EFFECTS OF DEFOLIATING STIPA-TENUIS AND PIPTOCHAETIUM NAPOSTAENSE AT DIFFERENT PHENOLOGICAL STAGES - ROOT-GROWTH

During 1991-1993, root growth after a single annual defoliation at dif ferent phenological stages (vegetative or during early, late or post-i nternode elongation) to 5-7 cm stubble height was determined on field- grown plants of Stipa tenuis and Piptochaetium napostaense, two import ant forage grasses in the semi-arid, temperate rangelands of Central A rgentina. Nondefoliated plants served as controls. Root growth was est imated by root length density measurements obtained through destructiv e harvesting, and by using a root periscope to monitor changes in the number of roots intercepting glass tubes buried in the soil. The root periscope allowed registration of the angle of each root interception, which made it possible to measure the root system dynamics (root appe arance and disappearance). The effects of defoliating S. tenuis and P. napostaense at different phenological stages on root growth varied wi th the methodology used. Root growth of both species was not reduced b y defoliation when it was estimated from root length density determina tions. However, it:was lower (p < 0.05) on plants which were defoliate d late compared to those not defoliated during the growing season when it was estimated using the root periscope. Lack of agreement in the r esults obtained by the two methods could be associated with various pr oblems inherent in the use of the root periscope. This instrument was, however, useful in providing a rapid evaluation of root system dynami cs (through root appearance and disappearance) in both species. A grea ter cumulative root appearance and disappearance, for example, occurre d after plants were defoliated early rather than late in the growing s eason. This may have contributed, at least in part, to the overcompens ation in forage production which was observed on plants defoliated at the vegetative stage during 2 consecutive years of study. (C) 1997 Aca demic Press Limited.