Fruit photosynthesis in Satsuma mandarin

Abstract To clarify detailed characteristics of fruit photosynthesis, possible gas exchange pathway and photosynthetic response to different environments were investigated in Satsuma mandarin ( Citrus unshiu ). About 300 mm −2 stomata were present on fruit surface during young stages (∼10–30 mm diameter fruit) and each stoma increased in size until approximately 88 days after full bloom (DAFB), while the stomata collapsed steadily thereafter; more than 50% stomata deformed at 153 DAFB. The transpiration rate of the fruit appeared to match with stoma development and its intactness rather than the density. Gross photosynthetic rate of the rind increased gradually with increasing CO 2 up to 500 ppm but decreased at higher concentrations, which may resemble C 4 photosynthesis. In contrast, leaf photosynthesis increased constantly with CO 2 increment. Although both fruit and leaf photosynthesis were accelerated by rising photosynthetic photon flux density (PPFD), fruit photosynthesis was greater under considerably lower PPFD from 13.5 to 68 μmol m −2  s −1 . Thus, Satsuma mandarin fruit appears to incorporate CO 2 through fully developed and non-collapsed stomata, and subject it to fruit photosynthesis, which may be characterized as intermediate status among C 3 , C 4 and shade plant photosynthesis. The device of fruit photosynthesis may develop differently from its leaf to capture CO 2 efficiently.