EFFECTS OF DEFOLIATING STIPA-TENUIS AND PIPTOCHAETIUM NAPOSTAENSE AT DIFFERENT PHENOLOGICAL STAGES - AXILLARY BUD VIABILITY AND GROWTH

The size and number of axillary buds at various degrees of respiratory activity, and their relationship to the re-establishment of photosynt hetic surface area after a single annual defoliation during different phenological stages were determined for two perennial tussock grasses, Stipa tenuis and Piptochaetium napostaense, under rain-fed conditions . Treatments were repeated annually on the same plants and their respo nses evaluated during 1991-1993. Bud respiratory activity was examined using the tetrazolium test and the vital stain Evans' blue which allo wed separation of the bud pool at any stem base into three categories of bud viability: metabolically active, dormant and/or dead. At the en d of the second season of successive defoliations, plants of both spec ies that had defoliated late or after internode elongation had a great er number of respiratorily inactive buds than undefoliated plants or p lants defoliated at the vegetative stage. This must have contributed t o the lower production of dry matter after clipping on later rather th an earlier defoliated tussocks. The effects of late defoliations on bu d metabolic activity, however, were only transient. At the beginning o f the growing season following two consecutive years of treatments, th e proportion of previous-year stem bases which had produced green till ers, and the number of metabolically active axillary buds and green ti llers of similar height plus total green leaf length and number, and s ubsequent tiller number and dry weight per plant, were similar under a il defoliation treatments. Even when stem bases of these species had o nly two to three axillary buds, the large number of tillers that can b e produced per stem base (up to 20 daughter tillers) must help these s pecies to rapidly re-establish a photosynthetic canopy after defoliati on and allow them to persist in communities which have been exposed to overgrazing for several decades. (C) 1997 Academic Press Limited.