Agronomy Journal 93:989-991 (2001)
© 2001 American Society of Agronomy
NOTES AND UNIQUE PHENOMENA
Comparison of Two Methods for Estimating Leaf Area Index on Common Bean
Waldir Cintra de Jesus, Jr.a,
Francisco Xavier Ribeiro do Vale*,a,
Reginaldo Resende Coelhoa and
Luiz Claúdio Costab
a Departamento de Fitopatologia
b Departamento de Engenharia Agrícola, Universidade Federal de Viçosa, 36571-000 Viçosa, Minas Gerais State, Brazil
* Corresponding author (dovale{at}mail.ufv.br)
Received for publication October 17, 2000.
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ABSTRACT
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Measurement of leaf area index (LAI) is important in studies of plant growth. Based on this concept, the analysis of crop growth has moved from classical growth analysis to mechanistic models, via functional growth analysis. Leaf area index is a major variable in mechanistic crop growth models and in models that attempt to simulate loss caused by pathogens. A field experiment was carried out from May through August 1998, using common bean (Phaseolus vulgaris L. cv. Carioca), in a randomized complete block design with two methods of LAI evaluation and three replications. The objective of this study was to compare the applicability of the LAI-2000 Plant Canopy Analyzer with the central leaflet width method used to estimate LAI in common bean. The high correlation (r2 = 0.97) observed between the two methods used to estimate LAI shows agreement between them. The results of this study indicated that the LAI-2000 can be used to estimate the LAI of common bean.
Abbreviations: HAA, healthy leaf area absorption • HAD, healthy leaf area duration • LAI, leaf area index
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INTRODUCTION
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THE QUANTITATIVE EFFECTS of environmental factors on plant growth have been studied for more than 80 yr. The basic component of this study, relative growth rate, was introduced by Blackman (1919). Watson (1947) was the first to recognize that the analysis of crop growth should be based on unit of field area rather than per plant, and introduced the concept of leaf area index (LAI), the plan area of leaves per unit area of ground beneath them. From then on, the analysis of crop growth has moved from classical growth analysis to mechanistic models, via functional growth analysis (Radford, 1967; Monteith, 1977; Jones and Kiniry, 1986; Tei et al., 1996; Costa et al., 1997). The concept of LAI is now a major variable in mechanistic crop growth models (Goudriaan and van Laar, 1994) and in models that attempt to simulate loss caused by pathogens (Waggoner and Berger, 1987).
The leaf area of a crop is a determinant factor in mechanisms such as radiation interception and water and energy exchange. Therefore, accurate measurements of LAI are essential to understand the interaction between crop growth and environment.
The determination of LAI in phytopathological studies has become indispensable in calculating healthy leaf area duration (HAD, days) and healthy leaf area absorption (HAA, MJ m-2) in crop loss assessment (Waggoner and Berger, 1987). Several papers have been published considering the relationships among disease, healthy LAI, and yield (Bergamin Filho et al., 1997; Carneiro et al., 1997; Silva et al., 1998).
Direct measurement of LAI in crops is commonly carried out by measuring manually, or with equipment, the total leaf area of the plant in relation to the soil area covered by these leaves. This evaluation is difficult and time consuming, as demonstrated by Daughtry and Hollinger (1984) in an experiment that examined the trade-off between labor costs and measurement accuracy of several direct methods for measuring LAI in corn (Zea mays L.). Direct measurements are generally collected at the end of the growing season and are prone to very large errors that may lead to inconclusive results in crop growth analysis (Monteith, 1981; Costa et al., 1997).
Indirect techniques, which are based on the close coupling between radiation penetration and canopy structure, are a good alternative to the direct LAI measurement techniques. The measurement of canopy gap fractions (the fraction of sky visible through the canopy) at various angles is a particularly powerful approach. Gap fractions have been measured using fisheye photographs (Anderson, 1971), by traversing a sunward-pointed sensor beneath the canopy (Ross, 1981; Lang and Yuequin, 1985; Perry et al., 1988), by linear light sensors (Walker et al., 1988), and by pushing metal probes through the canopy (Wilson and Reeve, 1959). A more detailed discussion of various direct and indirect techniques can be found in Goel and Norman (1990).
In studies of common bean, LAI may also be estimated using an empirical relationship between the maximum width of the central leaflet and total leaf area using a regression equation (Iamauti, 1995). This method is accurate (Iamauti, 1995) but also very time consuming, laborious, and difficult to carry out in the field. These surveys might be done more efficiently using other techniques. Fast and accurate alternatives for LAI assessment would improve parameter estimation under field conditions.
The objective of this study was to compare estimates of LAI by the LAI-2000 Plant Canopy Analyzer (LI-COR, 1990) with those obtained by central leaflet width method in common bean.
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MATERIALS AND METHODS
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The experiment was carried out at the Vila Gianete experimental field of the Federal University of Viçosa, Minas Gerais State, Brazil from May through August 1998, using common bean cultivar Carioca. A randomized complete block design was used with two treatments and three replications. Each plot (16m2) consisted of eight rows, 4 m long, spaced 0.5 m apart. Twelve seeds were sown, and 10 plants were allowed to grow per linear meter of row. The plots were maintained with conventional cultural practices used in commercial fields, including planting and topdressing fertilization, insecticide sprays, weeding, and, when necessary, overhead irrigation. The methods evaluated were (i) estimation of LAI using the central leaflet width method and (ii) estimation of LAI using the LAI-2000. In the central leaflet width method, the leaf area of all leaves on each marked plant was estimated weekly, starting from the appearance of the first trifoliolate leaf. For this, the maximum width of the central leaflet of each leaf was measured with a ruler. The leaf area was estimated for each plant from the empirical relationship (Iamauti, 1995):
where LA is leaf area (cm2) and L is the maximum width of the central leaflet of each leaf (cm). The LAI values were obtained by dividing the total leaf area of each plant on each day of assessment by the area of the soil occupied per plant (0.05 m2).
Estimation of LAI with the LAI-2000 Plant Canopy Analyzer was made using the recommendations made by LI-COR (1990). During the morning (0800–1000 h), a single above-canopy radiation measurement with five below-canopy readings in an equally spaced (20 cm) transect across the row and within the quadrant, with five subsamples per plot, was used to compute the LAI. A 90° view restrictor was used in all measurements to prevent direct sunlight from reaching the sensor and to occlude the operator from the field of view.
The results obtained were analyzed by plotting the data and determining the correlation between the two methods.
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RESULTS AND DISCUSSION
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The high positive correlation (r2 = 0.97) observed between the two methods used to evaluate the growth of the host plant through LAI shows great association between them (Fig. 1a and 1b). The machine resulted in greater LAIs than the manual method, especially for LAIs near 1. At 73 d after planting, it was observed that the plants were in senescence, and the leaf area was low (Fig. 1a). So we hypothesize that the branch area to the indirect estimate of LAIs at that time contributed to their overestimation. Lower LAI values near 1 estimated by the machine could result in an ideal 1:1 line. These results conform with those of Gower and Norman (1991), Welles and Norman (1991), Deblonde et al. (1994), Smolander and Stenberg (1996), Stenberg (1996), Welles and Cohen (1996), and Herbert and Fownes (1997), which demonstrated applicability of the LAI-2000 Plant Canopy Analyzer for estimating LAI in coniferous trees.
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Fig. 1. (a) Leaf area index (LAI) progress curves (± standard deviation) for the different estimation methods and (b) comparison of central leaflet width method of estimating LAI (conventional) values with indirect estimates obtained using the LAI-2000.
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The LAI-2000 Plant Canopy Analyzer has several advantages over other instruments that indirectly determine LAI. The equipment appears to provide accurate estimates over a broad range of LAIs (Gower and Norman, 1991).
The results obtained in this study show that the equipment, LAI-2000, can be used to estimate the LAI of common bean. The use of this equipment to calculate LAI saves time (Gower and Norman, 1991) and may permit extrapolation to HAD and HAA in the quantification of yield loss due to common bean diseases.
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SUMMARY
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This study attempted to identify the applicability that may be expected when estimating common bean LAI with the LAI-2000. The advantage of the equipment is that it permits nondestructive and rapid in situ estimates of LAI. This allows for day-to-day estimates of LAI to be obtained throughout the growing season on the same plants without need for extensive field plots and/or labor-intensive leaf area harvesting and sampling. It also removes the potential error associated with canopy LAI variability by allowing for flagging of sampling locations.
It appears that the LAI-2000 can estimate common bean LAI. The use of this equipment to calculate LAI saves time and may aid in calculation of HAD and HAA for quantifying yield loss due to common bean diseases.
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ACKNOWLEDGMENTS
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This research was partially supported by the European Commission (Project ERBIC18CT96-0037), FAPEMIG, and FINEP. First author was supported by CAPES. We thank J. Marois (University of Florida) for the review of this paper and suggestions for its improvement.
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ABSTRACT
INTRODUCTION
