Systemic Regulation Of Structure And Photosynthetic Characteristics In Developing Leaves In Sorghum Seedlings Under Salt Stress

The influences of systemic signal from mature leaves on the photosynthetic characteristics of developing leaves in sorghum seedlings were investigated. Gas exchange, chlorophyll fluorescence, anatomical structure, ultrastructure and soluble sugar content were measured to explore the possible mechanism of systemic regulation for developing leaves induced by light environments of mature leaves. The main results obtained are as follows:1. Compared with the plants under ambient condition (A), no significant changes were observed in stomatal density, carbon assimilation, stomatal conductance and photoinhibition extent in the newly developed leaves on plants with young leaves shaded (YS) under strong light. In contrast, significant decreases in stomotal density, adaxial mesophyll thickness and surface area of bundle sheath cell were observed in the newly developed leaves on plants with mature leaves shaded (MS) or with whole plant shaded (S), and increased susceptibility of photosystem II to high light resulted from depressed stomatal conductance and carbon assimilation was also observed in the newly developed leaves on the plants treated with MS and S.2. Newly developed leaves on plants with treatments of A and MS exhibited sun chloroplasts with thin thylakoid, while newly developed leaves on plants with treatments of YS and S exhibited shade chloroplasts with thick thylakoid.3. Positive correlations were observed between Pn, Gs in mature leaves and adaxial mesophyll thickness, adaxial stomatal density in the newly developed leaves, respectively. In addition, positive correlation (R2=0.86) was also observed between surface area of bundle sheath (Sb) cell in the newly developed leaves and photosynthetic rate in mature leaves.4. It was demonstrated that there were systemic signals from mature leaves to affect anatomical structure of newly developed leaves; the decreased stomatal density on adaxial surface, adaxial mesophyll thickness and surface area of bundle sheath cell might be responsible for the depressed carbon assimilation capability and stomatal conductance in leaves of sorghum seedlings with different shading treatments. However, the development of chloroplasts in newly developed leaves was independent of the systemic regulation of mature leaves, it was the local light environments that determined chloroplasts to develop into sun or shade type.5. 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, leaf thickness declined while the stomatal density on both adaxial and abaxial surfaces increased. Under salt-stressed condition, changes in leaf structure and photosynthetic performance showed a similar tendency as that under non-salt stress condition but a further decline in photosynthetic performance.6. Under salt stress, treatment with MS significantly enhanced decrease in photosynthetic ability and photoinhibition. Accordingly, we concluded that there is a systemic regulation on both structure and function of newly developed leaves under both salt stress condition and non stress condition. The enhancement of the light intensity of mature leaves is important to improve photosynthetic capacity and to alleviate photoinhibition in newly developed leaves under salt stress.