EVALUATION OF VISUAL RATING-SCALES FOR GREEN LEAF RETENTION IN SORGHUM

Visual ratings that can be taken quickly in the field provide information for evaluation and selection that could not otherwise be obtained due to time or cost constraints of quantitative techniques. We evaluated the accuracy of two subjective visual rating systems that assess green leaf retention in sorghum [Sorghum bicolor (L.) Moench] under drought stress in the field: Green leaf retention (GLR) estimates the percentage of total laminar leaf area that is green; number of green leaves (NGL) estimates the number of green leaves. The relationships between GLR, NGL, and two parameters of green leaf area based on measured leaf area (green leaf area or GLA and percent green leaf area or PGA) were examined by correlation and regression analyses. The simple coefficients of correlation (r) for the relationships were GLR/GLA, 0.824; GLR/PGA, 0.926; NGL/GLA, 0.956; and NGL/PGA, 0.923. Linear regressions of these relationships had slopes significantly greater than zero. The GLR/GLA was analyzed as a second-order relationship, with R2 = 0.821; GLR/PGA, NGL/GLA, and NGL/PGA were analyzed as linear relationships, with R2 = 0.857, 0.913, and 0.852, respectively. The R2 values for these relationships were high for biological or agronomic data, confirming the reliability of these ratings for assessing green leaf retention and supporting the validity of such visual rating systems used in sorghum breeding programs.