Water Stress and Seasonal Effects on Rubber Quality in Irrigated Guayule

Guayule (line N396) was harvested monthly from January through July, 1984, from irrigated plots grown in Tucson, AZ. Irrigations were scheduled according to the crop water stress index (CWSI) to maintain three irrigation regimes. Plots were irrigated when their respective CWSI values reached or exceeded 0.3 (wet), 0.6 (medium), and 0.9 (dry). Individual plants from four replicated plots of each regime were analyzed for rubber molecular weight by gel permeation chromatography. The molecular weight (M_w) values for all three irrigation regimes increased from January through March during the plants’ winter quiescent period. Molecular weight decreased between March and April for all regimes when active growth resumed. By May, M_w had increased above March values. Between irrigation regimes, significant differences in M_w were observed only in January and June, with the dry plots yielding the highest molecular weight in both instances.