Microclimate, Photosynthesis and Growth of Lucerne (Medicago sativa L.). I. Microclimate and Photosynthesis

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Abstract
Measurements of microclimate and photosynthesis of lucerne var. Europe were made in the field during the spring of 1976. The maximum rate of canopy gross photosynthesis (14.3 g CO 2 m −2 h −1 , I = ∞) was 2.5 times greater than that of S 24 perennial ryegrass at the same LAI. This difference was due to differences in individual leaf photosynthesis. The photosynthetic rate of the youngest fully expanded leaf of lucerne remained constant throughout the experimental period at 3.6 g CO 2 m −2 h −1 (300 W m −2 ). Measurements of soil water potential profiles indicated that lucerne extracted water from the soil to a depth of at least 800 mm, with a region of maximum uptake between 400 and 600 mm. This capability, with a moderate mean leaf resistance of 460 s m −1 , conferred a high assimilation efficiency on lucerne, with a mean water use efficiency of 34 g H 2 O lost per gram of carbohydrate assimilated, compared with 200 g H 2 O per gram of carbohydrate for S 24.