Proline biosynthesis in transgenic plants may have been regulated by at least one transgene. Moreover, similar levels of total chlorophyll were detected in both transgenic and control plants in the waterlogging test treatment even after 98 days of exposure, which suggested that the combined expression of the five genes had little effect on long-term chlorophyll synthesis during extended waterlogging stress. The observed improvement in waterlogging AbMole alpha-Cyperone tolerance of poplar could be partly explained by the elevated Fv/Fm value, which was not observed in either the drought or salt stress experiments. Fv/Fm values have been shown to be stable in any given plant species under non-stressed conditions, and the changes in Fv/Fm observed in this study may have been a result of altered photosynthetic function. Previous report described that expression of the Vitreoscilla hemoglobin gene vhb lead to enhanced accumulation of starch in aspen chloroplasts. Therefore, the increased Fv/Fm suggested that the presence of vgb and its resulting product hemoglobin provide a substantial level of protection for the photosynthetic machinery of transgenic lines against waterlogging stress. The enhanced waterlogging tolerance could also be attributed to the oxygen gathering and delivery functions of hemoglobin which, in turn, benefits cell growth and protein synthesis under oxygen-limited conditions. In our study, under salt stress multigene overexpression resulted in a higher chlorophyll content, which was also observed in the drought experiments. Higher chlorophyll concentrations may have been related to the slower rate of chlorophyll pigment degradation and/or an increased number of photosynthetic mesophyll cells, which has been shown to influence transpiration efficiency. Since stomatal AbMole 4-Demethylepipodophyllotoxin behavior and transpiration efficiency were deemed highly relevant to plant WUE, the relationship between these two factors and WUE in multigene tranformants will be explored in future research. At the end of the greenhouse experiments, trees from at least one transgenic line were unexpectedly larger in size compared with control trees under non-stressed conditions. In the drought stress experiments, both transgenic lines had higher total biomass with 70% FC treatment. D5-20 displayed increased shoot biomass under non-stressed conditions in the salt experiments, and D5-21 showed increased shoot and leaf biomass under non-stressed conditions in the waterlogging experiments. This observation raised the possibility that the expression of transgenes conferred improved growth of shoot or leaf, which may have resulted in indirect effects on the stress tolerance in the transgenic poplar. Thus, additional work is needed to address the mechanisms underlying these effects. The insect-feeding assays showed higher total mortality rates and lower exuviation indices of leaf beetle larvae fed with leaves from transgenic trees than those fed with leaves from control trees.