Broom Snakeweed Responses to Drought: II. Root Growth, Carbon Allocation, and Mortality

Abstract

The effects of soil water deficit on root growth, carbon allocation, and plant mortality of broom snakeweed (Gutierrezia sarothrae (Pursh) Britt and Rusby) were studied during the spring-summer growing season in plants subjected to different soil water regimes. As soil water deficit developed, root length density decreased, indicating that water deficit 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 length density was more closely correlated with green tissue dry weight/-stem 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 area was more sensitive than stem growth to water deficit and (2) carbon 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 potential of -7.5 MPa). The leaf relative water content just before death was about 0.50. Soil water content in the top 20 or 30 cm of the profile is the single most important factor determining mortality of the snakeweed plant and can be used in making decisions in snakeweed control programs.