Abstract
Some epiphytic Hymenophyllaceae are restricted to lower parts of the host (<60 cm; 10–100 μmol photons m-2 s-1) in a secondary forest of Southern Chile; other species occupy the whole host height (≥10 m; max PPFD >1000 μmol photons m-2 s-1). Our aim was to study the photosynthetic light responses of two Hymenophyllaceae species in relation to their contrasting distribution. We determined light tolerance of Hymenoglossum cruentum and Hymenophyllum dentatum by measuring gas exchange, PSI and PSII light energy partitioning, NPQ components, and pigment contents. H. dentatum showed lower maximum photosynthesis rates (Amax) than H. cruentum, but the former species kept its net rates (An) near Amax across a wide light range. In contrast, in the latter one, An declined at PPFDs >60 μmol photons m-2 s-1. H. cruentum, the shadiest plant, showed higher chlorophyll contents than H. dentatum. Differences in energy partitioning at PSI and PSII were consistent with gas exchange results. H. dentatum exhibited a higher light compensation point of the partitioning of absorbed energy between photochemical Y(PSII) and non-photochemical Y(NPQ) processes. Hence, both species allocated energy mainly toward photochemistry instead of heat dissipation at their light saturation points. Above saturation, H. cruentum had higher heat dissipation than H. dentatum. PSI yield (YPSI) remained higher in H. dentatum than H. cruentum in a wider light range. In both species, the main cause of heat dissipation at PSI was a donor side limitation. An early dynamic photo-inhibition of PSII may have caused an over reduction of the Qa+ pool decreasing the efficiency of electron donation to PSI. In H. dentatum, a slight increase in heat dissipation due to acceptor side limitation of PSI was observed above 300 μmol photons m-2s-1. Differences in photosynthetic responses to light suggest that light tolerance and species plasticity could explain their contrasting vertical distribution.
Highlights
Light energy is an essential resource for photosynthesis, both extreme low and high light intensity can limit plant performance [1]
H. cruentum declined its An with an increase in photosynthetic photon flux density (PPFD), especially above 60 μmol photons m-2 s-1, where An sharply decreased to less than 10% of Amax (Fig 3; S2 and S3 Tables)
Exceed its optimum activity (24.6 μmol photons m-2 s-1). This is supported by the concomitant sharp increase in the fraction of energy dissipated as heat (YNPQ), which overpassed YPSII at 50 μmol photons m-2s-1, and by the highest saturated NPQ observed in H. cruentum (Fig 5c and 5d)
Summary
Light energy is an essential resource for photosynthesis, both extreme low and high light intensity can limit plant performance [1]. To relax back to its ground form, the energy captured by them can basically have one of three fates: (1) it can be re-emitted as fluorescence; (2) it can be used to drive photochemical processes (e.g. photosynthesis, photorespiration, water-water cycle, etc.), or (3) it can be dissipated non-photochemically as heat [2]. These three processes have a competitive kinetics, in such a way that any increase in the efficiency of one will result in a decrease in the yield of the other two. If harmless mechanisms of photoprotection (i.e. such as non-photochemical quenching) are insufficient to deal with an excess of absorbed energy and the prevention of photoinhibition, this excess will conduct to damaging free radicals formation (e.g. superoxide anion, hydrogen peroxide, hydroxyl radical, peroxyl radical and singlet oxygen), and to the subsequent photo-oxidative destruction of the photosynthetic apparatus [10, 11]
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