Heterogeneity of Light Use Across the Leaf Lamina

Light intensity reaching a leaf can fluctuate rapidly and can vary strongly within a short distance, for example in forest understorey situations, where often light flecks penetrating canopy gaps are the main light source. Moreover, leaf tissue which is utilizing light energy for growth processes also shows a high degree of heterogeneity. How does a plant manage to coordinate growth processes of such different tissues as vein tissue consisting of largely differentiated sclerenchymatic elements providing mechanical stability and fully functional xylem and phloem elements and largely undifferentiated parenchymatic tissue of intercostal leaf regions?

To address the connection between heterogeneity of light use and growth distribution, analyses of the spatial distribution of growth and photosynthetic efficiency were performed in developing leaves of the tropical understorey species Coccoloba uvifera and Sanchezia nobilis (Walter et al. 2004). Although base-tip growth gradients and spatial heterogeneities of growth were observed across the leaf lamina, photosynthetic efficiency was distributed homogeneously between base and tip. Yet, differences in the distribution of potential quantum yield Fv/Fm were found between veins and intercostal tissue, indicating differences in the development of the photosynthetic apparatus in those tissues that might reflect a differential demand for locally produced photosynthates. Immediately after leaf unfolding, Fv/Fm was higher in vein tissue compared to intercostal tissue. Gradually, this difference reversed and in full grown leaves, Fv/Fm was higher in intercostal tissue than in vein tissue. This indicates that in early phases of post-emergent leaf development, an increased amount of photosynthates might be required for energy-consuming growth processes within the leaf vein tissue, while differentiation of the pho-tosynthetic machinery can proceed more slowly in intercostal tissue. There, carbohydrates can be supplied in sufficient amount via phloem import.

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