Seasonal changes in temperature and light drive photosynthetic acclimation in a lichen.

Abstract

Lobaria pulmonaria (L.) Hoffm. is an epiphytic lichen common to temperate deciduous forests where it copes with repeated seasonal cycles of temperature and light over many years of growth. At field-ambient temperatures and light levels, realised photosystem II electron transport, estimated by chlorophyll fluorometry, was low both in summer and winter, with transient peaks in spring and autumn. In contrast, potential electron transport and non-photochemical quenching measured at a fixed 20°C peaked in winter, showing the importance of temperature in driving photosynthesis to low levels in the winter despite an acclimation of electron transport to exploit the high ambient light. Chlorophyll content was highest in the dark summer months, when relatively low light levels were required to saturate electron transport. The amount of RuBisCO large subunit (LSU) and rbcL transcript changed seasonally and coincidentally, being high both in spring and late autumn, and low in the summer. The levels of rbcL transcript, estimated by reverse-transcriptase PCR, and its associated hyper-abundant protein, and the activity of PSII changed dramatically through seasonal changes in light and temperature, but the lichens maintained a relatively constant rate of maximum CO2 uptake per RuBisCO LSU from April through November despite these environmental changes. L. pulmonaria acclimates between light and temperature stress in the bright winter months to light-limitation in the dark summer months. Transition intervals in the spring and autumn, with warm, bright and wet conditions, are likely most favourable for growth.