BROOM SNAKEWEED RESPONSES TO DROUGHT .2. ROOT-GROWTH, CARBON ALLOCATION, AND MORTALITY

The effects of soil water deficit on root growth, carbon allocation, a nd plant mortality of broom snakeweed (Gutierrezia sarothrae (Pursh) B ritt and Rusby) were studied during the spring-summer growing season i n plants subjected to different soil water regimes. As soil water defi cit developed, root length density decreased, indicating that water de ficit reduced root proliferation. Root/shoot ratio remained unchanged (p>0.05) as soil water potential decreased from -0.023 MPa to -2 MPa; but it became higher (p<0.05) in extremely stressed plants (-3.4 MPa), indicating that root growth was favored over shoot growth. Root lengt h density was more closely correlated with green tissue dry weight/-st em dry weight ratio (r = 0.82, p<0.0001) than with root/shoot ratio (r = 0.52, p<0.05). This suggests that (1) expansion of photosynthetic a rea was more sensitive than stem growth to water deficit and (2) carbo n allocation within the shoot was more sensitive to water deficit than allocation between root and shoot. Plants died when gravimetric soil water decreased to around 0.03 g g-1 (equivalent to a soil water poten tial of -7.5 MPa). The leaf relative water content just before death w as about 0.50. Soil water content in the top 20 or 30 cm of the profil e is the single most important factor determining mortality of the sna keweed plant and can be used in making decisions in snakeweed control programs.