Developmental pattern of grapevine (Vitis vinifera L.) berry cuticular wax: Differentiation between epicuticular crystals and underlying wax

Abstract

The grapevine berry surface is covered by a cuticle consisting of cutin and various lipophilic wax compounds. The latter build the main barrier for transpirational water loss and protect the fruit against environmental factors e.g. pests, mechanical impacts or radiation. The integrety of the fruit surface is one important key factor for post-harvest quality and storage of fruits. Nonetheless, the developmental pattern of cuticular wax was so far only investigated for a very limited number of fruits. Therefore, we performed comparative investigations on the compositional and morphological nature of epicuticular wax crystals and underlying wax during fruit development in Vitis vinifera. The main compound oleanolic acid belongs to the pentacyclic triterpenoids, which occur very early in the development in high amounts inside the cuticle. The amount increases until veraison and decreases further during ripening. In general, very-long chain aliphatic (VLCA) compounds are present in much smaller amounts and alcohols and aldehydes follow the same trend during development. In contrast, the amount of fatty acids constantly increases from fruit set to ripening while wax esters only occur in significant amount at veraison and increase further. Wax crystals at the fruit surface are solely composed of VLCAs and the morphology changes during development according to the compositional changes of the VLCA wax compounds. The remarkable compositional differences between epicuticular wax crystals and the underlying wax are important to understand in terms of studying grape-pest interactions or the influence of environmental factors, since only wax crystals directly face the environment.

Fig 1. Morphology of grape berries (uppercase) and the respective cryo-scanning electron micrographs (8.000x) of the epicuticular wax crystals (lowercase) of V. vinifera cv. Müller Thurgau (left) and cv. Blauer Spätburgunder (right) during development (category 1–7).

Fig 2. Surface of a berry in full maturity with epicuticular wax crystals recorded with cryo-scanning electron microscope (8.000x).

A, In the cultivar Müller-Thurgau epicuticular wax appears as membranous platelets with distinct threadlike extensions. B, Cultivar Blauer Spätburgunder (Pinot noir) shows platelets with rounded margins and on the upper side with crenated edges.

Fig 3. Developmental pattern of different substance classes within the total wax (wax crystals plus remaining wax) of V. vinifera cv. Müller Thurgau (left) and cv. Blauer Spätburgunder (right) berries (N = 5, mean ± SD).

Fig 4. Amount of epicuticular wax crystals (μg cm^-2) and compositional changes concerning different substance classes during berry development of V. vinifera cv. Müller Thurgau (left) and cv. Blauer Spätburgunder (right) (N = 5, mean ± SD).

Fig 5. Amount (μg cm^-2) of remaining wax after mechanical removal of the wax crystals and compositional changes concerning different substance classes during berry development of V. vinifera cv. Müller Thurgau (left) and cv. Blauer Spätburgunder (right) (N = 5, mean ± SD).