Ultrastructural Changes in Chloroplast and Phloem of Rice Leaves during Senescense under Different Nitrogen Supply.

Abstract

Rice seedlings of 7.2 in leaf age were transplanted into pots containing culture solution. The control plants received the complete solution and the N-deficient plants received the -N solution. Samples for electron microscopy were cut from the central part of the 7th leaf blades 0, 15 and 34 days after expansion (DAE) . In the control plants, a large amount of starch grains in the chloroplasts from leaves 0 DAE was disappeared and the inner membrane system of chloroplast was more developed at 15 DAE. The decrease in inner membranes and the contraction of chloroplasts were observed at 34 DAE. In the N-deficient plants, however, the decrease in inner membranes and the contraction of chloroplast were occurred at 15 DAE, without disappearance of the starch grains. The protophloem sieve element-companion cell complexes of small bundles showed an apparent degeneration at 0 DAE, but the degeneration was not observed in the metaphloem, in which the P-plastids appeared in the thick-walled sieve elements. In the control plants, a part of the metaphloem had begun to degenerate at 15DAE and the greater part of the metaphloem was degenerating at 34 DAE. In the N-deficient plants, however, an extensive degeneration was observed at 15 DAE. The degeneration of phloem was characterized by the plasmolysis of sieve elements and companion cells, the swelling and breakdown of mitochondria, and the accumulation of osmiophilic globules in the companion cells. It can be considered that the degeneration of metaphloem is closely related to the decline of transport activity in the senescent leaf.