As briefly summarized in the introduction, the differentiation of leaves into sun and shade types is regulated remotely by mature leaves via long-distance signaling. In this section, we focus on the morphology of sun and shade leaves and their acclimation to the environment. Then, we introduce several important findings that indicate unique regulatory mechanism(s) that underlie the formation of sun and shade leaves.
One of the most significant anatomical differences between sun and shade leaves is the thickness of the lamina. Sun leaves are at least 1.5- to 3-fold thicker than shade leaves (Bjorkman 1981). Sun leaves show increased elongation and/or additional cell layers in the mesophyll, especially in the palisade tissue. The shape of cells in the palisade tissue is cylindrical in sun leaves, but sometimes funnelled or rounded in shade leaves (Haberlandt 1914). The height of the epidermal cells is not significantly different in these leaf types, but the contribution of the epidermis to the leaf thickness is lower in sun leaves because their lamina are thicker (Chabot and Chabot 1977; Dengler 1980; Yano and Terashima 2004). The cuticle of sun leaves is more abundant than that of shade leaves, preventing evaporation under high irradiance (Os-born and Taylor 1990; Ashton and Berlyn 1994). Stomata are also affected by light, showing increased numbers and density (see reviews by Ticha 1982 and Givnish 1988). In addition to light, the stomatal density and/or the stom-atal index, which is the proportion of stomata among the epidermal cells, are affected by CO2 (Woodward 1987; Woodward et al. 2002). These structural differences, which affect photosynthetic performance as determined by the conductivity of CO2 and optics (Richter and Fukshansky 1998; Terashima et al. 2001), are the result of leaf developmental regulation.
For plants to establish leaves that are adapted to the light environment, the first step is the recognition of the light environment. This raises questions as to whether the light quality or quantity affect the differentiation of sun and shade leaves, which light sensory system is involved, and where does the plant sense light.
There is no direct evidence regarding the issue of whether light quality or quantity is more important in controlling the development of sun and shade leaves. However, several lines of physiological and genetic evidence suggest that light quantity is a major stimulus. White clover (Trifolium repens L.) grown under the same photosynthetically-active photon flux density (PPFD) but with various ratios of red and far-red light, which governs the balance between Pfr (active) and Pr (inactive) phytochromes, showed the same leaf thickness index (leaf mass per unit area in mgmm-2, specific leaf area in mm2 mg-1 leaf DM) but different petiole elongation (Smith 1994; Heraut-Bron et al. 1999, 2001). The idea that light quantity is more important in the stimulation was supported by a genetic analysis. Loss-of-function mutant of PHYB, which encodes the main type-II phytochrome in Arabidopsis, also showed leaf thickening with an increase in the light intensity (Kim et al. 2005). On blue light receptors it was revealed that single and double mutants of CRYTOCHROMES (cry1) and cry2 and the PHOTOTROPINS (phot1) mutant showed the same leaf thickening as much as WT plants did (Weston et al. 2000; Lopéz-Juez et al. 2007). Thus, known photoreceptors are probably not involved in the differentiation of sun and shade leaves, although no results have been reported from double mutants of phototropins or disruptants of multiple phytochromes.
If light quantity is the triggering stimulus, does the plant monitor the instantaneous light intensity or the total amount of daily light? Chabot et al. (1979) compared the leaf morphology and photosynthesis efficiency of Fragaria virginiana leaves exposed to different light regimes, changing the light intensity and span (maintaining the photoperiod using the background light) but with plants receiving the same quantity of light. Although the instantaneous light intensities among the regimes differed by five-fold, the leaf thickness was not affected. However, the leaf thickness increased with increasing light span while the light intensity was held constant. Thus, the daily light amount regulates differentiation into sun or shade leaves.
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