Root and Top Growth of Irrigated and Nonirrigated Grain Sorghum 1

Limited field research has related grain sorghum [Sorghum bicolor (L.) Moench] root to top development at more than one stage of growth. This work investigated the relationship between depth and distribution of grain sorghum roots in the field and above-ground morphological development (plant height, leaf area index, and dry matter) during the growing season under irrigated and nonirrigated conditions. Soil cores were taken by machine throughout the season, and roots were washed free from soil using a 35-mesh screen to collect roots. Soil water content was estimated in the 15 to 150 cm soil profile by neutron moderation and in the 0 to 15 cm layer by gravimetric sampling throughout the season. Rapid root penetration was characteristic of root development early in the growing season. Traces of roots were observed at depths of 140 to 150 cm 6 weeks after emergence. Root dry matter per 1.08 dm2 of soil surface reached a maximum of 17.1 and 12.3 g at 9 and 10 weeks after emergence in the irrigated and nonirrigated sorghum, respectively. Maximum l/3-row root dry matter accumulation occurred about 8 weeks after emergence. A difference between irrigated and nonirrigated treatments was the increased quantity of irrigated roots in the top 15 cm of soil. A greater proportion of total root dry matter accumulated at the deeper depths in the nonirrigated than in the irrigated sorghum. Leaf dry matter increased minimally after the 7th week, but stem dry matter increased and peaked 8 weeks after emergence in both treatments. Stem dry matter declined in both treatments during panicle development. Maximum rate of dry matter accumulation of both roots and tops was higher in the irrigated treatment.