Abstract
Photosynthetic rate and shoot characteristics were investigated in relation to the tree vigor on 6 cultivars of 5 or 7-year-old very early-ripening Satsuma mandarin (Citrus unshiu Marc.), to discuss the differences of the ecological characteristics among the cultivars.
1. There were no distinctive differences in thickness of the leaf and the palisade tissue among the cultivars. However, in 'Hashimoto wase', both the leaf and the palisade tissue were somewhat thinner than those of the other cultivars. Stomatal density of the leaf varied among the cultivars. Density of stomata was the highest in 'Tanaka wase' and lowest in 'Hashimoto wase' of all the cultivars studied.
2. Photosynthetic rates determined from June to July were higher and maintained so in 'Ichifumi wase' and 'Kusumoto wase'. In 'Tanaka wase' and 'Douwaki wase', photosynthetic rates decreased markedly during this period. It seemed that 'Tanaka wase' and 'Douwaki wase' are not adapted to high temperatures.
3. Differences in the seasonal changes of photosynthetic rates among the cultivars were not clear. In all the cultivars, photosynthetic rates reached maximum at September, then decreased gradually. However, in 'Tanaka wase', 'Yamakawa wase' and 'Kusumoto wase' the rate of decrease of photosynthetic rate after September seemed slower than that of the other cultivars. The maximum photosynthetic rates was high in 'Tanaka wase', 'Kusumoto wase', 'Ichifumi wase' and 'Yamakawa wase', whereas in 'Hashimoto wase', the maximum photosynthetic rate was always the lowest.
4. Shoot growth after summer pruning was greater in 'Ichifumi wase' and 'Kusumoto wase' than in that of other cultivars. Furthermore, in 'Ichifumi wase', the average final shoot length was the longest which resulted in the greatest total shoot growth. 'Douwaki wase' and 'Hashimoto wase' produced fewer shoots, so that their total shoot growth was minimal.
The correlation between photosynthetic rate and total shoot growth indicates that tree vigor is related to photosynthetic rate as measured under field conditions.
