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CO₂ Exchange of Subterranean Clover in Variable Light Environments¹

An experiment was designed to compare the CO₂ balances of communities of subterranean clover (Trifolium subterraneum L.) plants exposed to variable and constant photon fluxes. Their CO₂ exchange rates (CER) were monitored continuously during a 4-day test. During 2 of those days the communities were exposed to photosynthetic photon flux densities (PPFD) which were varied over the range 0.26 to 2.00 mE(einsteins) .s^–1.m^–2 in a pattern that simulated the diurnal pattern of a clear day at latitude 30°. During the other 2 days the PPFD was constant at 1.20 mE.s^–1.m^–2. The photoperiod was 12 hours throughout. The PPFD was adjusted so that the integrated totals of CO₂ taken up during the daytime (D) were identical for the two treatments. The daytime CO₂ uptake patterns were different, reflecting the differences in PPFD distribution. The patterns and the totals of CO₂ released during the night-time (N) were found to be almost identical for the two treatments. Consequently, the diurnal CO₂ balance (D-N) of the plants was not affected by the different patterns of photon flux. Therefore, the normal practice in controlled environment studies of using a constant photon flux level throughout each photoperiod produces carbon balance data which are also valid in environments with variable photon fluxes.

Key Words: Carbon dioxide balance • Photosynthesis • Respiration • Trifolium subterraneum L.

² Graduate students and associate professor, Dep. of Soil and Crop Sciences.

Received for publication December 17, 1976.