Variations in tension wood characteristics of Populus alba under alternate bending, nitrogen fertilization, and gibberellin treatments

Reaction wood formation (tension wood) in trees such as poplar is a response to stress and environmental factors. Tension wood is a rich source of cellulose that can be used for products including paper or biofuels and is thus a target product in forestry. This study aimed to evaluate the formation of tension wood in two-year-old saplings of Populus alba by using alternate bending, nitrogen fertilization, and gibberellin hormone. Saplings were bent alternately in one or another direction every month during the growing season, fertilized twice at the beginning and in the middle of the growing season, and treated with gibberellin early in the growing season. The physical and anatomical characteristics of the wood were studied after the end of the growing season. Evaluation of transverse sections of specimens stained with safranin/Astra-blue showed that, compared with straight saplings, alternate bending saplings had a wider tension wood area in the growth ring and clear formation of a gelatinous layer. The wood of alternate bending saplings with nitrogen fertilization and gibberellin hormone had a higher wood density, greater longitudinal shrinkage, and less radial and tangential shrinkage than saplings with other treatments. Moreover, the alternate bending saplings treated with nitrogen fertilization and gibberellin hormone had tension wood with the largest vessels, the lowest vessel density, and the smallest total vessel lumen area than saplings with other treatments. Wood fibers of treated saplings also had the thickest wall with the smallest fiber and lumen diameters. Overall, the bending treatment with the addition of nitrogen fertilization and gibberellin hormone was the most effective for the stimulation of tension wood formation in terms of volume and intensity.