Abstract: The influences of a systemic signal, from mature leaves, on the photosynthetic characteristics of developing leaves in sorghum seedlings under salt stress were carefully investigated. Salt stress significantly decreased water potential, net photosynthetic rate, stomatal conductance and slightly increased extent of photoinhibition both in mature and newly developed leaves. In newly developed leaves, stomatal density of both adaxial and abaxial surfaces increased. Under non-salt stress condition, water potential both in mature and newly developed leaves was hardly affected when mature leaves exposed to weak light. However, carbon assimilation, stomatal conductance, soluble sugar content decreased markedly and the extent of photoinhibition increased largely in mature leaves and newly developed leaves due to the shading of mature leaves. Meanwhile, stomatal density, mesophyll thickness and surface area of bundle sheath decreased in newly developed leaves when light intensity decreased for mature leaves. In salt-stressed group, changes in photosynthetic structure and function had a similar tendency with that in non-salt stress group, but a further decline in photosynthesis was observed. Two-way ANOVA among these four treatments showed that there was a synergistic effect between salt stress and high irradiance which impaired the photosynthetic ability more heavily and enhanced photoinhibition. Accordingly, we concluded that there is a systemic regulation on the photosynthetic structure and performance of newly developed leaves under salt stress. The enhancement of the light intensity of mature leaves is very important for the improvement of development, carbon assimilation and the alleviation of photoinhibition in newly developed leaves under salt stress.

Key words: Salt stress, Photosynthetic characteristics, Systemic regulation, Photoinhibition, Sorghum