MEASUREMENT OF COTTON GROWTH PARAMETERS AND LEAF AREA INDEX: COMPARISON OF METHODS

S.K. Hicks, R.J. Lascano, and M.T. Forsythe

ABSTRACT

Measurement of cotton shoot growth and leaf area on a land area basis are necessary inputs for models that predict cotton yield and water use. Cotton plant growth on a per unit land area basis is often estimated by measuring growth parameters on individual plants and then extrapolating the measurements to a land area basis by multiplying by a variable determined on a land area basis such as population density. The objectives of this study were: (1) to compare two methods of extrapolating individual plant growth measurement to a per unit land area basis, and (2) to evaluate an instrument developed by LI-COR (LAI-2000) that estimates leaf area index (LAI) by measuring light attenuation as it passes through a plant canopy. The two methods of extrapolating individual plant measurement to a per unit land area basis were:(I) by multiplying by the population density, and (2) by determining the relationship between leaf dry matter and leaf area, leaf area and total shoot mass, fruit mass and total mass, fruit number and fruit mass, and the total leaf mass from a one meter of row samples. The first method was found to overestimate cotton growth and LAI by as much as 40% due to selection of plants that were larger than the population mean. The second method of growth estimation predicted crop growth within 10% of the actual per unit land area measurements. The second estimation method predicted LAI from the total leaf mass harvested from a one meter row sample and the average leaf area per unit leaf mass determined from individually sampled plants (ISP). The total shoot mass was then determined from the estimated LAI and average amount of shoot mass per unit leaf area determined from ISP. Total bolls mass was estimated from the predicted total shoot mass and the average amount of boll mass per unit total shoot mass determined from ISP. Number of bolls per unit land area was then estimated from the predicted boll mass and the average mass per boll determined from ISP. The LAI-2000 was found to be a useful tool for estimation of LAI, however, care must be taken in developing a sampling scheme that takes into consideration the major assumptions the instrument uses to estimate LAI from light attenuation. The major sampling considerations that must be evaluated are: (1) the amount of canopy gap, (2) leaf orientation (due to heliotropic leaf movement and other factors such as prevailing wind direction), and (3) the use of a shading system to provide diffuse light in the area that the instrument is sampling. The closest agreement between LAI estimations with the LAI 2000 and actual LAI measurements was found when measurements were made at noon when leaf orientation is most random. Plant lodging due to the prevailing wind direction and heavy boll load in the late season resulted in a non random leaf orientation and erroneous LAI estimations by the LAI-2000.





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Document last modified July 8, 2004