INDIRECT MEASUREMENT OF LEAF-AREA INDEX IN PIMA COTTON (GOSSYPIUM-BARBADENSE L) USING A COMMERCIAL GAP INVERSION METHOD

Leaf area index (LAI) is an important indicator of canopy development and function, but is time consuming to measure directly. In this paper the authors investigate an indirect and non-destructive method of det ermining LAI in field-grown Pima cotton (Gossypium barbadense L.) plan ted in rows, using a commercially available instrument (LiCor LAI-2000 , Plant Canopy Analyzer (PCA); Lincoln, NE, USA). The PCA uses a circu lar sensor head, with a full 360 degrees field of view, consisting of five sensor rings, each viewing a narrow range of zenith angles. Excel lent agreement was obtained between destructively determined LAI and i ndirectly measured LAI, over a range of LAI induced by irrigation trea tments. However, the near 1:1 relationship between the two methods req uired specific measurement and calculation protocols. All measurements were obtained at dusk. Only a 90 degrees sector (25%) of all five sen sor rings was exposed, directed along the rows and, when possible, awa y from the setting sun. Measurements were obtained at four positions a cross the row/inter-row, which were averaged to characterize the canop y. Underestimation of LAI was observed when data from all five sensor rings were used in the calculation. This bias was removed when data fr om only the central three, most vertical, rings were used. This protoc ol empirically compensated for significant violations of the theory un derlying the indirect measurement technique. As most row crops similar ly violate the theory, this protocol could have widespread utility.