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The nature of sink strength in orange fruit and changes occurring during drought stress were investigated. Potted trees of `Hamlin' orange [Citrus sinensis (L.) Osbeck] grafted on Troyer citrange [Citrus sinensis × Poncirus trifoliata (L.) Raf.] were irrigated using a microsprinkler system creating either well-watered or water-stressed conditions, as determined by stem water potentials. Fruit were harvested every other week from trees of both well-watered and drought-stressed treatments during the final stage of fruit development when sugars accumulate rapidly. Fruit quality indices and activities of sucrose synthase (SuSy), invertase, sucrose-P synthase, sucrose-P phosphatase, V-ATPase, and V-PPase were measured. Acids and soluble sugar concentrations were elevated in drought-stressed fruit, whereas juice pH decreased in those same fruit. Results indicate that increased sink strength in fruit from stressed trees was accompanied by an increase in SuSy activity and lowered juice pH. The remaining enzymes examined in this experiment showed no changes in activity between control and treated fruit, as was the case for plasmalemma and tonoplast sucrose carriers. Based on the present data, we conclude that SuSy and vacuolar pH are the predominant factors controlling photoassimilate accumulation in orange fruit under enhanced sink conditions brought about by imposition of a mild drought stress.
Guanfacine and clonidine were combined with ethephon or metsulfuron-methyl in the spray tank and applied as foliar sprays to Citrus sinensis L. Osb. `Valencia', Citrus madurensis Loureiro (calamondin), and Prunus persica `Elberta' to determine their effects on leaf loss, fruit detachment force (FDF), immature fruit loss, and twig dieback. In `Valencia' orange, `Elberta' peach and calamondin, guanfacine and clonidine effectively reduced ethephon-induced defoliation in all three tree species, whereas only guanfacine was effective with metsulfuron-methyl applications in `Valencia'. The ability of ethephon to reduce FDF in `Valencia' was only minimally impaired by guanfacine but not impaired by clonidine. Both guanfacine and clonidine diminished the capacity of metsulfuron-methyl to reduce FDF. Guanfacine reduced immature fruit loss of `Valencia' caused by metsulfuron-methyl and reduced twig-dieback. Leaf loss was reduced whether guanfacine or clonidine were applied with ethephon, or 24 hours or 17 days before ethephon application. Guanfacine and clonidine reduced leaf loss induced by continuous exposure of potted calamondin trees to ethylene, and leaf loss was similar with guanfacine and 1-methylcyclopropene (1-MCP) treatments. In separate experiments, guanfacine and clonidine were unable to block ethylene perception in Arabidopsis seedlings and petunia flowers but promoted rooting in coleus and tomato vegetative cuttings, suggesting that these compounds have auxin-like activity. The results demonstrate the potential to enhance selectivity of abscission agents with guanfacine and clonidine. Chemical names used: 2-[(2,6-dichlorophenyl)amino]-2-imidazoline, clonidine; 5-chloro-3-methyl-4-nitro-pyrazole, CMN-P; [(2,6-dichlorophenyl)acetyl]guanidine, guanfacine; [(2-chloroethyl)phosphonic acid, ethephon; indole-3-butyric acid, IBA; 1-methylcyclopropene, 1-MCP.
Coronatine is a polyketide phytotoxin produced by several plant pathogenic Pseudomonas spp. The effect of coronatine on abscission in Citrus sinensis L. Osbeck `Hamlin' and `Valencia' orange fruit, leaves, fruitlets, and flowers was determined. Coronatine at 200 mg·L-1 significantly reduced fruit detachment force of mature fruit, and did not cause fruitlet or flower loss in `Valencia'. Cumulative leaf loss was 18% with coronatine treatment. Coronafacic acid or coronamic acid, precursors to coronatine in Pseudomonas syringae, did not cause mature fruit abscission. Ethylene production in mature fruit and leaves was stimulated by coronatine treatment, and 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) and 12-oxo-phytodienoate reductase (12-oxo-PDAR) gene expression was upregulated. A slight chlorosis developed in the canopy of whole trees sprayed with coronatine, and chlorophyll content was reduced relative to adjuvant-treated controls. Leaves formed after coronatine application were not chlorotic and had chlorophyll contents similar to controls. Comparison of coronatine to the abscission compounds methyl jasmonate, 5-chloro-3-methyl-4-nitro-pyrazole and ethephon indicated differences in ethylene production and ACO and 12-oxo-PDAR gene expression between treatments. Leaf loss, chlorophyll reduction and low coronatine yield during fermentation must be overcome for coronatine to be seriously considered as an abscission material for citrus.
Satsuma mandarins (Citrus unshiu) have been produced intermittently along the Gulf Coast for over a century. However, very little is known about the market potential for this citrus fruit in today's consumer markets. This study evaluated consumer preferences for seven external attributes over a range of levels: price ($1.07, $2.18, or $4.39/kg), color (green-yellow, yellow-orange, or orange), size (5.08, 6.35, or 7.62 cm in diameter), seediness (0, 3, or 7 seeds), blemishes (0, 1.91, or 3 cm in diameter), production region label (Alabama or U.S.A.), and organic production (yes or no). Consumers from grocery stores in nine cities in Alabama and Georgia were asked to evaluate 20 photographs of various combinations of these attribute levels using a seven-point intention-to-buy scale. 605 useable surveys were collected and a conjoint analysis was conducted to determine the strength of preference for the attribute levels and the relative importance for attributes. Three consumer segments were identified by cluster analysis of strengths of preferences: the no-blemish segment (37% of sample), the price-sensitive segment (23% of sample), and the no-seeds segment (41% of sample). A multinomial logit analysis identified several demographic, socioeconomic, and usage variables as significant determinants of segment membership.