Effects of defoliatingStipa tenuisandPiptochaetium napostaenseat 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 photosynthetic surface area after a single annual defoliation during different phenological stages were determined for two perennial tussock grasses,Stipa tenuisandPiptochaetium napostaense, under rain-fed conditions. Treatments were repeated annually on the same plants and their responses evaluated during 1991–1993. Bud respiratory activity was examined using the tetrazolium test and the vital stain Evansʹ blue which allowed separation of the bud pool at any stem base into three categories of bud viability: metabolically active, dormant and/or dead. At the end of the second season of successive defoliations, plants of both species that had defoliated late or after internode elongation had a greater number of respiratorily inactive buds than undefoliated plants or plants defoliated at the vegetative stage. This must have contributed to the lower production of dry matter after clipping on later rather than earlier defoliated tussocks. The effects of late defoliations on bud metabolic activity, however, were only transient. At the beginning of the growing season following two consecutive years of treatments, the proportion of previous-year stem bases which had produced green tillers, and the number of metabolically active axillary buds and green tillers of similar height plus total green leaf length and number, and subsequent tiller number and dry weight per plant, were similar under all defoliation treatments. Even when stem bases of these species had only two to three axillary buds, the large number of tillers that can be produced per stem base (up to 20 daughter tillers) must help these species to rapidly re-establish a photosynthetic canopy after defoliation and allow them to persist in communities which have been exposed to overgrazing for several decades.