Cutin Synthesis in Developing, Field-Grown Apple Fruit Examined by External Feeding of Labelled Precursors

Abstract

An intact skin is essential in high-quality apples. Ongoing deposition of cuticular material during fruit development may decrease microcracking. Our objective was to establish a system for quantifying cutin and wax deposition in developing apple fruit. Oleic acid (¹³C and ¹⁴C labelled) and palmitic acid (¹⁴C labelled) were fed to developing apples and the amounts incorporated in the cutin and wax fractions were quantified. The incorporation of ¹⁴C oleic acid (C18) was significantly higher than that of ¹⁴C palmitic acid (C16) and the incorporation in the cutin fraction exceeded that in the wax fraction. The amount of precursor incorporated in the cutin increased asymptotically with time, but the amount in the wax fraction remained about constant. Increasing the concentration of the precursor applied generally increased incorporation. Incorporation in the cutin fraction was high during early development (43 days after full bloom) and decreased towards maturity. Incorporation was higher from a dilute donor solution (infinite dose feeding) than from a donor solution subjected to drying (finite dose feeding) or from perfusion of the precursor by injection. Feeding the skin of a developing apple with oleic acid resulted in significant incorporation in the cutin fraction under both laboratory and field conditions.

Keywords: Malus × domestica; cuticle; cutin; epidermis; hypodermis; oleic acid; palmitic acid; wax.

Figure 1

Stability of fatty acid solutions. Time course of change in radioactivity in the lower layers of an oleic acid solution in a scintillation vial without shaking (solution height 12 mm, solution volume 5 mL). Both solutions had 19,269 (±160) dpm total radioactivity and 0 or 200 mg L⁻¹ of un-labelled (cold) oleic acid. Time on the x-axis is log-transformed and re-plotted in the inset. The values are means ± SEs of five replications.

Figure 2

Incorporation of ¹⁴C (% of total applied) of oleic acid and palmitic acid in cuticular membrane (CM) (a), cutin (b) and wax (c) as affected by application method. ¹⁴C labelled oleic or palmitic acid was applied to the excised exocarp segments (ES) of young fruit of ‘Idared’ apples sampled at 43 days after full bloom (DAFB). The values are means ± SEs of 15 replications.

Figure 3

Time course of ¹⁴C incorporation (% of total applied) in cuticular membrane (CM) (a), cutin (b) and wax (c). ¹⁴C labelled oleic acid was applied to the excised exocarp segments (ES) of young ‘Idared’ apples, sampled at 62 days after full bloom (DAFB), using three different application methods. The values are means ± SEs of 15 replications.

Figure 4

Effect of fatty acid concentration of feeding solution in the incorporation of ¹⁴C in absolute amount (ng; main figures) and in relative amount (% of total applied; inset figures) in cuticular membrane (CM) (a), cutin (b) and wax (c). ¹⁴C labelled oleic acid was applied to the excised exocarp segments (ES) of young ‘Idared’ apples, sampled at 83 days after full bloom (DAFB), using three different application methods. The concentrations of fatty acid in the feeding solutions were adjusted using un-labelled (cold) oleic acid. The values are means ± SEs of 15 replications.

Figure 5

Effect of fruit development stage on ¹⁴C incorporation (% of total applied) in cuticular membrane (CM) (a), cutin (b) and wax (c). ¹⁴C labelled oleic acid was applied to the excised exocarp segments (ES) of ‘Idared’ apples, sampled at various stages of development between 43 days after full bloom (DAFB) and maturity (140 DAFB), using three different application methods. The values are means ± SEs of 15 replications.