Hormonal changes during non-climacteric ripening in strawberry

Abstract

In contrast to climacteric fruits, where ethylene is known to be pivotal, the regulation of ripening in non-climacteric fruits is not well understood. In the non-climacteric strawberry (Fragaria anannassa), auxin and abscisic acid (ABA) are thought to be important, but the roles of other hormones suggested to be involved in fruit development and ripening are not clear. Here changes in the levels of indole-3-acetic acid (IAA), ABA, GA1, and castasterone from anthesis to fully ripened fruit are reported. The levels of IAA and GA1 rise early in fruit development before dropping to low levels prior to colour accumulation. Castasterone levels are highest at anthesis and drop to very low levels well before ripening commences, suggesting that brassinosteroids do not play an important role in ripening in strawberry. ABA levels are low at anthesis and gradually rise through development and ripening. The synthetic auxin, 1-naphthaleneacetic acid (NAA), can delay ripening, but the application of GA3, the gibberellin biosythesis inhibitor paclobutrazol, and ABA had no significant effect. IAA and ABA levels are higher in the developing achenes than in the receptacle tissue and may be important for receptacle enlargement and ripening, and seed maturation, respectively. Contrary to a recent report, the biologically active GA4 was not detected. The pattern of changes in the levels of the hormones are different from those reported in another well studied non-climateric fruit, grape, suggesting that a single consistent pattern of hormone changes does not occur in this group of fruit during ripening.

Fig. 1.

(A) The pattern of strawberry fruit development as measured by changes in diameter and fresh weight. (B) The various stages of fruit development: FL, flower; SG, small green; LG, large green; SW, small white; LW, large white; P, pink/turning; R, red. (C) Diagrammatic representation of the changes in hormone levels at the different stages of fruit development based on the results in Fig. 2.

Fig. 2.

Changes in the levels of IAA (A), GA₁ (B), ABA (C), and CS (D) during strawberry fruit development. Values represent the mean ±SE derived from two individual replicates. The asterisk (*) represents values that were below the detection limit, while (^) denotes that the value was obtained from a single replicate only.

Fig. 3.

Levels of ABA (A) and IAA (B) at the combined white stages of strawberry fruit development as analysed by UPLC-MS. The fresh weight of the achenes was 0.93±0.22g and that of the receptacles 10.8±0.97g. Shown are the means ±SE from three replicates.

Fig. 4.

Effect of NAA (A), PCIB (B), GA₃, and paclobutrazol (C), and ABA (D) on ripening, as measured by colour change. Each point represents the mean for 8–12 replicates; vertical bars indicate standard errors. (This figure is available in colour at JXB online.)