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

Tiller demography and growth were determined for defoliated and nondef oliated plants of Stipa tenuis and Piptochaetium napostaense under nat ural field conditions in 1991 and 1992. Defoliated plants were clipped once a year to a stubble height of 5-7 cm at one of the following phe nological stages: vegetative (V), or during early (E), late (L) or pos t (P) internode elongation. Defoliation at the different phenological stages hastened tiller senescence in 1991 but not in 1992, which might be associated with lower precipitation during the first year. Rates o f leaf production for both species during late autumn-early winter wer e greater after plants were defoliated at V and/or E than for nondefol iated controls in 1992. In addition, tillers of S. tenuis and P. napos taense exhibited greater relative growth rates for height or green lea f blade, stem plus sheath and total lengths after they had been defoli ated at V and/or E than when not defoliated or defoliated later in the growing season of 1991 and 1992. During 1991, total dry matter produc tion was similar in all defoliation treatments for both species. In 19 92, however,defoliation at the V stage was earlier than in 1991 and al l active meristems remained on both species after defoliation. This, a nd the relative growth rate increments observed for these plants contr ibuted to their overcompensation of tissue lost by defoliation, and th us a greater amount of dry matter was produced than in the nondefoliat ed controls. At the same time, the lowest dry matter production among all treatments was observed after plants of both species were defoliat ed late during internode elongation. Removal of all active meristems, the occurrence of low soil water potentials at this time, the existenc e of a greater number of metabolically inactive buds on these plants, and the lower time available for regrowth must have contributed to thi s response. (C) 1997 Academic Press Limited.