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12.1 - Photosynthesis in sun and shade

In low light, plants need to absorb maximum light for photosynthesis if they are to survive. In high light the problem is reversed. Plants need to maximise their capacity for utilising their abundant light energy. At the same time, the plants have to deal with excess sunlight when their photosynthetic capacity is exceeded. As a consequence of such unrelenting selection pressures, plants have evolved a variety of features that optimise light interception, absorption and processing, according to the light environment in which they had evolved and adapted (Figure 12.2). Adaptation implies a genetically determined capability to adjust attributes, i.e., acclimate to either sun or shade. Such acclimation calls for adjustment in one or more attributes concerned with interception and utilisation of sunlight. Common features of either sun or shade plants are outlined below, and the advantage to plants growing in different light environments is discussed. Field applications are illustrated with examples of sun/shade acclimation and sunfleck utilisation in rainforest plants.


Figure 12.2 A comparison of photosynthetic structure and function in sun and shade plants. Major characteristics are described for cells, leaves and whole plants. (Original drawing courtesy S.A. Robinson)

Initial steps of photosynthesis involve interception and absorption of photons by photosynthetic organs; subsequent steps are involved with utilisation or dissipation of the absorbed quantum energy. Interception of light varies according to size, angle, orientation and surface features of the photosynthetic organ(s) and is also influenced by the arrangement of photosynthetic tissue within those organs (Figure 12.2).