Response of English Oak (Quercus Robur L.) Trees to Biostimulants Application in the Urban Environment

The influence of two biostimulants on leaf gas exchanges and leaf characteristics of English oak (Quercus robur L.) was evaluated in a urban environment over two growing seasons. Trunk diameter and shoot growth were also monitored. Treatments included 1) control (no biostimulants); 2) Root Grow WP, a commercial hydrosoluble powder made of a mix of humic acids, marine algae, Yucca extracts, vitamins, amino acids, and nitrogen-fixing bacteria; and 3) Mycobacter DP, a commercial granular mix of endo- and ectomycorrhizal fungi, biostimulants bacteria, humic acids, carbohydrates, vitamins, acrylamide, Yucca extracts, and marine algae. No statistical differences were found on trunk diameter increase and shoot growth. Photosynthesis, evaporation rate, and water-use efficiency were higher in plants treated with biostimulants than control plants on every observation date. No differences were found regarding leaf fresh/dry weight ratio and specific leaf weight, while chlorophyll content, leaf area, and leaf dry weight were higher in the treated plants, especially in the second year after planting. Nitrogen concentration in leaf tissue was higher in the treated trees, while the content of the other mineral nutrients was not affected by the products. No differences emerged between the two biostimulants used. Though this study supports the utility of biostimulants application as an aid to promote plant establishment in adverse sites, especially in terms of plant physiology, it is difficult to make specific recommendations on their application and on the choice of the best biostimulant product for Quercus robur in our conditions.