A possible optical role of the bundle sheath extensions of the heterobaricleaves of Vitis vinifera and Quercus coccifera

Heterobaric leaves are characterized by transparent regions in their lamina , due to the occurrence of bundle sheath extensions. Fused silica fibre-opt ic microprobes were used to monitor light gradients and part of the spectra l regime along the bundle sheath extensions, as well as along the mesophyll in the heterobaric leaves of two representative plants, one mesomorphic (V itis vinifera L.) and one xeromorphic (Quercus coccifera L.). It was found that the attenuation of collimated visible light by the bundle sheath exten sions of both plants was weaker than the attenuation by the photosynthetic parenchyma layers. However, only a small portion of the amount of light tha t strikes the leaf surface is transmitted through these structures. The ada xial epidermis covering the bundle sheath extensions, as well as the mesoph yll, afforded similar effective protection against UV radiation in both tis sues. The relative amount of the forward-scattered visible light inside the bundle sheath extensions approached that detected by the microprobe at the adaxial illuminated leaf surface. Moreover, light transmitted through the bundle sheath extensions was enriched mainly in the blue and red regions, c ompared to light transmitted through the photosynthetic tissue. The time co urse of photosynthetic starch formation in the leaves of V. vinifera detect ed by iodine staining showed that the accumulation of starch during the fir st minutes of illumination was high within photosynthetic parenchyma cells adjacent to the bundle sheath extensions. The data showed that bundle sheat h extensions act as transparent 'windows' which enrich the neighbouring mes ophyll areas with high levels of photosynthetically active radiation (400-7 00 nm). The phenomenon was more pronounced in the thick and compact sclerop hyllous leaves of Q. coccifera by virtue of the greater abundance of bundle sheath extensions as compared to that in V. vinifera. The enhancement of t he light micro-environment within the deep internal layers of the mesophyll may affect the photosynthetic performance of such leaves, giving adaptive advantages.