PLANT DEVELOPMENT, WATER RELATIONS, AND CARBON ALLOCATION OF HEART-PODDED HOARY CRESS

Attempts to control the noxious weed heart-podded hoary cress [Cardaria draba (L.) Desv.] in the Pacific Northwest have been relatively ineffective, and this species has expanded its distribution from irrigated hayland onto adjacent semiarid rangelands. The objective of this study was to define the seasonal pattern of development and biological activity of heart-podded hoary cress growing on semiarid rangeland. The study was conducted in the field over a 2-yr period on a terrace site that has deep Cumulic Haploxeroll soil and an adjacent upland site that has a shallow Lithic Argixeroll soil. Soil water content, phenology, xylem pressure potential and leaf conductance were measured at both sites. Above-and belowground biomass and spring C allocation pattern, determined by labeling plants with C-13, were measured at the terrace location. Regardless of location, no heart-podded hoary cress seedlings were found; all shoots developed from rhizome buds. The majority (76%) of heart-podded hoary cress biomass was located belowground. Although drought occurred earlier on the upland than the terrace site, phenology was similar on the two sites. Carbon was translocated to belowground tissues within 1 h of labeling and reached peak accumulation within 24 h. The greatest C-13 enrichment of roots and rhizomes occurred during the flowering stage. The peak in belowground C allocation occurred at a stage when leaf conductance was declining rapidly. The short period of maximum C allocation to belowground tissue, the large proportion of belowground tissue, and the wide variation of phenology among plants at a given time may account for the difficulty in chemically controlling this species.