Sodium and calcium uptake, transport and allocation in Populus euphratica and Populus x canescens in response to salinity

The salinization of soils caused by increasing NaCl concentrations is a major problem for today’s agriculture. Since P. euphratica is able to grow on sodic soils, it has been used for reforestation in saline areas. Because of this attribute there is a strong interest in elucidating this plants´ mechanisms and strategies to cope with high external NaCl concentrations. Therefore, the main objectives of this study were to analyse the uptake, transport and allocation of Na+ in P. euphratica and P. x canescens. This study also addresses the question whether Na+ recirculation in poplar takes place. Since the maintenance of Ca2+ transport under salinity was proposed to be decisive for NaCl tolerance of plants, a further aim of this work was to investigate Ca2+ uptake and Ca2+ distribution in poplar. The influence of salinity on the performance of P. euphratica and the NaCl sensitive P. x canescens was analysed. Both poplar species were exposed to up to 150 mM NaCl and growth measurements were carried out. P. x canescens was very sensitive and died at 150 mM NaCl whereas P. euphratica showed growth reduction but no significant leaf injury. Element analysis of old and young leaves revealed no significant differences between old leaves of P. x canescens and P. euphratica, but a higher Na+ accumulation in new leaves of P. x canescens than in those of P. euphratica. This suggests that P. euphratica is able to protect young, developing leaves against excess Na+. Long term exposure to NaCl levels leading to death of P. x canescens resulted in P. euphratica in morphological adaptation: roots of P. euphratica showed thickenings with incr! eased root diameter. This increase was caused by an increased number of cell layers in the cortex and not by an increased cell volume. Since Ca2+ deficiency and Cl--ions could be eliminated as the cause of this root swelling, and P. x canescens showed no thickening of roots under the same NaCl regime; thus the induction of “cob-like” thickening was concluded to be a Na+ ion- and species-specific adaptation in P. euphratica roots. To determine the function of cob roots for Na+ uptake under salinity, Na+ influx in adapted and non-adapted root tips of P. euphratica was measured using radioactive labelling. A decreased Na+ influx in adapted roots in relation to the fresh mass of the roots was measured, indicating that cob roots decrease Na+ accumulation of P. euphratica. Because cob roots of P. euphratica decreased Na+ uptake, their function for NaCl tolerance was tested. P. euphratica plants were adapted to high Na+ concentrations, subsequently grown in Na+-free hydroponic solution to remove excess sodium and then exposed to NaCl shock. Measurements of radioactively labelled Na+ uptake during the NaCl shock treatment showed an increased uptake in non-adapted plants compared to adapted plants. Chlorophyll fluorescence measurements and the plants performance showed an increased NaCl toler! ance in pre-treated plants. These results demonstrate that cob roots function in increasing NaCl tolerance of P. euphratica. Because of the differences in salt sensitivity, P. x canescens and P. euphratica plants were exposed to maximum external NaCl concentrations which they could withstand for longer periods of time. To determine Na+ uptake, Na+ release and Na+ xylem transport rates under salinity, split root experiments were conducted for both species, in which radioactive labelling was added to one root part and Na+ translocation to the other parts was determined during a chase period. Na+ uptake was 3-fold higher in P. x canescens than in P. euphratica, even though P. euphratica was exposed to 2-fold higher external NaCl concentrations than P. x canescens. The higher uptake of Na+ resulted in a 2-fold enhanced Na+ xylem transport per wood area in P. x canescens than in P. euphratica. Furthermore, both poplars differ in their strategy of Na+ retention and Na+ release. Analysis of Na+ allocation by imaging of the 22Na+ distribution revealed that P. euphratica retained Na+ in roots. In contrast, the 22Na+ label was present only in a small part of the coarse roots of P. x canescens. However, P. x canescens released a main fraction of incorporated Na+ into a non labelled root compartment, indicating Na+ recirculation. These results also demonstrate an increased Na+ accumulation and Na+ binding to roots of P. euphratica, suggesting that roots may function as a Na+ storage tissue. To investigate Na+ phloem transport, leaf feeding experiments were conducted using radioactive 22Na+. Since 22Na+ was detected in the shoot and in phloem sap collected below the feeding leaf, Na+ recirculation via the phloem took place in both poplars. Na+ phloem transport rates per bark area were 3-fold higher in P. euphratica than in P. x canescens under salinity. In P. x canescens phloem transport decreased under salinity and this was accompanied by decreased transcript levels of HKT1;1 in bark tissue of P. x canescens. This suggests that HKT1;1 plays a role in Na+ phloem transport in bark tissue of poplar. The maintenance of Ca2+ shoot transport is crucial for plants under salinity. Therefore Ca2+ uptake and Ca2+ distribution were analysed in both poplar species. Autoradiograms of both poplar species were used to image the distribution of radioactive Ca2+. Neither P. x canescens nor P. euphratica transported previously incorporated Ca2+ into new leaves formed during subsequent exposure to salinity. When the radioactively labelled 45Ca2+ was added during NaCl exposure, a diminished Ca2+ uptake was detected. However, this Ca2+ was preferentially allocated to new leaves in P. euphratica, indicating that this species adapted its pattern of Ca2+ distribution, to sustain Ca2+ nutrition to the developing organs. In conclusion, the present study shows multiple differences in Na+ uptake and Na+ distribution between P. euphratica and P. x canescens under salinity. It was shown that P. euphratica had a higher Na+ exclusion and a lower Na+ xylem transport rate than P. x canescens pointing out that Na+ xylem loading is a major control point for Na+ transport in poplar. Further experiments are required to elucidate the molecular basis of these differences because Na+ xylem loading is a major reason for differences in NaCl tolerance in the two poplar species.