. Water use efficiency (WUE) has subsequently doubled from 3.20 for the control plants to 8.93 for 30‰ treatment. Under arid leaf conditions, the stomatal factor (S_L) was more limiting to photosynthesis than its biochemical components (73.4 to 26.6%, respectively, at 30‰). It is concluded that salinity causes a drastic decline in photosynthetic gas exchange in H. tiliaceus leaves through its intrinsic and stomatal components, and that the apparent phenotypic plasticity represented by the leaf area modulation is unlikely to be the mechanism by which H. tiliaceus avoids salt stress.