![]() In a temperate pine plantation, stomatal responses generally limited sunfleck responses, but to different extents in different species ( Naumberg & Ellsworth, 2000). In a tropical ecosystem, stomatal response times determined the photosynthetic response times to sunflecks when water was limiting, but stomata exhibited little or no control when water was not limiting ( Allen & Pearcy, 2000a). Previous studies in diverse ecosystems (tropical rainforests, Allen & Pearcy, 2000a temperate pine plantations, Naumberg & Ellsworth, 2000) have shown considerable variation in the relative contribution of stomatal control to sunfleck utilisation. ![]() The efficient use of sunflecks is thus a function of the rapidity with which biochemical and stomatal limitations are removed due to, inter alia, light activation of enzymes and stomatal opening, and/or the maintenance of high enzyme activation and wide-open stomata during low-light periods ( Pepin & Livingston, 1997 Allen & Pearcy, 2000a,b Naumberg & Ellsworth, 2000 Timm et al., 2002). The stomatal limitation arises because stomata typically close at least partially in the shade and, again, it takes some time for the stomata to re-open and thereby reduce the limitation on supply of CO 2 for photosynthesis. Appreciable periods of time are required for enzyme re-activation and to build the pools of intermediates. Biochemistry is initially limiting because after prolonged shading, ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and other photosynthetic enzymes are deactivated, and because of the small size of the pools of Calvin cycle intermediates. Pearcy (1987) estimated that sunflecks resulted in 30–60% of the total daily carbon gain, and noted that the utilisation of sun flecks is of paramount importance for the overall assimilation balance of understorey plants.įor leaves acclimated to shade, the photosynthetic response to a sudden increase in irradiance (via sunflecks) is not instantaneous owing to biochemical and stomatal limitations. Depending on the ecosystem in question, sunflecks may account for between 20 and 80% of the total irradiance ( Watling et al., 1997). In this layer, the light environment is characterised by diffuse irradiance of low intensity interrupted by intense sunflecks that might last from a second or less to 15 min or more. The understorey of some evergreen closed forests is amongst the darkest environments habitable by higher plants ( Allen & Pearcy, 2000a). Average irradiance on the floor of a closed forest may be as little as 0.5% of full sunlight whilst leaves in the upper canopy receive full sunlight ( Björkman & Ludlow, 1972). Irradiance generally decreases vertically from the top of the canopy to forest floor. The more persistent reduction in photochemical efficiency of upper canopy leaves, which means less efficient light use in subsequent shade periods, but stronger protection from high light, may be related to the generally higher irradiance and longer duration of sunflecks in the upper canopy, but potentially reduces carbon gain during shade periods by 30%.Ĭlosed forests are often spatially complex and the light environment can also be spatially and temporally variable. Poor drought tolerance and achieving a positive carbon balance in a shaded canopy may be functionally related to high stomatal conductance in the shade in N. These processes were reversed within 30 min in coppice leaves, but this took longer in upper canopy leaves. ![]() During simulated sunflecks, zeaxanthin was formed rapidly and photochemical efficiency was reduced. Stomatal conductance was relatively high in the shade and stomata did not directly control photosynthetic induction under these conditions. Photosynthetic induction was rapid compared with tropical and northern hemisphere species. Gas exchange and chlorophyll fluorescence were measured during a 10 min simulated sunfleck and, in the ensuing dark treatment, we examined the recovery of PS II efficiency and the conversion state of xanthophyll cycle pigments. Shaded leaves were exposed to a sudden increase in irradiance from 20 to 1500 µmol m −2 s −1. Responses to simulated sunflecks were examined in upper canopy and coppice leaves of Nothofagus cunninghamii growing in an old-growth rainforest gully in Victoria, Australia.
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