A Field Technique for Screening for Genotypic Differences in Root Growth ¹ (original) (raw)

There is an interest in breeding for deep, extensive root systems to increase crop yields in semiarid environments. Our objective was to develop a field technique with which large numbers of genotypes could be tested for this trait. The experiments were conducted with cowpea (Vigna unguiculata [L.] Walp.) grown on stored soil moisture in the field (coarse-loamy, mixed, thermic Haplic Durixeralf). The herbicide, metribuzin, was banded into the root zone at sowing at specific depths and lateral distances from the seed rows. In the main field trial, lateral distances of 46, 61, and 76 cm were tested in combination with vertical herbicide bands of 30 and 15 cm at average vertical depths of 53 and 60 cm, respectively, and with herbicide rates of 4.5 and 9.0 kg active ingredient ha" 1 in the band. Plant leaves were observed for herbicide symptoms. With this technique, we succeeded in detecting the progress of root growth in the field. Herbicide symptoms consistently developed soonest in plants which were closest to the herbicide band, in either the horizontal or vertical direction. We also succeeded in detecting significant genotypic differences in mean numbers of days to first herbicide symptoms among five cowpea genotypes; California Blackeye #5 and Grant developed symptoms the earliest, 8006 and PI302457 developed symptoms the latest, and PI293579 was intermediate. With four replicates of two-row plots, mean genotypic differences of 2 to 3 days were usually significant at the 5% level. The ranking of genotypes obtained with this herbicide-band technique was consistent with estimates of relative depth of effective rooting obtained from soil moisture extraction measurements. Prior to using this technique in different environments, or to screen other crop species, tests should be conducted to determine the most appropriate herbicide, rate of application, and band location in the soil profile. This technique requires fairly uniform soil throughout the root zone, and relatively uniform genetic material. We conclude that this herbicide-band technique can be used to screen large numbers of genotypes in the field for the presence of rapid root growth. For short season crops, rate of root growth in the first 2 months in the field should be a good predictor of final extensiveness and depth of rooting.