Sugar Regulation of Plastid Interconversions in Epicarp of Citrus Fruit

Abstract

In a previous paper (9) it was demonstrated that transformation between chloroplasts and chromoplasts in citrus epicarp could be controlled in vitro by manipulating sugar and nitrogen supplies in agar media upon which pericarp segments were cultured. High concentrations of sucrose in media usually promoted degreening and inhibited regreening, whereas nitrogen as nitrate and certain amino acids acted in opposition to sucrose by inhibiting degreening and promoting regreening. In the presence ofincreasing nitrogen concentrations, the sucrose effect was much reduced and, when nitrogen supplies were sufficiently high, high concentrations of sucrose sometimes promoted regreening.In addition to promoting loss ofChl, high sugar concentrations in media increased epicarp sugar concentrations, and in media lacking nitrogen high concentrations of sugars caused a marked reduction in epicarp amino acid concentrations. In those in vitro experiments there was a significant negative correlation between Chl and the molar ratio of sugars to amino acids in the epicarp.Based on these observations, it is hypothesized that most citrus fruit degreen in response to a reduced flux in nitrogen to the fruit accompanied by increased concentrations of sugars in the epicarp, both usually induced by cool temperatures. Regreening of late season citrus fruit in the spring and summer could be attributed to renewed nitrogen flux and a reduction in sugar concentrations. Degreening of species such as C. madurensis Lour., could also be attributed to elevated sugar concentrations and reduced nitrogen content, but cool temperatures would not be the causative factor.To further examine the relationship between plastid transformations and sugar and nitrogen status, Chl, sucrose, reducing sugars, total nitrogen, and amino acids were monitored in the epicarp of C. sinensis (L.), Osbeck cv Valencia and pericarp of C. madurensis Lour. fruit degreening in situ, and in the epicarp of regreening C. sinensis fruit. Also, pericarp sections from C. sinensis fruit at different stages of development were tested for their response to sugars and nitrate in vitro. The results, reported here, suggest that accumulation ofsugars in the epicarp is indeed a major factor regulating plastid metamorphosis in citrus fruit, and that while the abundance of nitrogen is an influential factor, nitrogen flux is not a major factor in seasonal changes in plastid form. MATERIALS

Physiology of citrus fruiting

“…The process of external citrus fruit ripening is mostly dependent upon the conversion of chloro-to chromoplasts and involves the progressive loss of chlorophylls and the gain of carotenoids, thus changing peel color from green to orange (Huff, 1983(Huff, , 1984. The changes associated with external ripening are essentially comparable to the senescence of vegetative chlorophyllous tissues and in citrus fruits are influenced by environmental conditions, nutrient availability and hormones (Goldschmidt, 1988;Iglesias et al, 2001).…”

“…It has also been suggested that color break of citrus fruit is controlled by nutrients. Based on data collected from in vitro studies on citrus epicarp, Huff (1983Huff ( , 1984 suggested that citrus fruit might partially de-green in response to the accumulation of sugars. It has specifically been shown that the chloroto chromoplast conversion in citrus fruit epicarps is stimulated by sucrose accumulation after an initial decrease in peel nitrogen content (Iglesias et al, 2001).…”

Control endógeno y exógeno de la maduración externa de los frutos cítricos

“…La evolución de clorofilas estudiada in situ en tres zonas del fruto (peduncular, ecuatorial y estilar) de naranjo dulce 'Valencia', es irregular en la superficie del mismo. Durante el período de degradación de clorofilas, la zona peduncular retiene este pigmento por más tiempo que la zona estilar y el reverdecimiento también se da más fuertemente, obteniendo mayor concentración de clorofilas en dicha zona que en la distal (Huff, 1984). Se han encontrado excepciones a este modo de evolución del color como es el caso del naranjo dulce 'Navelate'.…”

“…Cuando ocurre reverdecimiento, éste consiste en regenerar la construcción de clorofila y las membranas de los cloroplastos y restaurar parcialmente las actividades fotosintéticas (Thomson et al, 1967;Saks et al, 1988). Existen cultivares como la naranja `Valencia' (Coggins y Lewis, 1962;Huff, 1984) y el tangor 'Ortanique' (Gravina et al, 2004) que pasado el período de maduración, si los frutos permanecen en la planta reverdecen, y en el caso de 'Valencia' el proceso se acelera con aplicaciones exógenas de sal potásica de ácido giberélico (Coggins y Lewis, 1962) y de nitrógeno (Jones y Embleton, 1959;Embleton et al, 1973).…”

“…Algunos autores no han encontrado cambios en el contenido de proteínas totales (Lewis et al, 1967), N total (Huff, 1984, Win et al, 2006 o aminoácidos en C.sinensis y C.madurensis (Huff, 1984) durante la maduración de los frutos. Collado et al (1996), sin embargo, encuentran mayor concentración de proteínas totales en El calcio es un mineral esencial en lo que respecta al mantenimiento de las paredes celulares, en especial por reducir la permeabilidad de la cutícula (Zaragoza et al, 1996), y está demostrada su eficacia en el control de algunos desórdenes fisiológicos relacionados con la senescencia, como el 'creasing' (Treeby et al, 2000) y el 'peel pitting' (Zaragoza et al, 1996).…”