The time-response of the CO2-exchange of both leaf surfaces was measured separately. Leaves ofPrimula palinuriandZea mayswere used for the study. After short dark-periods (3 min) the stomata are not closed. Consequently CO2-uptake starts quickly after re-illumination and reaches the steady-state value very rapidly. The time-response of stripped leaves of Primula and of normal leaves after short dark-periods is identical. Accordingly, the conclusion seems to be evident that in both cases we are measuring the time-response of photosynthesis, which is not influenced by stomatal reactions. After long dark-periods (60 min) the stomata are closed. After re-illumination the CO2released by respiration is immediately reassimilated. There is a distinct lag-phase in time-response which is more or less located in the CO2-compensation point. This lag-phase is of different length for both leaf surfaces, and is interpreted as being the lag-phase of stomatal opening reactions. The consequence of the observed different time response of photosynthesis and stomatal reactions is discussed: under non-steady-state conditions photosynthesis is limited by slow stomatal opening reactions.
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