Genetic Variation in Root and Shoot Growth of Sorghum in Hydroponics1

Abstract

Thirty sorghum (Sorghum bicolor L. Moench) genotypes were grown in hydroponics and characteristics of their shoot and root growth compared to determine if genetic variability exists in the balance of growth between shoots and roots and within the crown root system. A demonstration that such variability exists is a necessary first step in systematic improvement of root systems.All genotypes were compared relative to RTx2536, a standard common to all experiments. Comparisons were based on the ratios of leaf area (LA) to crown root length (RL) and shoot dry weight (S) to root dry weight (R), which reflect overall shoot to root balance, and on the ratio of RL to total root volume (RV) which describes the balance between crown root axes and lateral root branches. Standard breeding lines RTx7078, BTx3197, BTx378 and RTx7000 were similar to RTx2536 in all three ratio comparisons. This similarity may reflect the narrow genetic base available to breeders when these lines were developed, or similar environments during selection.Recent breeding efforts have yielded lines which differ from standard lines as measured by the three ratios. Eight of the 30 entries had LA/RL ratios which differed from RTx2536; seven ratios were higher while only one was lower. Of the seven entries with higher LA/RL ratios, six also had higher S/R ratios. Seven entries had RL/RV ratios different from RTx2536, four entries had higher ratios, and three had lower ratios. Genotypes with low RL/RV ratios did not have more or fewer crown roots and the frequency of lateral roots was not different from RTx2536, but growth and branching of lateral roots was more extensive. We conclude that sufficient variability exists in shoot and root growth characteristics to warrant further study on the problem of systematic improvement of sorghum root systems which may allow superior crop performance in specific environments.