Different leaf surface characteristics of three grape cultivars affect leaf optical properties as measured with fibre optics: possible implication in stress tolerance

Young leaves of three grape cultivars having different surface characteristics (cv. Athiri, pubescent; cv. Soultanina, glabrous green; and cv. Fraoula, glabrous red-brown) only during the early stages of their development, were used to investigate the potential, differential effect of a trichome layer or a pigmented epidermis on the light microenvironment within the mesophyll. The penetration of forward propagated 310, 360 and 430 nm radiation into the leaf tissues was monitored using a quartz fibre-optic microprobe. The mesophyll of the young leaves of Athiri was much better protected from ultraviolet-A, ultraviolet-B and high visible radiation compared to the other two cultivars. Abaxial, as well as adaxial trichome layers attenuated almost all incident radiation at 310 nm and 360 nm and a considerable part of the blue light (430 nm). No significant differences in light attenuation from epidermal and mesophyll layers between the other two cultivars were observed. The adaxial epidermis of young and dehaired leaves of cv. Athiri was largely ineffective in absorbing the incident ultraviolet- B radiation. In addition, the dehaired lamina of such leaves exhibited 80% lower relative concentration of ultraviolet-absorbing compounds per leaf surface area, compared to leaves of similar ontogenetic stage in the cv. Soultanina. It is proposed that the occurrence of a dense trichome layer in young leaves, besides other proposed functions, may play a protective role against not only ultraviolet radiation damage, but also against high insolation. This protection could be advantageous under stress conditions during leaf development.