12.1 Photosynthesis in sun and shade

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Plants have adapted to an extraordinarily wide range of light environments, from the deep shade of rainforest understoreys and underwater habitats to the high-radiation environments of deserts and mountain tops. Exploitation of such a wide diversity of habitats is possible because plants have evolved various mechanisms to optimise their use of sunlight. Many plants also exhibit great plasticity in their response to changes in light availability within a particular habitat. This potential for acclimation enables them to exploit more variable environments than plants with a narrower range of responses to light. This section will cover some features that make a plant suited to either a high or low light environment.

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