Variable responses of mesophyll conductance to substomatal carbon dioxide concentration in common bean and soybean

Some reports indicate that mesophyll conductance (g (m)) to carbon dioxide varies greatly with the substomatal carbon dioxide concentration (C (i)) during the measurement, while other reports indicate little or no change in g (m) with C (i). I used the oxygen sensitivity of photosynthesis to determine the response of g (m) to C (i) over the range of about 100 to 300 mu mol mol(-1) C (i) at constant temperature in common bean (Phaseolus vulgaris) and soybean (Glycine max) grown over a range of temperatures and photosynthetic photon flux densities (PPFD). In soybean grown and measured at high PPFD there was only a slight, approximately 15% decrease in g (m) with C (i) over the range of 100 to 300 mu mol mol(-1). With lower PPFD during the measurement of g (m), and especially with low PPFD during plant growth, there was a larger decrease in g (m) with C (i) in soybean. In common bean, the same range in C (i) resulted in about a 60% decrease in g (m) for plants grown and measured at high PPFD, with an even larger decrease for plants at low growth or measurement PPFD. Growth temperatures of 20 to 30A degrees C had little influence on the response of g (m) to C (i) or its absolute value in either species. It is concluded that these two species differed substantially in the sensitivity of g (m) to C (i), and that PPFD but not temperature during leaf development strongly affected the response of g (m) to C (i).