CARBOHYDRATE AND MINERAL-NUTRITION OF ORANGE FRUITLETS IN RELATION TOGROWTH AND ABSCISSION

Carbohydrate and mineral nutrition was studied in relation to abscissi on in fruitlets from leafy inflorescences of the Washington navel oran ge (Citrus sinensis [L.] Osbeck). Differences in the growth rate of th e fruitlets permitted to predict abscission several weeks in advance. This allowed characterization of early differences in composition and behaviour of persisting and abscising fruitlets. Inflorescences with p ersisting fruitlets accumulated more mineral elements than inflorescen ces with abscising fruitlets, and for the phloem-mobile elements the e xcess accumulation was allocated to the fruitlets. Starch accumulated in the inflorescence leaves during early fruitlet growth, and this acc umulation was enhanced by the persisting fruitlets despite their highe r growth rate and mobilizing ability. The relations between the fruitl ets and the inflorescence leaves cannot be explained totally in terms of source sink relationships; a hormonal regulation of the leaves by t he fruitlets is postulated. Acid invertase activities and hexose conce ntration in the pericarp were higher in the abscising fruitlets. The l ower early growth rate of these fruitlets is thus not caused by a limi tation in carbohydrate supply. It seems more related to carbohydrate u tilization, probably hormonally mediated, as demonstrated by the highe r dependence on hormone supply for the growth in vitro of the endocarp explants.