Regulation of Color Break in Citrus Fruits. Changes in Pigment Profiling and Gene Expression Induced by Gibberellins and Nitrate, Two Ripening Retardants

Abstract

Citrus clementina fruits were repeatedly treated on-tree from mature green until breaker stages with either nitrate or gibberellin, two retardants of external ripening. The natural color break was characterized by a reduction in chlorophyll concentration, a decrease in beta,epsilon-carotenoids, beta-carotene, neoxanthin, and all-E-violaxanthin, and an increase in beta,beta-xanthophylls [mainly (9Z)-violaxanthin and beta-cryptoxanthin]. The two retardants delayed both chlorophyll depletion and total carotenoid accumulation and in addition altered carotenoid composition. Treated fruits maintained longer the typical carotenoid composition of green fruits and reduced beta,beta-xanthophyll accumulation. Natural degreening was accompanied by a marked decrease in transcript levels of 1-deoxy-d-xylulose 5-phosphate synthase (DXS) and geranylgeranyl reductase (CHL P) while, conversely, pheophorbide a oxygenase (PaO) and phytoene synthase (PSY) gene expression increased. Gibberellin and nitrate delayed the reduction of DXS expression and the induction of PaO and PSY transcript accumulation, while no differences in CHL P were observed. The data indicate that both ripening retardants repressed natural PaO and PSY expression, suggesting a mechanistic basis for the elevated levels of chlorophyll and lower carotenoid concentration resulting from the gibberellin and nitrogen treatments and the consequent color break delay in citrus fruit peels.

Physiology of citrus fruiting

“…In this work it is proposed that sugar regulation may operate via ethylene, whereas GA functions as a repressor of the ethylene-sucrose stimulation. Therefore, GAs as well as nitrates are color break retardants since both delay the chloro-to chromoplast transition (Alós et al, 2006) in contrastethylene (Rodrigo and Zacarías, 2007).…”

“…For example, it has been shown that chlorophyllase, a constitutively expressed gene, does not increase during natural fruit development (JacobWilk et al, 1999). In addition, the expression patterns of carotenoid biosynthetic genes and an associated abundance of carotenoids during ripening have recently been reported (Kato et al, 2004;Rodrigo et al, 2004;Alós et al, 2006). It is noteworthy that citrus peel contains the greatest diversity of carotenoids of any fruit studied to date and their specific accumulation patterns are responsible for the broad range of colors exhibited by PHYSIOLOGY OF CITRUS FRUITING citrus fruits (Gross, 1987).…”

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

“…Huff (1984), propone que el nitrógeno tiene un papel protector estabilizando los cloroplastos, pero que no promueve su formación. Considerando los antecedentes y nuestros resultados, proponemos que el nitrógeno, a pesar de ser un metabolito importante modulando la toma de color de los frutos cítricos (Jones y Embleton, 1959;Huff, 1983;Lee y Chapman, 1988;Quiñones et al, 2004;Alós et al 2006), no es un factor desencadenante del proceso.…”

“…Por otro lado, los frutos con el pedúnculo anillado permanecieron más verdes y presentaron mayores niveles de ABA en el flavedo que los control, al igual que la corteza del pedúnculo de la zona conectada al fruto (zona apical). La mayor concentración de N encontrada en los frutos anillados parece no haber afectado la ruta de biosíntesis de ABA, propuesta por Alós et al (2006), frente al agregado exógeno de nitratos.…”

“…Por otro lado, el ácido giberélico (GA 3 ) aplicado previo al cambio de color retrasa la pérdida de color de los frutos Eilati, 1970, Kuraoka et al, 1977) a través de un retraso en la degradación de clorofilas (Agustí et al, 1988;Alós et al, 2006) y en algunos casos en la síntesis de carotenoides (Lewis y Coggins, 1964;García-Luis et al, 1985). Adicionalmente, mantiene la firmeza de la piel, es decir, mantiene a ésta durante más tiempo en un estado juvenil, en distintas especies y cultivares de Citrus (Coggins et al, 1960a,b;Goldschmidt y Eilati, 1970, Kuraoka et al, 1977, Agustí et al, 1981García-Luis et al, 1985).…”