Graduate Thesis Or Dissertation
 

Abscisic acid levels in relation to stomatal behavior during drought and recovery in contrasting ecotypes of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)

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  • Foliage of Douglas-fir seedlings [Pseudotsuga menziesii (Mirb.) Franco] from two contrasting environments was sampled during progressive stages of two consecutive cycles of induced drought for leaf resistance, plant water potential , and abscisic acid content (ABA). One group of seedlings was placed in a controlled environment room while another group was kept outside at a nursery under natural conditions. During the first cycle of drought, the xeric and mesic ecotype seedlings maintained similar levels of transpiration. During a second cycle of drought, xeric ecotype seedlings from N.E. Washington, both in the growth room and nursery environments, delayed the initiation of stomatal closure until they reached greater levels of plant moisture stress than in the first drought cycle. There was no apparent change in the stonatal behavior of the mesic ecotype seedlings during a second cycle of drought. In both ecotypes, during the first drought cycle, a sharp increase in ABA content occurred simultaneous with the increase in leaf resistance within a narrow water potential range. However, after rewatering, high leaf resistances were not concurrent with high ABA content. In xeric ecotypes a threshold level of moisture stress again triggered an increase in ABA content, but leaf resistances remained low. The change in stomatal response induced by previous stress could not be explained by altered ABA levels. In mesic ecotypes in the growth room, ABA levels remained high after rewatering and slowly decreased with time, although recovery of pre-stress leaf resistance was mainly complete two days after rewatering. A rapid closure of stomata in these seedlings in response to a threshold level of moisture stress during a second drought cycle was not related to a corresponding change in ABA content.
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