EFFECTS OF STAGE OF GROWTH, IRRIGATION FREQUENCY AND GYPSUM TREATMENT ON CO2 EXCHANGE OF LUCERNE (MEDICAGO SATIVA L. ) GROWN ON A HEAVY CLAY SOIL.

Effects of weekly (W) and fortnightly (F) irrigation schedule on CO//2 assimilation by lucerne grown on untreated (C) and gypsum-ameliorated (G) heavy clay soil were investigated. Leaf are limited rates of assimilation during the initial stages of regrowth, but rates of up to 9g CO//2m** minus **2h** minus **1 were measures once full ground cover was achieved after approximately two weeks. High rates were maintained until the fifth week of regrowth (one week after full flower), after which there was a marked decline. During the fourth week of regrowth, afternoon rates of canopy photosynthesis under less frequent irrigation were less than those measured at similar irradiance during the morning. This was evidenced as hysteresis in the light response curves and was apparent in all treatment during the final stages of experiment. Results are appraised in terms of the response of lucerne to moisture deficits. Implications for above-ground dry matter production are also discussed.