Hormonal regulation of fruitlet abscission induced by carbohydrate shortage in citrus

The hormonal signals controlling fruitlet abscission induced by sugar shortage in citrus were identified in Satsuma mandarin, Citrus unshiu (Mak.) Marc, cv. Clausellina and cv. Okitsu. Sugar supply, hormonal responses and fruitlet abscission were manipulated through full, partial or selective leaf removals at anthesis and thereafter. In developing fruitlets, defoliations reduced soluble sugars (up to 98%), but did not induce nitrogen and water deficiencies. Defoliation-induced abscission was preceded by rises (up to 20-fold) in the levels of abscisic acid (ABA) and 1-aminocyclopropane-1-carboxylic acid (ACC) in fruitlets. Applications to defoliated plants showed that ABA increased ACC levels (2-fold) and accelerated fruitlet abscission, whereas norflurazon and 2-aminoethoxyvinyl glycine reduced ACC (up to 65%) and fruitlet abscission (up to 40%). Only the fun defoliation treatment reduced endogenous gibberellin Al (4-fold), whereas exogenous gibberellins had no effect on abscission. The data indicate that fruitlet abscission induced by carbon shortage in citrus is regulated by ABA and ACC originating in the fruits, while gibberellins are apparently implicated in the maintenance of growth. In this system, ABA may act as a sensor of the intensity of the nutrient shortage that modulates the levels of ACC and ethylene, the activator of abscission. This proposal identifies ABA and ACC as components of the self-regulatory mechanism that adjusts fruit load to carbon supply, and offers a physiological basis for the photoassimilate competition-induced abscission occurring under natural conditions

HORMONAL REGULATION OF FRUIT SET AND ABSCISSION IN CITRUS: CLASSICAL CONCEPTS AND NEW EVIDENCE

Evidence has been presented in citrus suggesting that initiation of fruit growth is promoted by mediators of hormonal nature, such as gibberellins, that are able to respond to internal and external stimuli. In seeded citrus, pollination increases GA levels and reduces ovary abscission, whereas exogenous GAs mimic the effects of pollination on fruit set of non-pollinated fruits. In seedless cultivars, a large number of observations indicate that gibberellins are one of the limiting and crucial factors regulating parthenocarpic fruit initiation. While cytokinins have also been involved in the improvement of fruit set, the role of auxins appear to be related to the control of fruit size and retardation of pre-harvest abscission. After hormone stimulation, fruit development is mostly supported by the disponibility of nutrients. It is also known that unfavorable environmental conditions, as well as a shortage of carbohydrates, activate synthesis of regulatory compounds involved in the processes of abscission, such as abscisic acid and ethylene. Abscission of developing fruits appears to be under hormonal regulation according to the sequence: ABA --> ACC --> ethylene --> fruit abscission

ACTA HORTICULTURAE

Evidence has been presented in citrus suggesting that initiation of fruit growth is promoted by mediators of hormonal nature, such as gibberellins, that are able to respond to internal and external stimuli. In seeded citrus, pollination increases GA levels and reduces ovary abscission, whereas exogenous GAs mimic the effects of pollination on fruit set of non-pollinated fruits. In seedless cultivars, a large number of observations indicate that gibberellins are one of the limiting and crucial factors regulating parthenocarpic fruit initiation. While cytokinins have also been involved in the improvement of fruit set, the role of auxins appear to be related to the control of fruit size and retardation of pre-harvest abscission. After hormone stimulation, fruit development is mostly supported by the disponibility of nutrients. It is also known that unfavorable environmental conditions, as well as a shortage of carbohydrates, activate synthesis of regulatory compounds involved in the processes of abscission, such as abscisic acid and ethylene. Abscission of developing fruits appears to be under hormonal regulation according to the sequence: ABA --> ACC --> ethylene --> fruit abscission

In vivo sucrose stimulation of colour change in citrus fruit epicarps: Interactions between nutritional and hormonal signals

During ripening, citrus fruit-peel undergoes 'colour break', a process characterized by the conversion of chloroplast to chromoplast. The process involves the progressive loss of chlorophylls and the gain of carotenoids, changing peel colour from green to orange. In the present work, the in vivo and in vitro effects of supplemented nutrients (sucrose and nitrogen) and phytohormones (gibberellins \GA\ and ethylene) on colour change in fruit epicarp of Satsuma mandarin (Citrus unshiu (Mak,) Marc,, cv, Okitsu), were studied, The rate of colour break was correlated positively with sucrose content and negatively with nitrogen content. The removal of leaves blocked natural sucrose build-up and nitrogen reduction in the peel. Defoliation also inhibited chlorophyll disappearance and carotenoid accumulation, thereby preventing colour break. In vivo sucrose supplementation promoted sucrose accumulation and advanced colour break. In both in vivo and in vitro experiments, colour change promoted by sucrose was unaffected by ethylene but delayed by GA,. in non-supplemented plants, ethylene accelerated colour break while GA, had no detectable effects. Ethylene inhibitors effectively counteracted the sucrose effects on colour change. Collectively, these results suggest that the chloroplast to chromoplast conversion in citrus fruit epicarps is stimulated by sucrose accumulation, The sugar regulation appears to operate via ethylene, whereas GA may act as a repressor of the sucrose-ethylene stimulation

Regulation of photosynthesis through source: sink imbalance in citrus is mediated by carbohydrate content in leaves

In citrus, the occurrence of a sink effect on photosynthesis (A) is controversial. Leaf carbohydrates and photosynthetic rates in field-grown trees of Satsuma mandarin (Citrus unshiu [Mak.] Marc.) cv. Okitsu, were measured to elucidate whether or not the demand for photoassimilates regulates A. The data indicated that the source-sink imbalances induced by different treatments altered both soluble (sucrose, glucose and fructose) and insoluble carbohydrates in leaves, as well as photosynthetic rates. In general, girdling and defruiting increased starch and reduced photosynthesis, whereas source-limiting conditions imposed through partial defoliations had the opposite effect. These results are compatible with the assumption that a lack of sink activity leads to carbohydrate accumulation and feedback inhibition of A, and vice versa. Further evidence supporting a source-sink effect on A was provided by measurements of the dry matter: leaf area ratio, since defoliations, for example, increased this ratio. The in vivo sucrose supplementation to plants with different source: sink ratios (control, defoliated, girdled and defruited plants) increased carbohydrates and reduced photosynthesis. This suggests that sugars may have, per se, the potential to repress photosynthetic rates in intact plants with active sinks. Based on these results we propose that sugar accumulation in citrus leaves causes a feedback inhibition of A

Crescimento de tangerinas 'Ponkan' em plantas submetidas ao raleio químico

Pesquisas comprovam que a relação entre o número de frutos e seu tamanho influencia o rendimento da produção e a qualidade dos frutos. Este trabalho foi conduzido com o objetivo de avaliar o desenvolvimento de tangerinas 'Ponkan', produzidas em plantas enxertadas sobre limoeiro 'Cravo', submetidas ao raleio químico com a aplicação de ethephon. As plantas foram pulverizadas com cinco concentrações de ethephon (0, 150, 300, 450, 600 mg L-1), aplicadas em dois estádios de desenvolvimento dos frutos (30 e 40 mm de diâmetro transversal). Em cada planta, foram selecionados quatro ramos, localizados nos quatro quadrantes, com apenas uma fruta, para avaliar seu desenvolvimento por meio do diâmetro transversal. O raleio químico com a aplicação de ethephon favoreceu a obtenção de tangerinas com maior tamanho. As concentrações a partir de 300 mg L-1 de ethephon foram as mais adequadas para promover o raleio da tangerina 'Ponkan', nas condições estudadas. O raleio realizado no estádio de 40 mm proporcionou a obtenção de frutos de maiores diâmetros

Crescimento de frutos da tangerineira 'Poncã' (Citrus reticulata Blanco) Fruit gowth of 'Ponkan' mandarin

Este trabalho foi realizado com o objetivo de avaliar o desenvolvimento de frutos da tangerineira Poncã, desde o pegamento até a colheita dos frutos, em Viçosa - Minas Gerais. O desenvolvimento do fruto seguiu uma curva do tipo sigmóide simples, sendo a fase I compreendida da antese até o 85º dia após o pleno florescimento, com um período de transição na fase II, que foi até o 101º dia após o pleno florescimento. A fase II teve início logo após a fase de transição, prolongando-se até o 251º dia após o pleno florescimento. A fase III, de amadurecimento do fruto, iniciou-se no 251º dia após o pleno florescimento e prolongou-se até a colheita dos frutos, a qual foi realizada no 276º dia após o pleno florescimento.<br>The objective of this work was to evaluate the fruit development of Ponkan mandarin (Citrus reticulata Blanco). Fruit development followed a simple sigmoid curve, with phase I extending from anthesis to the 85th day after full flowering, with a transition period to phase II that lasted up to the 101st day after full flowering. Phase II began soon after the transition phase, extending up to the 251st day after full flowering. Phase III, fruit ripening, began at the 251st day after full flowering and it was prolonged until harvest, which was carried out at the 276th day after full flowering