Browsing by Author "Altman, A."

The effect of a 20-day NaCl treatment on shoot growth, transpiration, ion uptake and transport, and intracellular ion compartmentation was investigated in regenerated plants of Populus euphratica Oliv. and Populus tomentosa Carr. Plants watered with 100 mM NaCl for 8 days and then 200 mM NaCl for 12 days exhibited soil NaCl concentrations of 60 and 95 mM, respectively. Unit transpiration rates and relative growth rates of P. tomentosa were restricted more by salinity as compared with P. euphratica. Salinized P. tomentosa exhibited leaf necrosis whereas no damage was seen in stressed P. euphratica. Compared with P. tomentosa, P. euphratica had considerably lower rates of net root uptake and transport of salt ions (Na+ and Cl-) to the shoots under salinity. The relatively lower unit transpiration rates of P. euphratica and the lower salt concentrations in the xylem of salinized P. euphratica contribute to its greater capacity for salt exclusion. X-ray microanalysis showed that P. euphratica had a greater ability to restrict radial salt transport in roots by blocking apoplasmic salt transport and sequestering more Cl- in cortical vacuoles. In addition, P. euphratica maintained higher K+ uptake and transport than P. tomentosa in the presence of high external Na+ concentrations.

The uptake and transport of salt ions (Na+, Cl-), macronutrients (K+, Ca2+, Mg2+) and abscisic acid (ABA) response to increasing soil salinity were examined in 2-year-old seedlings of Populus euphratica and a hybrid, P. talassica Kom x (P. euphratica + Salix alba L.). Leaf burn symptoms appeared in the hybrid after 8 days of exposure to salinity when soil NaCl concentration increased to 206 mM, whereas P. euphratica exhibited leaf damage after day 21 when soil NaCl exceeded 354 mM. Leaf necrosis was the result of excess salt accumulation since the injury followed an abrupt increase of endogenous salt levels. Compared with the hybrid, P. euphratica exhibited a greater capacity to exclude salt ions from leaves under increasing salinity, especially Cl-. Salt treatment altered nutrient balance of the hybrid, leaf K+, Ca2+ and Mg2+ concentrations significantly declined and the same trends were observed in roots with the exception of K+. Although K+ levels decreased in salinised P. euphratica, increasing salinity did not affect the levels of Ca2+ and Mg2+ in leaves, but did increase the uptake of these nutrients when salt stress was initiated. NaCl-induced increase of ABA concentration in xylem sap [ABA] was observed in the two tested genotypes, however xylem [ABA] increased more rapidly in P. euphratica and a fivefold increase of xylem [ABA] was recorded after the first day of exposure to salt stress. Therefore, we conclude that the increase of Ca2+ uptake may be associated with the rise of ABA, and thus contributes to membrane integrity maintenance, which enables I! euphratica to regulate uptake and transport of salt ions under high levels of external salinity in the longer term.

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 x euramericana cv. I-214 (cv. Italica) and a drought-tolerant genotype R 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 muM). Application of ABA to the transpiration stream accelerated leaf abscission in both genotypes with a threshold of approximately 1.7-1.8 muM. 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 muM, 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 mumol 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 prestress 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 oil 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.