Light acclimation of the charophyte Lamprothamniun papulosum

Two experimental designs were carried out to study how Lamprothamnium papul osum (Wallr.) J. Gr. is able to acclimate to underwater irradiance. Field m easurements showed that L. papulosum can acclimate quickly to daily changes in salar irradiance. Quantum yield of Photosystem II (PSII) photochemistry showed a fast decrease with increasing irradiances in the morning and an i ncrease with decreasing irradiances in the afternoon. L. papulosum recovere d its PSII photochemical yield completely within Ih after transfer into dar kness. Pigmentation analysis supported these results showing a distinct acc limation of pigment composition and pigment ratios in regard to both acclim ation irradiance and daily irradiance changes. Growth experiments in the la boratory with daily irradiance doses up to 21.6 mol photons m(-2) per day s howed increasing growth rates with increasing irradiances. The calculated d aily light dose needed for zero net growth was about 1 mol photons m(-2) pe r day. The determination of relative electron transport rates exhibited a p ronounced light acclimation in terms of the initial slope of the photosynth esis versus irradiance curve at limiting irradiances (a), the maximum photo synthesis rate at saturating irradiances (P-max) and the irradiance saturat ion point for photosynthesis (E-k = P-max/alpha) in dependency of the accli mation conditions. The correlation of E-k versus irradiance showed that lig ht acclimation mechanisms of L. papulosum decreased at irradiances below 5 mol photons m(-2) per day. Thus, an irradiance of more than 5 mol photons m (-2) per day is necessary for growth. The growth limits corresponded well t o the fact that L. papulosum is often referred to as a species which prefer s to grow in shallow water areas. (C) 2000 Elsevier Science B.V. All rights reserved.