Physiological Response and Mineral Concentration of Sago Palm under Diurnal Changes of NaCl Concentration in Culture Solution

Abstract

Sago palm seedlings were exposed to 2 cycles of diurnal change of NaCl concentration in the culture solution in a hydroponic system for 4 months. During the experiment, the emergence rate of new leaves in the treated plants tended to be slow, compared to that of the control plants. However the number of dead leaves was the same in both control and treated plants. The total N and P concentrations in the leaflets and petioles and the leaf area at each leaf position did not change with the treatment. The increase of the SPAD value was slow in the treated plants. The photosynthetic rate, transpiration rate and stomatal conductance in the treated plants decreased by about 40%. The Na+ concentration increased in all the plant parts with the treatment, especially in the roots and petioles at the lower leaf positions. The K+ concentration decreased in the roots and petioles, whereas it increased in some leaflets. Although a large difference in the decrease in Mg2+ concentration was found in the cortex, the difference was negligible in the stele of adventitious roots, petioles and leaflets. Based on these results, it is considered that sago palm plants growing in a brackish water area are able to maintain a low Na+ concentration in the leaflets of active leaves by storing Na+ mainly in the roots and petioles. The factor limiting the photosynthetic rate under NaCl stress was the reduction in stomatal conductance that resulted from a trade-off with the decrease in the transpiration rate for maintaining the water status in the leaves. Although chlorophyll production was depressed, the absorption of macronutrients was not inhibited by salt stress and there was no lack of materials such as N and Mg for chlorophyll production. Therefore, it was found that the chlorophyll concentration could increase up to high levels over a comparatively long time. These factors may account for the resistance of sago palm to salt stress and ability to grow even with a reduction of the growth rate.