CONTROL OF PAR-SATURATED CO2 EXCHANGE-RATE IN SOME C-3 AND CAM PLANTS

We measured PAR-saturated CO2 exchange rate (CER), and leaf N, P and c hlorophyll (Chl) concentrations in 21 plant species, selected to encom pass as broad a range in specific leaf area (SLA) as possible, and enc ompassing non-succulent C-3 as well as succulent CAM plants, We worked with plants growing under uniform conditions in the facilities of a b iological research station to ensure that any correlations found were due to inherent, genetically controlled, relationships between the mea sured parameters and not due to variations in resource availability in different habitats, We found CER to be strongly correlated to SLA, le af N concentration and Chl concentration, CER increased much faster wi th increasing leaf N concentration (CER approximate to N-3.1) than wit h increasing SLA (CER approximate to SLA(1.2)). CER also increased muc h faster with leaf N concentration than with increasing Chl concentrat ion (CER approximate to Chl(1.3)), indicating the photosynthetic N-use efficiency (NUE) to be higher for plants with high N concentration th an for plants with low N concentration (NUE approximate to N-2.1), Ana lysis of covariance showed that these relationships exist even when co mparing plants of widely different growth forms - succulent or non-suc culent, and of different photosynthetic pathways, as the C-3 and CAM p lants compared here, Testing against scaling coefficients calculated u sing dimensional analysis, showed that the scaling of N, Chl and CER a gainst SLA was not merely a result of diluting N and Chl with carbon i n thicker leaves but that SLA, probably through influencing light abso rption and/or CO2 diffusion pathway, played an independent role in con trolling CER.