GROUND COVER AND LEAF-AREA INDEX OF MAIZE AND SUGAR-BEET CROPS

Leaf area index (L) is an important variable characterizing the develo pment of a crop and its exchanges with the atmosphere. Direct measurem ents are destructive and tedious. The relations between L and the grou nd cover (C) can be described analytically by taking account of leaf a ngle distribution and leaf clumping. Because it is difficult to charac terize these two variables, we have studied the stability of empirical relations between L and C. Ground cover was measured from vertical-vi ew photographs. For both maize (Zen mays L, var Deal and sugar beet (B eta vulgaris, var Matador), we found the extinction coefficient (K) to be robust throughout crop development and only weakly affected by pop ulation density and sowing geometry. Thus, measurement of C appears to be a simple way to estimate L, once the relation has been established for the variety of interest, and excluding periods of water stress th at may modify crop geometry.