SEASONAL AND DIURNAL VARIATION IN THE COs FLUX AND COs--WATER FLUX RATIO OF ALFALFA*

Baldocchi, D.D., Verma, S.B. and Rosenberg, N.J., 1981. Seasonal and diurnal variation in the CO 2 flux and COs---water flux ratio of alfalfa. Agric. Meteorol., 23: 231--244. The seasonal and diurnal variation of CO2 flux (F c ) and COs--water flux ratio (CWFR) of alfalfa were studied using micrometeorological techniques during the summer of 1978 at a location in the east central Great Plains. Significant seasonal variation was found in both F c and CWFR. F c and CWFR were large in value during late spring. F c and CWFR decreased and reached a minimum during midsummer but recovered with the approach of fall. F c was great during late spring as a result of optimal environmental conditions and the spring growth flush. The midsummer minimum in CO 2 flux appears to have been due to a reduced [CO s ] gradient between the chloroplast and atmosphere, to oxygen inhibition of CO s uptake and thermal pre-conditioning. The diurnal course of F c was affected by irradiance and, possibly, by an accumulation of starch in the leaves. The diurnal course of CWFR was affected by net radiation and by sensible heat advection.