Ultrastructural Acclimation to the Light Environment

Chloroplasts of sun and shade leaves differ greatly with respect to their thylakoid systems (Fig. 3.6). Chloroplasts of sun leaves possess only thin grana, while chloroplasts of shade leaves show large grana stacks. The importance of the size of the thylakoid system in a chloroplast becomes obvious when the molecular structure of the thylakoid membranes and the functions of the photosynthetic protein complexes are considered. As photosystem II (PS II) has a larger antenna than photosystem I (PS I), over half of the total chlorophyll a, almost all of the chlorophyll b and most of the carotenoids (P-carotene and xanthophylls) are associated with PS II.

Fig. 3.6. Acclimation of chloroplast ultrastructure to the light environment. a Chloroplast from a sun leaf of tobacco. The small grana stacks are typical of chloroplasts from high-light leaves (Hall and Rao 1994). b Transverse section of a chloroplast from a shade leaf of snapdragon (Antirrhinum majus), showing enhanced thylakoid stacking (Strasburger 1983)

The largest part of a cross-section through a photosystem consists of its antennae. PS II and its antennae are located in the so-called appressed regions—the contact zones of the stacked thylakoids. The number of antenna complexes—especially the outer or mobile antennae, which feed excitation energy to the photosynthetic reaction centre—is variable. A shade chloroplast contains a very large number of light-harvesting systems relative to the reaction centres, owing to the large thylakoid stacks and big antennae. This corresponds to a lower chlorophyll a to chlorophyll b ratio, indicating a higher proportion of the light-harvesting chlorophyll b-containing antennae around photosystem II (Table 3.1) (Kitajima and Hogan 2003).

Chloroplasts of sun leaves, in contrast, contain smaller antennae and thus the ratio of antennae to reaction centres is smaller. The different organisation of the thylakoid systems of sun leaf and shade leaf chloroplasts can be considered a physiological acclimation to the ambient light environment, as both combinations allow optimal utilisation of the incident light. Such differences are observed not only in sun and shade leaves but also between the upper and lower mesophyll cells of dorsiventral leaves.

Chloroplasts of the better-illuminated upper side appear as sun leaf chloroplasts, while those of the lower side (usually the spongy parenchyma) show characteristics of shade leaf chloroplasts. As the thylakoid system of a chloroplast is a dynamic substructure, fixing a leaf upside down accordingly results in a reorganisation of the thylakoid systems of the mesophyll cells.

The range of light energies known to be used for photosynthesis is huge. A champion of using low photosynthetic light intensities is a green sulphur bacterium (Chlorobium phaeobacteroides) from the Black Sea, collected from an 80-m depth, where the light intensity is 3-10 nmol quanta m^-2 s^-1. Thus, a single bacterium receives no more than 300 photons per second, whereas 10¹⁶ photons per second impinge on a mediumsized leaf of a terrestrial plant. Owing to high concentrations of light-harvesting pigments and very low maintenance energy requirements, the sulphur bacterium can survive but takes years to double (Overmann et al. 1992).

In spite of its acclimation to the low-light environment, the efficiency of energy transfer in its photosynthetic apparatus (chlorosomes) is no more than about 60% of the absorbed radiation. Even fastgrowing terrestrial plants with optimised photosynthetic membranes use less than 50% of the absorbed photosynthetic active radiation for photosynthetic CO₂ assimilation (Scholes et al. 2011) (Fig. 3.7).

Fig. 3.7. Gradients of chlorophyll, light absorption and photosynthetic activity across a spinach leaf. a Thin crosssection (Modified from Munns et al. (2010)). b Profiles of light intensity, absorbed light, chlorophyll and photosynthetic ¹⁴CO₂ fixation across a spinach leaf. CO₂ fixation follows best the profile of absorbed light (based on Nishio et al. (1993) and Evans (1995))

On average, a dorsiventral leaf (e.g. of spinach) absorbs about 85% of the incident light between 400 and 700 nm. This is mainly due to extension of the light path in the leaf by scattering; depending on the epicuticular fine structure, up to 10% is reflected and the remaining (-5%) is transmitted.