Are bryophytes shade plants?: Photosynthetic light responses and proportions of chlorophyll a, chlorophyll b and total carotenoids

Background and Aims Data are presented from 39 species of mosses and 16 liverworts for ratios of chlorophylls and total carotenoids, and light saturation of photosynthetic electron flow or photosynthetic CO2 uptake, in relation to the postulate that bryophyte cells in general show shade-plant characteristics. Methods pigment concentrations were measured by spectrophotometer in 80 % acetone extracts. Light-saturation curves were constructed by (modulated) chlorophyll florescence and for some species by infra-red gas analysis. Key Results The pigment measurements were widely variable but broadly in line with the findings of previous authors. Median value's (mosses/liverworts) were: total chlorophyll, 1.64/3-76 mg g(-1); chlorophyll a : b 2(.)29/1(.)99; chlorophylls: carotenoids, 4(.)74/6(.)75). The PPFD values at 95 % saturation (estimated from fitted curves) also ranged widely, but were almost all <1000 mumol m(-2) s(-1); the median for mosses was 583 and for liverworts 214 mumol m(-2) s(-1). The two highest PPFD95% values were from Polytrichum species with lamella systems forming a ventilated photosynthetic tissue. Total chlorophyll, chlorophyll a : b and chlorophylls : carotenoids all correlated significantly with PPFD95%. Conclusions Bryophytes include but are not inherently shade plants. Light-saturation levels for species of open sun-exposed habitats are lower than for vascular sun plants and are probably limited by CO2 diffusion into unistratose leaves this limit can only be exceeded by bryophytes with ventilated photosynthetic tissues which provide increased area for CO2 uptake. (C) 2004 Annals of Botany Company.