Xylem abscisic acid accelerates leaf abscission by modulating polyamine and ethylene synthesis in water-stressed intact poplar

We investigated the effects of endogenous and exogenous abscisic acid (ABA) on polyamines (PAs) and ethylene synthesis and their relevance to leaf senescence in 1-year-old intact cuttings of a drought-sensitive poplar genotype Populus 2 euramericana cv. I-214 (cv. Italica) and a drought-tolerant genotype P. popularis 35-44 (P. popularis). P. popularis exhibited a transitory and moderate increase in xylem ABA concentrations in response to water stress and no leaf abscission during the period of drought (30% of field capacity; the soil water potential was -2.108 MPa). In contrast, leaf shedding occurred in stressed cv. Italica following a sustained increase in xylem ABA concentrations (up to 1.73 µM). Application of ABA to the transpiration stream accelerated leaf abscission in both genotypes with a threshold of approximately 1.7-1.8 µM. Initial elevation of xylem ABA concentrations reduced PA levels but enhanced ethylene synthesis simultaneously at the onset of water stress or the ABA treatment. Moreover, sharp increases in ABA concentrations, i.e. over 1.7 µM, severely restricted PA synthesis in aged leaves in the longer term (within 3 days). Leaves abscised following a progressive decline of PA levels, when putrescine decreased below approximately 0.5 µmol g-1 fresh weight, and spermidine and spermine became almost undetectable by HPLC. Therefore, we concluded that a drastic reduction in the level of PAs might increase the sensitivity of the leaf to ethylene, thus accelerating defoliation, even though the ethylene emission of stressed plants returned to pre-stress values by day 3. Young leaves were not shed during the period corresponding to the increase in xylem ABA, which appeared to result from the lesser reductions of PA. Compared with P. popularis, the inhibitory effect of ABA on PA synthesis was more pronounced in the drought-sensitive genotype cv. Italica. On the other hand, cv. Italica plants typically produced more ethylene than P. popularis. Taken together, these observations might explain our previous finding that ABA-treated plants of cv. Italica experienced more leaf loss than P. popularis (Chen et al. 1997).