Photoinhibitory damage to forest floor herbs can occur in sunflecks. This imposes an energy cost on the plant, relative to hypothetical non-photoinhibitable plant, due to the decreased photosynthesis during the sunfleck and during the recovery period after the sunfleck, and to the cost of synthesizing, maintaining and operating the protein synthesis apparatus which repairs photodamage. Some forest floor herbs, such as Oxalis oregana Nutt., avoid photoinhibition by leaflet folding when exposed to high photon flux densities. However, such folding incurs an energy penalty in terms of the cost of synthesizing, maintaining and operating the leaflet-folding (pulvinar) apparatus and as a result of decreased photosynthesis during leaf unfolding after the sunfleck due to reduced light interception in low-light conditions. In addition, the repair machinery is also present in 0. oregana. Using data from the literature the costs of repair (no leaf folding) and avoidance (leaf folding) are computed for a shade environment with (a) no sunflecks, (b) two sunflecks of full sunlight lasting 2 min each and separated by 2 h and (c) two sunflecks of full sunlight lasting 30 min each and separated by 2 h. The first two shade environments give an energetic advantage to the plant which has the repair mechanism only, while the plant with the avoidance mechanism as well is energetically superior in longer sunflecks. The conclusions are discussed in relation to the speed of response which is selectively useful in pulvinar and stomatal movements, and the relative rarity of the leaf-folding mechanism of avoiding photoinhibition. Key-words: Avoidance of photoinhibition, D1 (QB) polypeptide, forest floor herbs, leaf folding, Oxalis oregana, photoinhibition, protein synthesis, pulvini, repair of photodamage, sunflecks Introduction The harvesting, and photochemical transduction, of photons at a rate sufficient to permit rapid growth can be a dangerous enterprise. Oxygenevolving photolithotrophs are universally prone to photoinhibition. This phenomenon involves a reduced capacity for light-saturated and for lightlimited photosynthesis after exposure to high photon flux densities (PFD) between 400-700nm (Powles, 1984). The inhibition can be reversed by exposure to low photon flux densities or to darkness (Powles, 1984). Reversal of photoinhibition involves a repair process which has an energy cost for synthesis and maintenance of the proteinsynthesizing machinery, and for its operation (Raven & Samuelsson, 1986). To this cost may be added the decrement in photosynthetic energy trapping during photoinhibition to give a total energy cost of a photoinhibitory episode. The cost may be compared to that of a hypothetical 'photoinhibition-proof' plant with otherwise identical characteristics living in the same environment. In addition to the universal occurrence of repair, there are a variety of mechanisms whereby plants partially or completely avoid photoinhibition in a given environment (Powles, 1984; Whitelam & Codd, 1986). The avoidance mechanisms all involve an energy cost for their synthesis, maintenance and operation, and may also, in a fluctuating light environment, involve some loss of photon trapping capacity at low photon flux densities. The avoidance mechanisms are listed (non-exclusively) in Tables 1-3; we note that the mechanisms vary greatly in the time taken for their operation, and in their resource costs and their implications for photosynthesis at low photon flux densities. Photoinhibition is not a laboratory artifact; it frequently occurs in the natural environment (Powles, 1984; Whitelam & Codd, 1986; Osmond, 1987). There is accordingly the possibility that the capacity to avoid and to repair photoinhibition can contribute to fitness. This paper has the limited objective of comparing, using data culled from the literature, the net energy gain by a 'repairer' and an 'avoider' in a given environment with fluctuating This content downloaded from 207.46.13.57 on Fri, 09 Sep 2016 04:30:00 UTC All use subject to http://about.jstor.org/terms 6 Table 1. Possible mechanisms of avoidance of photoinhibition based on reducing light incident on plant, organ or J. A. Raven plastid, or fraction of incident light which is absorbed by the photosynthetic pigments. 1. Mobility of whole cell Samuelsson & Richardson (1982); or organisms (negative phototaxis) Raven & Richardson (1984); by flagellar or gliding motility Osborne & Raven (1986) to environment with lower PFD 2. Movement of part of attached macrophyte relative to another part reducing area for photon interception/increasing self shading (a) Nastic movement using pulvini Bjdrkman & Powles (1981); Powles & (e.g. leaflets of Oxalis spp.) Bjbrkman (1981); Ludlow & Bjbrkman (1984) (b) Paraheliotropism Osborne & Raven (1986) (c) Musuclar responses in Wilkens (1986) symbioses such as Tridacna/ Symbiodinium (d) Wilting, leaflet cupping Chiariello, Field & Mooney (1987) upon water loss (e) Oscillation of photosynthetic Raven (1984a) organs in wind, currents or waves 3. Movement of change of shape of plastids, decreasing area for photon interception/increasing self shading (a) Plastid taxis Schorer-Mortel (1972); Haupt (1982); (b) Plastid shape/volume change Raven (1984a) 4. Decreased areal density of Raven (1984ab); Geider (1987) photosynthetic pigments mol pigment (M2 area exposed to light)-1 Table 2. Possible mechanisms of avoidance of photoinhibition based on a reduction in the fraction of light absorbed which can cause excess excitation of photoreaction two.
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John A. Raven (1989) studied this question.