Rudimentary leaf abortion with the development of panicle in litchi: changes in ultrastructure, antioxidant enzymes and phytohormones.

摘要

Floral bud of litchi (Litchi chinensis Sonn.) is a mixed bud that consists of leaf primordia, rudimentary leaves and panicle primordia. According to field observation, the fate of rudimentary leaves depends upon the environment and development of the panicle primordia. Vigorous development of panicle results in abortion of rudimentary leaves. In this study, three types of rudimentary leaves, the upper leaves (UL) which subtended a developing panicle primordium and were bound to abscise after a period of panicle development, the abscising lower leaves (AL) which had an emerging axillary panicle and would abscise under a light external force, and the non-abscising lower leaves (NAL) which subtended a vegetative bud and would develop normally, were sampled from the terminal shoots of cvs. 'Nuomici' and 'Feizixiao'. Ultra-structure of the petiole base in the three types of rudimentary leaves in 'Nuomici' was observed. Signs of cell autolysis, e.g., tonoplast disruption and fusion of vacuole and cytoplasm, were found in localized cells in UL, while massive cell death was observed in AL, suggesting that the two types of rudimentary leaves were at different stages of abortion. The UL was at an early stage of abortion, whereas AL at a late stage of abortion. Cell autolysis was not observed in the nonvascular tissues in NAL. In 'Nuomici', AL had a significantly lower superoxide dismutase (SOD) activity compared to the UL and the NAL, while the latter two had no significant difference in the enzyme activity. In 'Feizixiao', however, no significant difference was found among the three types of rudimentary leaves, although AL had the lowest SOD activity. Peroxidase (POD) activity in AL in both cultivars was significantly higher than that in NAL. The UL had a lower POD activity in 'Nuomici', but a higher activity in 'Feizixiao' compared to NAL. Ascorbate peroxidase (APX) activity was the highest in NAL, and there was no significant difference between the UL and AL. H2O2 concentration was the highest in AL in the two cultivars. The NAL had higher levels of zeatin plus zeatin riboside (ZRs), auxin (IAA) and gibberellin (GA4+7), and lower levels of abscisic acid (ABA) than AL. Total concentration of IAA, GA4+7 and ZRs (TC) and the ratio of (IAA+GA4+7+ZRs)/ABA (RTC) in NAL were higher than those in UL and AL in the two cultivars. The RTC in UL was higher than that in AL. The results suggested that abortion of the rudimentary leaves caused by panicle development may involve programmed cell death, reduced antioxidant activity and increased H2O2 and share similar hormonal control (reduced ZRs, IAA and GA4+7 and increased ABA) to the senescence and abscission of mature leaves.