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

Some reports indicate that mesophyll conductance ( g) to carbon dioxide varies greatly with the substomatal carbon dioxide concentration ( C) during the measurement, while other reports indicate little or no change in g with C. I used the oxygen sensitivity of photosynthesis to determine the response of g to C over the range of about 100 to 300 μmol mol C 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 with C over the range of 100 to 300 μmol mol. With lower PPFD during the measurement of g, and especially with low PPFD during plant growth, there was a larger decrease in g with C in soybean. In common bean, the same range in C resulted in about a 60% decrease in g 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 30°C had little influence on the response of g to C or its absolute value in either species. It is concluded that these two species differed substantially in the sensitivity of g to C, and that PPFD but not temperature during leaf development strongly affected the response of g to C. [ABSTRACT FROM AUTHOR]