| Populus euphratica is an ecological pioneer and a dominant tree of riparian floodplain in semi-arid environments, the success of seedling establishment is challenged by the ability of their roots to maintain contact with the water table depth and associated capillary fringe. However, the response of the root systems of riparian plants to hydrological fluctuations at the seedling stage has not been fully elucidated. Investigation of tree roots is laborious and time consuming. As soil heterogeneity and the difficulty in extracting integrity tree root systems without damaging them, seedlings root system structure and development are difficult to assess and are poorly understood in field environments. For this reason, the study of root topology and function in response to environmental fluctuations has been lagged behind the study of aboveground. Water table depths and sediment types are the most important environmental factors. Many studies have investigated the individual effects of water table depths or sediment types on the performance of wetland plants, but few have examined the combined influence of these processes. We conducted an experiment in which P. euphratica seedlings, grown in both clay and clay/sandy sediments, were subjected to a range of hydrological conditions over a period of 75 days. We compared the root traits, biomass allocation and growth of P. euphratica. We hypothesized that adaptive phenotypic plasticity is likely to develop or be advantageous in seedlings of this species to allow them to adapt desert floodplain environments. Topological analysis was carried out to investigate branching patterns as basic determinants of root architecture. The main results were as following:1. P. euphratica seedlings growth characteristicsBoth the plant height and diameter cumulative growth curve of P. euphratica seedlings under both sediment types in the whole growth stage presented a ―S‖ curve and showed a same trend: substantial increase at the begaining, linear decrease after that, then increase slightly. Water table depths have no significant effects on internode.In both sediment types, inundation significantly reduced seedling biomass partitioning to the roots at the end of the experiment(p < 0.05), and the seedlings subjected to inundation therefore showed a significantly lower root/shoot ratio than the other three treatments. In particular, the seedlings subjected to the inundation treatment allocated less biomass to the roots than to the shoots in clay sediment(R/S = 0.45) and in clay/sand sediment(R/S = 0.37), R/S ratio in both sediment types are less than 0.5, while the seedlings subjected to the remaining three treatments concentrated more resources in the root system(root/shoot >1)(R/S ratio: 30 cm < 50 cm < 70 cm). In both sediment types, the R/S ratio increased as water table depth decreased, which consistent with optimal partitioning theory.2. Root growth characteristics of P. euphratica seedlingsSediments types had no effects on all root traits we measured in the same water table depth(p < 0.05). Inundation significantly reduced seedling biomass partitioning to the roots and the relative root biomass allocation increased as water table depth decreased(p < 0.05). Relative biomass of root in clay sediment and clay/sand sediment were 31.43% and 26.28%, respectively. While the relative biomass of root subjected to the remaining three treatments were more than 50%(relative biomass of root: 30 cm < 50 cm < 70 cm), consistent with optimal partitioning theory. The tap root depths were all around its water table depths, which was a trade-off in growth process.3. Root topological characte ristics of P. euphratica seedlings responsed todifferent treatmentsThe value of qa and qb were significant different at the same water table depth,(p < 0.05). Water table depth had no effects on root topological structure of P. euphratica seedlings(p < 0.05), which was inconsistent with our hypotheses 1.Sediment types had significant effects on P. euphratica seedlings root topological structure(p < 0.05), which was consistent with our hypotheses 2.In contrast with the 50 cm and 70 cm water table depth(TI > 0.7), topological index(TI) of inundation and 30 cm water table depth are relatively small(TI < 0.7). The TI increased as water table depth decreased, which means the root topological structure changes from dichotomous to herringbone branching.Inundation had the biggest TI in clay/sand sediment(TI=0.733), TI for the treatments were all less than 0.7, TI increased as water table depth decreased, which meant the root topological structure changes from herringbone to dichotomous branching. And this trend was comsistent with our hypothesis 2(there were some significantly differences of root topological plasticity in contrasting sediment types), which meant that P. euphratica seedlings root had different adaptive strategies under same water table depth.4. Root plasticity adaption of P. euphratica seedlingsGrowth was significantly reduced by inundation. However, rather than following relatively fixed trait and allocation patterns, the seedlings displayed adaptive mechanisms involving the development of adventitious roots to enhance plant stability and oxygen obtainment, together with a lower proportion of root biomass. At the whole-plant level, at deeper water table depths, seedlings allocated more biomass to the roots. And total root length increased with decreasing water table depths, regardless of the sediment, which was consistent with optimal partitioning theory. The sediment type had a significant effect on seedling root traits. The root traits of P. euphratica was very different in different sediment types under the same hydrological conditions, showing much greater first-order root number in clay sediment under shallower water table conditions, whereas rooting depth was greater in clay/river sand sediment under deep water table conditions. In clay sediment, seedlings responded to lower water availability via greater root elongation, while the root surface area was increased through increasing the total root length in clay/river sand sediment, suggesting that seedlings facing deeper water tables were not always likely to increase their root surface area to obtain more water. Our results indicated that P. euphratica seedlings were able to adapt to a range of water table conditions through plastic responses in root traits and biomass allocation, consistent with our hypotheses 3. |
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