Carbon Dioxide Exchange in Lichens: Low Carbon Dioxide Compensation Levels and Lack of Apparent Photorespiratory Activity in Some Lichens

Abstract

A survey of thirteen species of lichen, ten being from the Stictaceae, revealed that the majority (10) had low carbon dioxide compensation levels ( 50 l CO2 1-1), high photorespiratory activity and RuBP car- boxylase as primary CO2 acceptor. It is apparent from the literature that lichens have been considered to be normal C3 plants (Hill 1976). In fact, although there have been many studies of gross carbon dioxide exchange, particularly with respect to water relations (Bewley 1979), there is a shortage of physiologically or biochemically orientated work. To date, only one estimate of CO2 compensation level appears to have been published, and that was a high 170 pl CO2 -1 for Xanthoria parietina (Collins & Farrar 1978). In this study we are able to show that some lichens do not behave as typical C3 plants but appear to have no, or very low, apparent photorespiratory activity. The results come from studies of carbon dioxide compensation levels and carbon dioxide exchange in the presence and absence of oxygen. In many ways, the lichens behave similarly to some algal species that also show low compensation levels (Birmingham & Colman 1979). An assessment of the relationship between net carbon dioxide exchange and external carbon dioxide concentration was made using an ADC series 225 infra-red gas analyser (IRGA) and a closed- loop gas circulation system. Experiments were carried out at a thallus temperature of 20 ?+ 1C and a saturating irradiance of 150 gE m-2 s-1. Lichen thalli were blotted before use, and had been held moist at 100% RH, 16TC and 70 gE m-2 s-1 for at least six hours to minimise any resaturation respiration. A rapid survey technique was also used in which lichens, collected wet from the field, were incubated in a 30 cm3 vessel kept at a thallus temperature of 20TC by a water bath. One-cm3 gas samples were taken at the start of incubation and again after two hours and analysed on the IRGA, which had been modified to take injection samples (Larson & Kershaw 1975; Snelgar et al. 1980). Initial results using Pseudocyphellaria billardierii, a green algal lichen with cephalodia, demonstrated that the lichen had a carbon dioxide compensation level consistently below 15 1 l CO2 1-1, and occasionally approaching 0 -l CO2 1-1. Furthermore, the lowering of the oxygen level from 21% to about 1% had little effect on net photosynthesis (Table 1). A survey of several species showed many to have low compensation levels but a few to have levels expected of a C3 plant (Table 2). Detailed work using P. delisea, a high compensation-level species, showed that net photosynthesis was markedly sensitive to the 007-2745/80/505-507$0.55/0