644 J. AMER. SOC. HORT. SCI. 125(5):644–652. 2000. J. AMER. SOC. HORT. SCI. 125(5):644–652. 2000. Defoliation and Fruit Removal Effects on Papaya Fruit Production, Sugar Accumulation, and Sucrose Metabolism Lili Zhou, 1 David A. Christopher, 2 and Robert E. Paull 3 Department of Tropical Plant and Soil Sciences, College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, 3190 Maile Way, Honolulu, HI 96822-2279 ADDITIONAL INDEX WORDS. sucrose phosphate synthase, sucrose synthase, acid invertase, continual defoliation, fruit thinning, Carica papaya ABSTRACT. 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), sucrose phosphate synthase (SPS), sucrose synthase (SS), and acid invertase (AI) enzyme activities in response to defoliation and fruit thinning were determined. The effects of defoliation and fruit thinning varied with weather conditions, plant growth conditions, and cultivar. Removal of 75% of the leaves significantly reduced new flower production and fruit set, and decreased ripe fruit total soluble solids (TSS), while 50% defoliation did not reduce 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 than when the same number of leaves was removed from the oldest to younger leaves. Continual removal of old leaves reduced new fruit set, fruit weight, and TSS in the 168 day experimental period. Fruit thinning increased new fruit set and ripe fruit TSS. Larger fruit size, faster fruit development, lower respiration rate, and higher sugar contents and AI activity were observed in immature (young) fruit when old fruit were removed. AI activity was reduced during early fruit development and increased again in mature fruit in plants subjected to defoliation, and suggested a role for AI in mature fruit sugar accumulation, while SS activity declined significantly in fruit 154 and 175 days after anthesis and in mature fruit when plants were subjected to continual defoliation. SPS activity was not affected significantly by defoliation or fruit thinning. Source–sink balance was critical for papaya fruit set, development, and sugar accumulation and each mature leaf was able to provide photoassimilate for about three fruit. hermaphroditic species (Awada, 1967; Spears and May, 1988; Wilson, 1983). Papaya (Carica papaya) is a herbaceous, dicotyledonous plant with a single main stem, terminating with a crown of large palmately lobbed leaves (Nakasone, 1986). Most cultivars have flowers borne in a modified cymose inflorescence that appear in every leaf axis just below the growing point (Nakasone, 1986). Plants flower and fruit continuously after flower initiation commences and the leaves generally senesce and abscise before the fruit reaches maturity. The availability of carbohydrate exported from leaves to fruit deter- mines papaya fruit production and sweetness. However, foliage injury can occur in papaya because of insects [e.g., broad mite (Hemitarsonemus latus Banks)], diseases such as powdery mildew (Oidium caricae F. Noack), and papaya ring spot virus (Marler et al., 1993; Nakasone, 1986), and strong winds. This can lead to fruit with reduced sweetness that fail to meet the commercial grade standard of 11.5% TSS (Paull et al., 1997). The relationship between papaya leaf area, fruit production, and sweetness is only partially understood. Defoliation increases pa- paya staminate flower number and decreases trunk growth and leaf dry weight (DW), whereas deflowering decreases staminate flower number and increases trunk growth and leaf DW (Awada, 1967). Papaya leaf pruning to 15 functional leaves does not affect fruit production or TSS of the fruit (Ito, 1976), while thinning papaya to one fruit per node increases fruit size and has no effect on fruit sugar (Martinez, 1988). However, the number of mature leaves or their total area to fruit number or weight was not reported in the aforemen- tioned papers. The critical leaf to fruit ratio or whether there are cultivar differences that impact fruit production and sweetness are unknown. The time from loss of papaya source leaves before fruit set and mature fruit size and sweetness are affected has not been determined. As new leaves are formed following defoliation, the Received for publication 3 Aug. 1999. Accepted for publication 2 June 2000. College of Tropical Agriculture and Human Resources journal series 4467. This research was funded by USDA-CSREES Grants 96-34135-2842 and 98-34135- 6458. The research represents a portion of a dissertation submitted by the L. Zhou for the PhD in Horticulture. We thank Gail Uruu and Nancy Chen for technical assistance. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. 1 Postdoctoral fellow. 2 Associate professor, Department of Molecular Biosciences and Biosystems Engineering. 3 Professor; to whom reprint requests should be addressed. e-mail: paull@hawaii.edu. Plants with a large leaf area often have increased photosynthetic capacity and at a given fruit load can have higher fruit total soluble solids (TSS) levels (Hubbard et al., 1990; Welles and Buitelaar, 1988). An optimum leaf number and area for development of individual fruit has been reported for kiwi fruit (Actinidia deliciosa C.S. Laing and A.R. Fergusson) (Antognozzi et al., 1992; Snelgar and Martin, 1997), mango (Mangifera indica L.) (Chacko et al., 1982), grapefruit (Citrus x paradisi MacFad.) (Fishler et al., 1983), apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] (Palmer et al., 1991) and sweet cherry (Prunus avium L.) (Roper and Loescher, 1987). The leaf to fruit ratio (source–sink ratio) also affects the final fruit size and composition of apples (Hansen, 1982) and plums [Prunus x domestica L.] (Toldam-Anderson and Hansen, 1993). Limiting carbohydrate export from leaves, naturally or artificially induced, reduces fruit size and quality in tomato (Lyco- persicon esculenteum Mill.) (Bertin, 1995), muskmelon [Cucumis melo L. (Reticulatus group)] (Hubbard et al., 1990), grape (Vitis vinifera L.) (Koblet et al., 1994), peach [Prunus persica (L.) Batsch (Peach group)] and pome fruits (Pavel and Dejong, 1993). Sexual expression is also altered by carbohydrate limitation in many