Spectral Observations for Estimating the Growth and Yield of Rice

Remote spectral observations of plant canopies provide information that may be useful for describing their growth and yield. In this paper we (1) present equations that describe how spectral reflectance observations, expressed as vegetation indices (VI), relate to leaf area index (L); fractional photosynthetically active radiation, PAR, absorbed (Fp); daily solar PAR absorbed (Sp); above-ground dry matter (DM); and, grain yield (YIELD), and (2) apply the equations to a rice (Oryza sativa L.) experiment conducted at Tsukuba in 1987. The 13 treatments consisted of incomplete combinations of 2 transplanting dates (21 May, 11 June), 6 nitrogen application rates (0, 2, 4, 6, 8 and 12 g/m²), and 3 cultivars (Nipponbare, Koshihikari, and Shinanomochi). Over the seasonal interval transplanting to physiological maturity of the grain, grain YIELD (g/m²) and cumulative daily absorbed photosynthetically active radiation (ΣSp, MJ/m²) were linear functions of the cummulative perpendicular vegetation index (ΣPVI). The efficiency of conversion to dry matter, e_c, was 2.9g DM/MJ for transplanting to 20 days after heading and ∼2.5g DM/MJ for transplanting to physiological maturity. YIELD was linearly related to dry matter at heading, DM_h (r² = 0.92) which could be better estimated by the reflectance difference vegetation index at heading, (R₁₁₀₀–R₁₆₅₀)_h (r² = 0.82) than by PVI at heading, PVI_h, (r² = 0.69). The functional relations suggested by the equations demonstrate how observable canopy attributes (L, DM) and plant processes (light interception, photosynthesis, growth) interrelate to VI and YIELD, and provide a basis for interpreting the variation in vegetation indices of rice canopies in terms of canopy development and grain yield.