Light is an essential source of energy for photoautotropic organisms. Plants are adapted to use this energy source as efficiently as possible and are accli mating towards alterations of light climate during their entire development. Phases of low light availability such as the night or shade periods can be "buffered" in terms of sustained growth activity via remobilization of energy from transient storage compounds such as starch, but they can also lead to photomorphogenetic growth reactions (Matsubara and Walter 2006). Light intensity directly affects assimilation rate and is hence the most important environmental factor with which plant growth rates are controlled over periods of weeks to seasons (Kruger and Volin 2006; Shipley 2006).
The spatial and temporal heterogeneity of the light climate induces a large number of responses on different levels of organization; especially in the shoot (Bjorkman 1981). Among them are state transitions in thylacoid membranes (Demmig-Adams and Adams 1992; Allen and Nilsson 1997; Niyogi 1999; Wollman 2001), ontogenetic modifications in sun and shade leaves of some plant species (Sack et al. 2003; Terashima et al. 2006) and acclimation responses of plant canopy architecture (Terashima and Hikosaka 1995; Evans and Poorter 2001; Frak et al. 2002; Niinemets et al. 2004a,b). This wide spectrum of phenotypic plasticity facilitates efficient acclimation of photosynthesis, resource allocation and biomass production to dynamically fluctuating light climates and habitat conditions (Schurr et al. 2006).
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