Are Bryophytes Shade Plants? Photosynthetic Light Responses and Proportions of Chlorophyll a, Chlorophyll b and Total Carotenoids

INTRODUCTION

Bryophytes, even those of open, exposed habitats, have been seen as showing features that would generally be regarded as characteristic of shade plants (Valanne, 1984). They typically have low chlorophyll a : b ratios, reported values mostly lying within the range from 1·5 to 3·0 (Rastorfer, 1972; Rao et al., 1979; Martin, 1980; Aro, 1982; Martin and Churchill, 1982; Kershaw and Webber, 1986). This implies that the light-harvesting chlorophyll a : b protein complex makes up a large proportion of the total chlorophyll present. Light-response curves for bryophytes commonly show saturation of photosynthesis at rather low irradiances. Saturation levels around 20 % of full sunlight have been found by a number of investigators for a wide range of bryophytes, including species of open, brightly lit habitats such as Bryum argenteum (Rastorfer, 1970), Racomitrium lanuginosum (Kallio and Heinonen, 1975), Grimmia pulvinata and Tortula ruralis (Alpert and Oechel, 1987). In mosses from oak–hickory forest and exposed sandstone outcrops in Kansas, Martin and Churchill (1982) found generally higher chlorophyll a : b ratios on the exposed outcrops than in the forest. The forest mosses showed a striking increase in total chlorophyll concentration, and a (weaker) tendency for chlorophyll a : b ratios to increase, following canopy closure in spring. Kershaw and Webber (1986) demonstrated strong seasonal changes in the chlorophyll content and light-responses of Brachythecium rutabulum in an abandoned apple orchard in Ontario, but little change in the chlorophyll a : b ratio.

It is intuitively reasonable that poikilohydric photosynthetic organisms should be adapted to function at relatively low light levels. During periods of bright, dry sunny weather bryophytes will generally be dry and metabolically inactive. Most of their photosynthesis takes place in rainy or cloudy weather, when irradiance may often be <20 % of that in full sun. For bryophytes of dry habitats, the major physiological need during transient exposures to bright sunshine as the plant dries out is likely to be for photoprotection rather than energy capture. The bryophytes that, prima facie, might be expected to be best adapted to photosynthesize under high light conditions are the species of mires, springs and other wet habitats, which remain constantly moist in full sun.

In the present study we set out to explore further how the ratios of chlorophyll a to chlorophyll b, and of total chlorophylls to total carotenoids, are related to habitat and to the light-response of photosynthesis in bryophytes. Chlorophyll fluorescence provided a ready means of estimating photosynthetic electron flow (Schreiber et al., 1995; Maxwell and Johnson, 2000), making the screening of the light responses of a wide range of species relatively quick and easy. Fluorescence measurements, of course, include photorespiratory as well as photosynthetic electron flow, but this does not invalidate comparisons within or between-species; it simply puts them on a different basis. There are various other problems that may arise and precautions that need to be taken in the use of fluorescence measurements, which are discussed below. On the positive side, chlorophyll fluorescence provides its own additional insights into the responses of the photosynthetic system to increasing irradiance. In particular, the photochemical quenching parameter q_P gives a measure of the oxidation state of the primary electron acceptor (Q_A), and non-photochemical quenching (NPQ) reflects the non-radiative dissipation of excitation energy (Björkman and Demmig-Adams, 1995; Gilmore, 1997). For comparison with the chlorophyll-fluorescence data, response curves of net photosynthesis and dark respiration were constructed for a few species by infra-red gas analysis.

The material studied, mostly collected in south-west England, covers a taxonomically wide range including examples of most of the major groups of bryophytes.

MATERIALS AND METHODS

RESULTS

The major results are presented in Tables 1–3; the measurements and the more immediate correlations between them are discussed briefly below. Wider and more general relationships and implications are considered in the Discussion.

DISCUSSION

How far do the results bear out the proposition that bryophytes are shade plants? The pigment data are widely variable, but generally in line with previous findings for bryophytes where comparable data are available. Mean chlorophyll a : b ratios for mosses in our data (2·39, s.d. = 0·51) are close to those of a sample of 31 species from Kansas (2·51, s.d. = 0·22) (Martin and Churchill, 1982). Eleven species from North Carolina ranged from 1·4 to 2·1 (Martin 1980), all within the range of our data. In a seasonal study of 15 aquatic bryophytes from Spain, Martínez-Abaigar et al. (1994) found an overall range of chlorophyll a : b ratios from 1·94 to 3·33. These figures for bryophytes are generally low by vascular-plant standards, but there is a large overlap between the two groups. In eight Malayan ferns, Nasrulhaq-Boyce and Haji Mohamed (1987) found chlorophyll a : b ratios from 2·1 to 2·9, with a slightly higher mean in sun (2·70) than in shade (2·33) species. Extreme-shade ferns can have chlorophyll a : b ratios well down in the bryophyte range; Nasrulhaq and Duckett (1991) found a ratio of 1·8 in the Malayan rainforest fern Teratophyllum (and 2·2 in the lycopod Selaginella willdenowii), and Johnson et al. (2000) found ratios of 1·58 and 2·00 in gametophytes and sporophytes, respectively, of Trichomanes speciosum. In a survey of 20 vascular-plant species (all angiosperms) from the Sheffield area, Johnson et al. (1993a) found chlorophyll a : b ratios ranging from 2·12 to 3·29 (mean 2·74, s.d. = 0·36); in light-acclimation experiments on six species from the same area, Murchie and Horton (1997) found average chlorophyll a : b ratios of 3·61 in moderate and 3·23 in low-light treatments. Johnson et al. (1993a) and Murchie and Horton (1997) found only a weak association between the chlorophyll a : b ratio and shade tolerance; the correlation between the chlorophyll a : b ratio and PPFD_95% was similarly weak among the mosses in our data, but was stronger in the liverworts and in the data as a whole.

Johnson et al. (1993a) found a very close correlation between total chlorophylls and total carotenoids in their sample of vascular plants, hence rather little variation in the chlorophyll : carotenoid ratio (mean 4·00, s.d. = 0·77). This contrasts with the higher mean and very much greater variability in the present bryophyte data. Johnson et al. (1993a) found highly significant correlations of the proportions of lutein (positive) and the xanthophyll-cycle pigments (negative) with shade tolerance, but Rosevear et al. (2001) concluded from growth experiments that environment is more important than genetic predisposition in determining leaf pigment content. Environmental influences undoubtedly play a part in bryophytes, but are very unlikely to be the sole (or even the main) cause of the variation seen here. Either way, it is reasonable to surmise (which invites testing) that at least a part of the very wide range in the chlorophyll : carotenoid ratio seen in bryophytes is due to large amounts of xanthophyll-cycle pigments in the species of sunny sites, particularly in view of the extreme NPQ levels shown by many of these plants at high irradiance (Demmig-Adams, 1998). The highest (saturation) values of NPQ found by Johnson et al. (1993b) in their sample of 20 vascular plants was approx. 4·5. At high irradiance, values of 10–15 or more are commonplace in bryophytes of sunny sites, implying very high levels of photoprotection.

It is clear from Fig. 2 that (with a few exceptions) even bryophytes from the most sun-exposed sites are not true ‘sun plants’. In bryophytes PPFD_95% rarely exceeds 1000 µmol m⁻² s⁻¹, whereas vascular sun plants commonly reach saturation only at irradiances approaching full sunlight. Measurements using the same methods as for the bryophytes gave PPFD_95% values of 1355 µmol m⁻² s⁻¹ for Fraxinus excelsior, 2125 µmol m⁻² s⁻¹ for Malus domestica ‘Discovery’ and 1349 µmol m⁻² s⁻¹ for Typha angustifolia (M. C. F. Proctor, unpubl. res.). However, it is also clear that there is a very wide range in the light responses of bryophytes, related to habitat – and that species with PPFD_95% in the range 600–1000 µmol m⁻² s⁻¹ hardly qualify as ‘shade plants’. Wide variation in the number of chloroplasts per cell between sun and shade mosses has been reported by Duckett and Renzaglia (1988) and Robertson and Duckett (in Nasrulhaq-Boyce and Duckett, 1991). On the evidence of our sample, liverworts are generally more shade-adapted than mosses, but with a large overlap between the two groups. There is no clear evidence of any significant general differences between mosses and liverworts in the interrelationships of total chlorophyll (mg g⁻¹), the chlorophyll a : b ratio, and the ratio of total chlorophylls to total carotenoids.

For fundamental physical reasons, small ectohydric plants with unistratose leaves, in which external capillary water plays a physiologically essential role, are inherently best adapted to function at rather low radiation incomes (Proctor and Tuba, 2002). On the one hand, high net radiation (W m⁻²) leads to high evaporation. On the other, high rates of photosynthetic CO₂ uptake at high PPFD (µmol m⁻² s⁻¹) are readily met by the deep ventilated mesophyll of a vascular sun plant, but may not be met by the unistratose (but relatively thick walled) leaves of a bryophyte (Nobel, 1977). This is likely to be true even taking into account a ‘leaf-area index’ of 5 or more for the bryophyte—analogous to the mesophyll : leaf area ratio for a leaf (Proctor, 2000; Nobel, 1974). It is probably the fundamental limitation of bryophytes growing in permanently wet situations in which water is not a problem. It may not be a significant ecological constraint in the field because bryophytes of wet places are largely confined to nutrient-poor habitats, since they are unable to compete with the tall dense growth of vascular plants in fertile sites.

The desiccation-tolerant species of dry sunny places face the additional constraint that they quickly dry out in sunshine and, for most of the time the sun is shining, they will be dry and metabolically inactive. Natural selection might therefore be expected to have optimized the photosynthetic system for rainy and overcast conditions leading to lower saturation levels for photosynthesis than in species of permanently wet places. The present data do not support this idea, although the mosses of sunny desiccation-prone sites tend to be those in which there is greatest uncertainty in their light responses (double asterisks in Table 1). The gas-exchange measurements (Fig. 3) confirm at least that the saturation levels in Table 1 cannot be too low, and are probably not greatly at variance with actual rates of photosynthesis. The high values of NPQ typically seen in these plants can be seen as reflecting the need for protection against the combined stress of bright light and repeated cycles of drying and re-wetting, but the high chlorophyll contents found in several of the species are surprising. Their physiology needs further critical study.

The Polytrichaceae, with a ventilated photosynthetic tissue formed by lamellae on the upper leaf surface, in effect provide a test of the hypothesis that the rate of photosynthesis of bryophytes with unistratose leaves is limited by CO₂ diffusion. With relatively high chlorophyll content, PPFD_95% extending far beyond the range normal for other mosses, and low NPQ and 1 – q_P, they more nearly approach the photosynthetic adaptation of vascular plants than any other bryophytes. The lamella systems of Polytrichaceae evidently correspond with a major shift in photosynthetic physiology relative to other mosses. But Polytrichaceae are typically also desiccation tolerant; the shoots dry out quickly and there is no indication that the lamellae significantly retard water loss. Thus the lamellae should be seen primarily as an adaptation enhancing CO₂ uptake at high irradiance for a given water loss, rather than as an adaptation to control water loss as such. The present data have little to say about the other group of bryophytes with ventilated photosynthetic tissues, the Marchantiales. World-wide, these liverworts are primarily plants of permanently or seasonally moist open habitats, from the lowland tropics to temperate and alpine regions. They are prominent, for instance, in the winter flora of the Mediterranean, but they extend into permanently moist woodland habitats, where Conocephalum is common on stream banks. On our limited sample, the Marchantiales do not stand out from the other liverworts as do the Polytrichales from other mosses. However, they vary widely in the development of ventilated photosynthetic tissue in the thallus – lacking in the deep shade-adapted Dumortiera hirsuta, well developed in, for example, Lunularia cruciata, Targionia hypophylla and species of Riccia – and they would repay further exploration (Green and Lange, 1995). Notably, the Polytrichales and the Marchantiales both include gregarious, fast-growing invasive species of open habitats. Polytrichum species often cover large tracts of ground after forest or moorland fire (Maltby et al., 1990), and Marchantia, Lunularia and Conocephalum can be troublesome weeds of flower pots and nursery gardens.

Sphagnum is a special case. The species have relatively low PPFD_95%, but high NPQ indicating a high level of photoprotection consistent with their unshaded habitat. The low saturation values for photosynthesis probably reflect high diffusion resistance to CO₂ uptake—but this may be only a secondary limitation because low availability of N and P impose their own constraints on possible rates of production in most bog habitats. For all bryophytes, the photosynthetic parameters must be, to some extent, interlinked by the combination of natural selection and physiological control processes. This is underlined by the narrow range of discrimination against the heavy isotope ¹³C over a broad range of bryophytes and C₃ flowering plants, in spite of wide variation in chlorophyll content, in morphology, and in the adaptations of different taxonomic groups and species to the availability of light and water (Rundel et al., 1979; Farquhar et al., 1989; Proctor et al., 1992; Price et al., 1997).

Supplementary Material