1.1.1 - Leaf Structure

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Figure 1.1 A scanning electron micrograph of an uncoated and rapidly frozen piece of tobacco leaf showing a hairy lower leaf surface and cross-sectional anatomy at low magnification. Epidermal outgrowths (hairs) offer some protection against insects, and contribute to formation of a boundary layer (unstirred air) adjacent to lower leaf surfaces. An electrical analogue (right side) shows a series of resistances (r) that would be experienced by CO2 molecules diffusing from outside (ambient) air to fixation sites inside chloroplasts. Subscript ‘b’ refers to boundary layer, ‘s’ to stomatal, ‘i’ to intercellular airspaces, ‘w’ to cell wall and liquid phase. Notional values for these resistances are given in units of m2 s mol-1, and emphasise the prominence of stomatal resistance within this series. Corresponding values for CO2 concentration are shown in µL L-1, and reflect photosynthetic assimilation within leaves generating a gradient for inward diffusion. In that case, subscript ‘a’ refers to ambient air, ‘s’ to leaf surface, ‘i’ to substomatal cavity, ‘w’ to mesophyll cell wall surface, ‘c’ to sites of carboxylation within chloroplasts. ci is routinely inferred from gas exchange measurements and used to construct A:ci curves for leaf photosynthesis. Scale bar = 100 µm. (Original illustration from Jian-Wei Yu and John Evans, unpublished)

In a typical herbaceous dicotyledon (Figure 1.1) lower leaf surfaces are covered with epidermal outgrowths, known to impede movement of small insects, but also contributing to formation of a boundary layer. This unstirred zone immediately adjacent to upper and lower epidermes varies in thickness according to surface relief, area and wind speed. Boundary layers are significant in leaf heat budgets and feature in the calculation of stomatal and internal conductances from measurements of leaf gas exchange.  

In transverse fracture (Figure 1.2A) the bifacial nature of leaf mesophyll is apparent with columnar palisade cells beneath the upper surface and irregular shaped cells forming the spongy mesophyll below. Large intercellular airspaces, particularly in the spongy mesophyll, facilitate gaseous diffusion. The lower surface of this leaf is shown in Figure 1.2B. On the left-hand side, the epidermis is present with its irregular array of stomata. Diagonally through the centre is a vein with broken-off hair cells and on the right the epidermis has been fractured off revealing spongy mesophyll cells. Light micrographs of sections cut parallel to the leaf surface (paradermal) through palisade (C) and spongy (D) tissue reveal chloroplasts lying in a single layer and covering most of the internal cell wall surface adjacent to airspaces. Significantly, they are rarely present on walls that adjoin another cell. Despite the appearance of close packing, palisade cell surfaces are generally exposed to intercellular airspace. Inward diffusion of CO2 to chloroplasts is thereby facilitated.

Leaves that develop in sunny environments and have high photosynthetic capacities are generally thicker than leaves from shaded environments. This is achieved with more elongate palisade cells and/or several layers of palisade cells. Thicker leaves in a sunny environment prove energy effective because enough photons reach chloroplasts in lower cell layers to keep their Rubisco gainfully employed. Such depth deploys sufficient Rubisco to confer a high photosynthetic capacity. By contrast, in a shaded habitiat, less Rubisco is required for a leaf with lower photosynthetic capacity, and this can be achieved with thinner leaves.

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Figure 1.2 A scanning electron micrograph of an uncoated and rapidly frozen piece of tobacco leaf fractured in (A) to reveal palisade mesophyll cells beneath the upper leaf surface and spongy mesophyll in the lower half. Chloroplasts can be clearly seen covering the inner faces of cell walls. Looking onto the lower surface (B), the epidermis and stomata are present on the left side of the vein, whereas the epidermis was fractured away on the right side, revealing spongy mesophyll tissue. Light micrographs (C, D) of sections cut parallel to the leaf surface are shown for palisade (C) and spongy mesophyll (D) with solid lines showing where the paradermal sections align with (A). Chloroplasts form a dense single layer covering the cell surfaces exposed to intercellular airspace, but are rarely present lining walls where two cells meet. Scale bar in (A) = 50 µm and in (B) = 200 µm. Magnification given in (A) also applies to (C) and (D). (Original illustration from John Evans and Susanne von Caemmerer. See Evans et al. 1994 for related material)

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