PLANT AND SOIL INFLUENCES ON ESTIMATING BIOMASS OF WHEAT IN PLANT-BREEDING PLOTS USING FIELD SPECTRAL RADIOMETERS

Field spectral radiometers were used to estimate the biomass of wheat at early growth stages, as wheat breeders require a rapid, non-destruc tive technique to rank wheat genotypes for early vigour. Under experim ental conditions, good relationships were obtained between reflectance and biomass prior to the wheat crop achieving a green area index of 1 .5. When used above different soil types, good results were achieved o n very uniform dark and light soils under experimental conditions, but greater differentiation between plots differing in biomass was achiev ed on darker soils. Similarly, under operational conditions in wheat b reeders: plots, the best results were achieved against a dark soil bac kground. Structural differences between plants also influenced solar r adiation reflectance. At the Merredin site with the dark soil backgrou nd, where the best correlation between reflectance and biomass was ach ieved, the relationship was much stronger for the more uniform genotyp es at the second stage of selection than for the more heterogeneous ge notypes at the first stage of selection. On these plots, the vegetatio n spectral indices NDVI (normalised difference vegetation index) and T SAVI (transformed soil-adjusted vegetation index) had a coefficient of determination 90-95% as good as the best regression using two waveban ds. To optimise the field spectroradiometry technique for estimating e arly biomass. it should be applied at a weed-free site, with a uniform dark soil background and on material that is relatively homogenous in structure. We conclude that, unless these precautions are taken, the technique will have limited utility in breeding programs.