Jeffrey W. Bates

Responses of photosynthesis to irradiance in bryophytes of the Azores laurel forest

Abstract Photosynthetic responses to light intensity were studied under laboratory conditions in seven bryophyte species from evergreen laurel forest, a threatened habitat, on Terceira island in the Azores. Four mosses (Andoa berthelotiana, Echinodium prolixum, Fissidens serrulatus, Myurium hochstetteri) and three liverworts (Bazzania azorica, Frullania tamarisci, Lepidozia cupressina) were selected to encompass a range of potential responses to variations in the forest light environment. Carbon dioxide exchange measurements were made, using an infra-red gas-analyser, at photosynthetic photon flux densities (PPFD) of 0-900 µmol m-2 s-1 and a mean temperature of 21°C in fully hydrated shoots. Most species achieved light saturation of photosynthesis below 30 µmol m-2 s-1, the lowest value being for A. berthelotiana (20 µmol m-2 s-1) and the highest for M. hochstetteri (68 µmol m-2 s-1). The liverwort F. tamarisci had the highest maximum photosynthetic rate (Pmax, 23 µmol CO2 g-1 h-1) whereas Pmax was lowest in the mosses E. prolixum and M. hochstetteri (10 µmol CO2 g-1 h-1). Dark respiration rate, a critical factor in toleration of shade by forest floor plants, was highest in the species with the highest values for Pmax. Compensation point was extremely low (7 µmol photons m-2 s-1) in Fissidens serrulatus, a species found in the deep shade of forest ravines and caves, and highest in M. hochstetteri a moss restricted to better illuminated habitats within and outside the forest. No photoinhibition was detected during the relatively short exposures to high irradiances. Comparison of these responses with data on the forest light environment indicates that, despite the possession of considerable shade adaptations, during winter in the evergreen laurel forest, low light levels may often limit photosynthetic rates of the bryophytes.

Chlorophyll-fluorescence measurements in bryophytes: evidence for three main types of light-curve response

Diffusion theory predicts that, except in the lower part of the daylight range, carbon dioxide supply will always be limiting for photosynthesis in a unistratose leaf. We have used chlorophyll fluorometry to survey the photosynthetic responses of numerous bryophytes to a range of light intensities employing the ‘light curve’ approach. Initially, as light intensity is increased in a stepwise manner, electron transport rate (ETR) in bryophytes follows a saturation curve closely fitted by a negative exponential function, y = A(1 – e–kx ), where y = ETR, x = light intensity (or photosynthetic photon flux density), A is the asymptote (ETR at infinitely high light intensity), k is a rate constant and e is the base of natural logarithms. The initial slope of the response curve, Ak, approximates maximum quantum efficiency (Fv/Fm) which is measured on dark-adapted plant material. However, at higher intensities ETR frequently veers away from the saturation curve owing to the onset of either photoinhibition or the dissipation of the excitation energy by a photoprotective mechanism, probably involving reduction of O2. In the latter case, the measurement of ETR significantly overestimates the rate of photosynthetic carbon fixation. We describe a simple approach that enables these instances of photoprotection and photoinhibition to be identified and discuss the wider significance of the results to the ecology of individual species.

Dialytrichia fragilifolia (Bryopsida: Pottiaceae) in Berkshire and Caernarvonshire, new to Britain

Abstract Dialytrichia fragilifolia (Bizot & Cl. Roux) F.Lara has been found at two localities near the River Thames in Berkshire (v.-c. 22), southern England and also identified as a herbarium specimen from Caernarvonshire (v.-c. 49), North Wales. A description is presented based on the British material which is sterile. This moss is similar to D. mucronata but possesses fragile leaves and a bistratose leaf border with markedly crenate margins, particularly in the apical one-third of the leaf. Mature shoots often have a brush-like apex produced by the persistent costas. It appears to require shadier and less-frequently flooded conditions than D. mucronata. An earlier record of Syntrichia (Tortula) fragilis from Mont-St-Michel in north-west France is shown to be erroneous; the specimen (in E) is also D. fragilifolia. A survey of numerous Dialytrichia specimens held at NMW revealed a further British specimen of D. fragilifolia and also drew attention to the widespread occurrence in Britain, previously overlooked, of D. mucronata plants with fragile leaves. The pattern of leaf fragility differs between the two species. Previously, D. fragilifolia was known only from southern France, Portugal and western Spain. The new records together with other recent finds in Europe extend the range of this moss significantly northwards in western Europe.