Barley and its environment. 7. Relationships between irradiance, leaf photosynthetic rate and stomatal conductance.
A technique for measuring photosynthetic rates of attached leaves using 14CO2 was compared, in a field crop of barley, with a non-destructive method which uses a mobile infra-red gas analyser and leaf chamber to measure fluxes of 12CO2. There was good agreement between the 2 methods for the response of photosynthetic rate (P) to irradiance on 2 environmentally different days. At saturating irradiances, the maximum rates (Pmax) decreased with increased depth in the canopy and with increased age of leaf. On 26 June 1972, total stomatal conductance was directly proportional to irradiance but on 14 July they were not proportional and stomatal conductance was less at equivalent irradiances. Up to 48% of the measured variability in Pmax on 26 June and up to 100% on 14 July could be explained by measured differences in stomatal conductance. It was concluded that the determinants of the response of leaves to rapid changes in the environment could be separated more clearly if measurements of P, using 14CO2, and stomatal conductance were made concurrently on the same sample.