INDIRECT DETERMINATION OF LEAF-AREA INDEX OF SUGAR-BEET CANOPIES IN COMPARISON TO DIRECT MEASUREMENT

The determination of the leaf area index of sugar beet canopies by using direct measurement techniques is very laborious. Indirect measurement techniques based on the theory of gap fractions offer a non-destructive and rapid determination of the leaf area index. In this study the performance of the LAI-2000 instrument from LI-COR(1) which measures the gap fraction at five zenith angle classes with an upward looking sensor was compared to a direct determination. A thinning rest and two measurement series in sugar beet with different population densities and varied nitrogen application were conducted in 1992 and 1993 at the site of Gottingen. The direct determination of leaf area index was based on a destructive sampling of leaves and measurements with a Delta-T-2 image analysis system (DIAS). The results of the thinning test and the measurement series in 1992 did not indicate a significant difference between the LAI-2000 and the direct determination. With correlation coefficients of 0.98 and 0.95 respectively, the relation between the two techniques was very close. The measurement series of 1993 showed a worse correspondence. In general the values of the LAI-2000 measurements tended to be a bit lower than those of the direct determination. The leaf area index of sugar beet can be determined with sufficient accuracy using the LAI-2000 instrument if light conditions are favourable and ii the measurement procedure is adapted correctly.