Fruit photosynthesis and phosphoenolpyruvate carboxylase activity as affected by lightproof fruit bagging in satsuma mandarin.

摘要

Effects of bagging on satsuma mandarin photosynthesis and phosphoenolpyruvate carboxylase activity (PEPC; enzyme code 4.1.1.31) were determined. Seasonal changes in gross photosynthesis ranged from 70 to 400 mumol d--2 h--1 O₂ with a peak at 99 days after full bloom (DAFB) when the assimilation rate of fruits was comparable to that of leaves. However, a peak showing net photosynthesis appeared at 112 DAFB due to high fruit respiration. When fruits were bagged at 85 DAFB, the net photosynthetic peak disappeared possibly as a result of the decline in chlorophyll content in peel. Sugars and organic acids content in bagged fruits were 0.3 and 0.16% less, respectively, than controls at the mature stage (204 DAFB). PEPC activity in peel was much higher than in leaves on a protein basis, increasing between 92 and 112 DAFB and with a peak of 72 units. PEPC activity peak was also 90% of the control after fruit bagging. Just before colour development, mandarins assimilated CO₂ actively through photosynthesis and PEPC. However, these activities were inhibited by bagging, likely resulting in a lower sugar content at harvest. The concomitant activation of PEPC and photosynthesis between 99 and 126 DAFB indicated that CO₂ fixed by PEPC may be used for photosynthesis by mandarins as photosynthesis in several fruits and vegetables (e.g. apples and peas) is considered to have an intermediate status among C3, non-autotrophic tissue and C4/CAM photosynthesis.