Leaf Anatomy and Drought

Leaves are complex structures mainly consisting of two dissimilar layers (spongy and palisade mesophyll) of photosynthesizing cells, with different packing and cell orientation, interspersed by vascular tissues, all between two epidermis, which are perforated by stomatal pores. This structure results in an appreciable volume, so that the pho-tosynthesizing cells and chloroplasts are located at some distance from the points of entry of CO2 . Once CO2 reaches the leaf surface, diffusion into the leaf depends on the stomatal resistance as the

Fig. 2.6 (a) Detailed micrograph of the abaxial surface of an olive leaf (bottom side up), presenting CO2 pathway of from ambient (Ca) through leaf surface (Cs) and intercellular air spaces (Ci) to the chloroplast (Cc). Boundary layer conductance (gb), stomatal conductance (gs), and mesophyll conductance (gm) are indicated. (b) Electron micrograph of a grapevine leaf where cell wall (cw), plasma membrane (pm), the chloroplast envelope (ce), and stroma thylakoid (st) can be observed. The differences in CO2 concentration between Ci and Cc depends on intercellular air space (gias), cell wall (gw), and Liquid (gliq) CO2 conductances. A grain of starch (s) and a plastoglobule (pg) can be also seen (reproduced from Flexas et al. 2008). Mesophyll conductance to CO2 : current knowledge and future prospects. Plant, Cell and Environment 31: 602-612; permission from John Wiley and Sons. Photos by A. Diaz-Espejo

Fig. 2.6 (a) Detailed micrograph of the abaxial surface of an olive leaf (bottom side up), presenting CO2 pathway of from ambient (Ca) through leaf surface (Cs) and intercellular air spaces (Ci) to the chloroplast (Cc). Boundary layer conductance (gb), stomatal conductance (gs), and mesophyll conductance (gm) are indicated. (b) Electron micrograph of a grapevine leaf where cell wall (cw), plasma membrane (pm), the chloroplast envelope (ce), and stroma thylakoid (st) can be observed. The differences in CO2 concentration between Ci and Cc depends on intercellular air space (gias), cell wall (gw), and Liquid (gliq) CO2 conductances. A grain of starch (s) and a plastoglobule (pg) can be also seen (reproduced from Flexas et al. 2008). Mesophyll conductance to CO2 : current knowledge and future prospects. Plant, Cell and Environment 31: 602-612; permission from John Wiley and Sons. Photos by A. Diaz-Espejo cuticle is usually regarded as effectively CO2 impermeable (Kerstiens 2006; Morison and Lawson 2007) . During photosynthesis, the CO2 entering the leaves through stomata has to diffuse from substomatal internal cavities to the sites of carboxylation inside the stroma through the leaf mesophyll. Therefore, understanding CO. diffusion in leaves is considered very important because the characteristics of the overall diffusion pathway are one of the determinants of the photosynthetic rate (Flexas et al. 2004) . It is generally assumed that the overall leaf internal diffusion conductance in the photosynthetic pathway (often refereed as mesophyll conductance gm) can be divided in at least two main components; a gaseous component through the intercellular air spaces (gias) and a liquid component referring to CO2 diffusion from the cell walls to the chloroplasts (gliq) (Fig. 2.6). Taking into consideration that both components can be significantly affected by the structure of the mesophyll as well as that abiotic stress considerably affects leaf anatomy it is obvious that abiotic stresses can influence photosynthetic performance through alterations in leaf structure.

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