Ultrastructural and Compositional Changes in Chloroplast Thylakoids of Leaves of Borya nitida during Humidity-sensitive Degreening

Abstract

The leaves of the desiccation tolerant plant Borya nitida, when dehydrated at 96% relative humidity, lose chlorophyll but retain viability. During the first 14 h of dehydration thylakoid stacking increased, probably because of increasing salt concentrations within the chloroplast as the cells lost water. After 18 h of dehydration the chloroplasts were partly swollen and unstacking of the thylakoids was taking place. This apparent reversal of the effects of dehydration was attributed to the conversion of starch to sugar. Swelling of the chloroplast and unstacking of the thylakoids continued until 22 h of leaf dehydration. A progressive rise in leaf variable chlorophyll fluorescence between 14 and 22 h indicated that a block began to form in the electron transfer system after photosystem II. Leaf fluorescence declined after 22 h of dehydration and parallelled the loss of chlorophyll, P700, chloroplast proteins and the degradation of thylakoid membranes. By 37 h of dehydration the leaves were yellow and the chloroplasts contained little internal structure. In the dried yellow leaves, cytochrome f and cytochrome b-559_HP were almost completely absent but cytochrome b₆ and b-559_LP were not, indicating that some thylakoid remnants were retained.