Insights Into Olive Fruit Surface Functions: A Comparison of Cuticular Composition, Water Permeability, and Surface Topography in Nine Cultivars During Maturation

Abstract

Olive (Olea europaea L.) growing has outstanding economic relevance in Spain, the main olive oil producer and exporter in the world. Fruit skin properties are very relevant for fruit and oil quality, water loss, and susceptibility to mechanical damage, rots, and infestations, but limited research focus has been placed on the cuticle of intact olive fruit. In this work, fruit samples from nine olive cultivars ("Arbequina," "Argudell," "Empeltre," "Farga," "Manzanilla," "Marfil," "Morrut," "Picual," and "Sevillenca") were harvested from an experimental orchard at three different ripening stages (green, turning, and ripe), and cuticular membranes were enzymatically isolated from fruit skin. The total contents of cuticular wax and cutin significantly differed among cultivars both in absolute and in relative terms. The wax to cutin ratio generally decreased along fruit maturation, with the exception of "Marfil" and "Picual." In contrast, increased water permeance values in ripe fruit were observed uniquely for "Argudell," "Morrut," and "Marfil" fruit. The toluidine blue test revealed surface discontinuities on green samples of "Argudell," "Empeltre," "Manzanilla," "Marfil," and "Sevillenca" fruit, but not on "Arbequina," "Farga," "Morrut," or "Picual." No apparent relationship was found between water permeability and total wax coverage or the results of the toluidine blue test. The composition of cuticular waxes and cutin monomers was analyzed in detail, and sections of fruit pericarp were stained in Sudan IV for microscopy observations. Skin surface topography was also studied by means of fringe projection, showing large differences in surface roughness among the cultivars, "Farga" and "Morrut" fruits displaying the most irregular surfaces. Cultivar-related differences in cuticle and surface features of fruit are presented and discussed.

Keywords: Olea europaea L.; cuticular wax; cutin; maturity stage; skin surface topography; water permeance.

Figure 1

An example of fruit surface differences between two olive cultivars (left, “Farga”; right, “Sevillenca”) at the green stage. (A, B): 3D-diagrams of raw data outputs from fringe projections. Blue and black areas represent background noise due to the shape and size of the olives, which did not cover the whole assessment window of the equipment. (C, D): Toluidine blue (TB) staining. (E, F): Sudan IV-stained cross-sections of fruit pericarp observed under a bright-field microscope (bar: 60 μm).