Defoliation and fruit removal effects on papaya fruit production, sugar accumulation, and sucrose metabolism

Papaya (Carica papaya L.) source size and sink strength were modified by a single defoliation or continual defoliation and fruit thinning, Fruit set, development, weight, total sugar (sum of sucrose, fructose, and glucose), s ucrose phosphate synthase (SPS), sucrose synthase (SS), and acid invertase (AI) enzyme activities in response to defoliation and fruit thinning were d etermined. The effects of defoliation and fruit thinning varied with weathe r conditions, plant growth conditions, and cultivar. Removal of 75% of the leaves significantly reduced new power production and fruit set, and decrea sed ripe fruit total soluble solids (TSS), while 50% defoliation did not re duce new fruit set or ripe fruit TSS, When every third leaf from the oldest leaf was not removed, the number of new flowers was reduced by 47% more th an when the same number of leaves was removed from the oldest to younger le aves. Continual removal of old leaves reduced new fruit set, fruit weight, and TSS in the 168 day experimental period. Fruit thinning increased new fr uit set and ripe fruit TSS, Larger fruit size, faster fruit development, lo wer respiration rate, and higher sugar contents and AI activity were observ ed in immature (young) fruit when old fruit were removed. AI activity was r educed during early fruit development and increased again in mature fruit i n plants subjected to defoliation, and suggested a role for iii in mature f ruit sugar accumulation, while SS activity declined significantly in fruit 154 and 175 days after anthesis and in mature fruit when plants mere subjec ted to continual defoliation, SPS activity was not affected significantly b y defoliation or fruit thinning. Source-sink balance was critical for papay a fruit set, development, and sugar accumulation and each mature leaf was a ble to provide photoassimilate for about three fruit.