Photosynthesis and resource distribution through plant canopies

Abstract

Plant canopies are characterized by dramatic gradients of light between canopy top and bottom, and interactions between light, temperature and water vapour deficits. This review summarizes current knowledge of potentials and limitations of acclimation of foliage photosynthetic capacity (Amax) and light-harvesting efficiency to complex environmental gradients within the canopies. Acclimation of Amax to high light availability involves accumulation of ratelimiting photosynthetic proteins per unit leaf area as the result of increases in leaf thickness in broad-leaved species and volume : total area ratio and mesophyll thickness in species with complex geometry of leaf cross-section. Enhancement of light-harvesting efficiency in low light occurs through increased chlorophyll production per unit dry mass, greater leaf area per unit dry mass investment in leaves and shoot architectural modifications that improve leaf exposure and reduce within-shoot shading. All these acclimation responses vary among species, resulting in species-specific use efficiencies of low and high light. In fast-growing canopies and in evergreen species, where foliage developed and acclimated to a certain light environment becomes shaded by newly developing foliage, leaf senescence, age-dependent changes in cell wall characteristics and limited foliage re-acclimation capacity can constrain adjustment of older leaves to modified light availabilities. The review further demonstrates that leaves in different canopy positions respond differently to dynamic fluctuations in light availability and to multiple environmental stresses. Foliage acclimated to high irradiance respond more plastically to rapid changes in leaf light environment, and is more resistant to co-occurring heat and water stress. However, in higher light, co-occurring stresses can more strongly curb the efficiency of foliage photosynthetic machinery through reductions in internal diffusion conductance to CO2. This review demonstrates strong foliage potential for acclimation to within-canopy environmental gradients, but also highlights complex constraints on acclimation and foliage functioning resulting from light ¥ foliage age interactions, multiple environmental stresses, dynamic light fluctuations and species-specific leaf and shoot structural constraints.

Seasonal variability of multiple leaf traits captured by leaf spectroscopy at two temperate deciduous forests

“…For example, shaded leaves display lower chlorophyll a to b ratio and higher LMA 94 compared with sunlit leaves (Niinemets, 2007). As such, it is important to not only 95 explore trait variation in space but also as in the vertical dimension to better capture 96 ecosystem responses to global change.…”

“…Leaf traits not only change with time, but also with the light environments, such 90 as the sun-lit or shaded light condition and the accompanying changes in microclimate, 91 affect leaf traits (Ellsworth and Reich 1993;Niinemets, 2007), as a consequence of 92 underlying fundamental evolutionary and ecophysiological constraints (Terashima et al 93 2001). For example, shaded leaves display lower chlorophyll a to b ratio and higher LMA 94 compared with sunlit leaves (Niinemets, 2007).…”

“…Leaf chlorophyll concentration represents the light harvesting potential and is related to 58 photosynthetic activity (Niinemets 2007;Laisk et al 2009), while accessory pigments 59 such as carotenoids protect leaves from damage when exposed to excessive sunlight 60 (Demmig-Adams and Adams 2000). Leaf mass per area (LMA) describes plants' 61 investment to leaves in terms of carbon and nutrients to optimize sunlight interception 62 (Poorter et al, 2009).…”

Photosynthetic acclimation within individual Typha latifolia leaf segments

“…The local light environment influences the morphological development of leaves in many species, resulting in comparatively thick leaves in bright locations Hikosaka, 1996;Niinemets, 2007). Fully expanded leaves have a limited capacity for morphological change (Sims and Pearcy, 1992;Evans and Porter, 2001;, and acclimation by these leaves requires biochemical changes in carboxylation, electron transport, and light harvesting, as well as modifications to chloroplast structure and orientation (Sebaa et al, 1987;De la Torre and Burkey, 1990a, b;Avalos and Mulkey, 1999;Frank et al, 2001;Murchie et al, 2002;Oguchi et al, 2003Oguchi et al, , 2005Walters, 2005).…”

“…Some species are restricted to sunny or shady locations, and the leaves of these plants are often genetically adapted to their characteristic light environment. The leaves of other species, including those that are naturally exposed to particularly variable light environments, acclimate to local conditions (Murchie and Horton, 1997;Shawna and De Lucia, 1998;Niinemets, 2007). Acclimation to extended (days or longer) changes in light enhances net assimilation and nitrogen use efficiency while decreasing vulnerability to high light stress (Anderson and Osmond, 1987;Pearcy, 1987;Pearcy and Sims, 1994;Dreccer et al, 2000;Niinemets and Valladares, 2004;.…”