Root growth, appearance and disappearance in perennial grasses: Effects of the timing of water stress with or without defoliation

Abstract

The effects of early and late defoliations were evaluated under different levels of soil water content on root growth, appearance and disappearance in Stipa clarazii Ball. S. tenuis Phil., and S. gynerioides Phil. Field studies were conducted in 1995. 1996 and early 1997. Stipa clarazii and S. tenuis are two important palatable perennial tussock grasses in temperate, semiarid rangelands of central Argentina. where S. gynerioides is one of the most abundant, unpalatable perennial grass species. We hypothesized that (1) root growth is reduced after defoliation at any phenological stage in S. clarazii and S. tenuis in comparison to undefoliated controls, (2) root growth. and root appearance and disappearance in all three species decrease as plant water stress increases, and (3) root growth associated with water stress in S. clarazii and S. tenuis is reduced comparatively less when plants are water-stressed earlier than later, or for a longer period of time during the growing season. Our results led us to reject hypothesis 1 and to accept hypotheses 2 and 3. Maintenance of root growth after defoliation in S. clarazii and S. tenuis would allow these species a greater soil exploration and resource finding to sustain regrowth in their native, semiarid environments. Key words: Root growth, appearance and disappearance, perennial grasses, water stress, defoliation, Stipa species