Container grown 5-year-old chestnut trees were used to evaluate the effect of Paclobutrazol (PBZ) treatment on anatomical and physiological activity.
PBZ was applied to the soil at the end of June using 1 and 2 g/tree, or as a foliar spray (0.5 g/l), repeating the treatment 3 times, with a 2-week interval.
Three years after the PBZ-treatments, the size of foliar-treated trees was not different in comparison to the control.
The effect of soil-treatment at 1 g/tree and especially at 2 g/tree was effective in reducing tree growth and total leaf area per tree.
The 2 g/tree soil treatment increased the thickness of the leaves.
In the spongy parenchyma the thickness was greater because of more cell layers, and the volume of its intercellular spaces was reduced.
The 1 g/tree soil treatment and foliar applications did not significantly influence leaf development.
PBZ application, and above all the soil treatment of 2 g/tree, greatly decreased primary xylem development, the thickness of the annual rings of the stem and the number and diameter of the vessels.
PBZ also reduced secondary phloem development.
The PBZ applications greatly inhibited root development, especially when the 2 g/tree soil-treatment was used.
Also in the roots, PBZ treatment reduced the number and diameter of the vessels.
In treated chestnuts, total nitrogen and carbohydrate contents, leaf photosynthetic rate, dark respiration and transpiration rate were similar to those of the untreated trees.
In the 2 g/tree soil-treatment, chlorophyll and carotenoid contents and specific leaf weight were significantly higher than the control.
In conclusion, the foliar-treatment (0.5 g/tree repeated 3 times) was able to control chestnut growth only during the first year.
A single PBZ soil-treatment at 2 g/tree caused excessive reduction in tree growth and total leaf surface area, not compensated for by an increase in photosynthetic rate, with negative consequences on the skeletal formation and productive performance of the tree.
A single soil-treatment of 1 g/tree could be a useful means to control the vigor of young chestnuts.