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| Dissertation / PhD Thesis/Book | PreJuSER-45486 |
2005
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 3-89336-403-X
Please use a persistent id in citations: http://hdl.handle.net/2128/487
Abstract: Gas exchange of leaves is generally considered as the interchange of gaseous compounds between the leaf interior and ambient air. Once inside the leaf, CO$_{2}$ can diffuse along its concentration gradients mainly regarded in the vertical direction of the blade towards the assimilating tissues. Lateral gas diffusion within intercellular air spaces may be much more effective than has been considered so far which depends on anatomical features of leaves. In heterobaric leaves, lateral diffusion is restricted by bundle-sheath extensions and the mesophyll is composed of closed compartments. Homobaric leaves, however, lack such extensions and the leaves have large interconnected intercellular air spaces. The specific internal gas diffusion properties of the leaves were characterized by gas conductivities. Gas conductivity was larger in lateral than in the vertical direction of homobaric leaf blades. However, there was a large variability of the size and property of the intercellular air space among different species. When `clamp-on´ leaf chambers were used it was found that lateral diffusion inside leaves seriously affected gas exchange measurements. The impact of lateral CO$_{2}$ diffusion on gas exchange measurement was substantial when exchange rates were low. Homobaric leaves showed internal lateral gas fluxes when an overpressure was applied to the leaf chamber which has been used in commercial gas exchange systems to minimise the effects of leaks in the leaf chamber. It was found here that overpressure affected CO$_{2}$ and H$_{2}$O exchange rates of homobaric leaves substantially larger than the theoretical direct impact of air pressure on gas exchange processes. Gas gradients inside leaves emerged when a leaf part was shaded and the adjacent area of the leaf blade illuminated. Respiratory CO$_{2}$ evolved in the shaded region diffused to the illuminated area were it was fixed by photosynthesis. These processes obviously increased the photosynthetic efficiency along the light/shade borderline as was visualized by chlorophyll fluorescence imaging techniques. The recycling of respiratory CO$_{2}$ from distant shaded areas was found to be larger when stomatal conductance was low as is the case under drought stress. Thus, when a homobaric leaf was illuminated by lightflecks, additional CO$_{2}$ increased the carbon gain, water use efficiency, and reduced light stress. It was hypothesized that homobaric leaf anatomy is a trait which has evolved under certain environmental conditions.
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