Abstract
The Plant Area Index (PAI) of rice, measured indirectly using a Plant Canopy Analyzer (PCA), was compared with that measured directly using a leaf area meter to determine the primary factor that causes the PAI to be underestimated on vertically oriented canopies. Comparison measurements were conducted on several growth stages of the rice in three different cropping seasons; i.e., the usual season (from May to September), the early season (from March to July) and the late season (from June to October) in a warm temperate climate.The ratio of PAI measured using a PCA to directly (PAIPCA/PAID) showed a seasonal variation from 0.62 to 1.07. The lowest values of PAIPCA/PAID (0.62-0.65) were observed near the flowering time of the rice in all cropping seasons, due to the deviation from random distribution of the foliage as a result of vertically oriented leaves, tillers and panicles. A simple method was proposed to correct the PCA measurements by expressing PAIPCA/PAID as a function of the mean tilt angle of the foliage. The corrected values using this function showed good agreement with the directly measured values. The correction method was confirmed by additional comparison measurements conducted in the usual cropping season in the following year.
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