1.1 - Leaf anatomy, light interception and gas exchange

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Leaves have evolved into a myriad of sizes and shapes, showing great variation in surface features and internal anatomy. Nevertheless, these organs all share a common function, namely to intercept sunlight and facilitate CO2 uptake while restricting water loss. The wide variety of shapes, sizes and internal structure that leaves display implies that many solutions exist to meet the mixed demands of leaf function under frequently adverse conditions.

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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 (Original illustration from Jian-Wei Yu and John Evans, unpublished)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 nature, photon irradiance (photon flux density) can fluctuate over three orders of magnitude and these changes can be rapid. However, plants have evolved with photosynthetic systems that operate most efficiently at low light. Such efficiency confers an obvious selective advantage under light limitation, but predisposes to photodamage under strong light. How then can leaves cope? First, some tolerance is achieved by distributing light over a large population of chloroplasts held in architectural arrays within mesophyll tissues. Second, each chloroplast can operate as a seemingly independent entity with respect to photochemistry and biochemistry and can vary allocation of resources between photon capture and capacity for CO2 assimilation in response to light climate. Such features confer great flexibility across a wide range of light environments where plants occur and are discussed in Chapter 12.

Photon absorption is astonishingly fast (single events lasting 10–15 s). Subsequent energy transduction into NADPH and ATP is relatively ‘slow’ (10–4 s), and is followed by CO2 fixation via Rubisco at a sedate pace of 3.5 events per second per active site, and is generally constrained by even slower diffusion processes. Distributing light absorption between many chloroplasts thus equalises effort over a huge population of these organelles, but also reduces diffusion limitations by allowing placement of chloroplasts at optimal locations within each cell.  The internal structure of leaves (Figures 1.1 and 1.2) reflects this need to maximise CO2 exchange between intercellular airspace and chloroplasts and to distribute light more uniformly with depth than would occur in an homogeneous solution of chlorophyll.

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