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

MATERIALS AND METHODS

RESULTS AND DISCUSSION

No significant differences were found between the biostimulant-treated trees and the control ones, regarding trunk diameter and shoot growth (Table 1), thus confirming findings by other authors who did not find appreciable increase in plant growth from biostimulant applications, either applied on container-grown plants or after transplanting (Laiche 1991, Kelting et al. 1997, 1998a).

In year 2 (2000), the biostimulant treatments positively affected leaf area and, to a limited extent, leaf dry weight, while no differences were detected regarding SLW and dry/fresh weight ratio (Table 2). Plant response to treatments in terms of leaf area was similar in both years of observation.

Table 3 shows that leaf gas exchanges (cumulative data of four sampling dates in both years) were higher in trees treated with the biostimulants in both years.

Water-use efficiency (WUE), calculated by dividing Pn by evaporation (E) rate measurements, showed that the treated trees also used water more efficiently than control ones, especially in the first year after planting. Therefore, based on gas exchange and water-use efficiency, treated English oak used water more efficiently while maintaining photosynthesis better than the controls. The role of the biostimulants is probably indirect and comes from their supposed influence on stimulating drought tolerance (Russo and Berlyn 1990). This can be of some advantage to the plants because it can contribute to the maintenance of a sufficient level of leaf gas exchange, which can be severely lowered in case of drought. Water deficit can also damage the photosynthetic apparatus because it dehydrates tissues, which after that respond more slowly to changes in environmental water status (Cormich 2000).

The chlorophyll content of the leaves, as shown in Table 4, was positively influenced by the treatments in both years, consistent with the treatment effects on leaf gas exchanges.

Leaf mineral analyses indicated that biostimulants increased N content, while no differences were found in the other mineral nutrients (Table. 5). These results were consistent with Garbaye and Churin (1996), who found a higher leaf nitrogen concentration in Tilia tomentosa trees inoculated with a mixed inoculum of mycorrhizal fungi, while the application of the single fungus species did not show any appreciable influence. The possible nutrient effect by the applied biostimulants is probably related to their content in humic acids, which positively affect water retention and act as a nutrient reserve because of their high exchange capacity and capability to form water-soluble complexes with metal ions, thus possibly enhancing the adsorption of some mineral nutrients by roots (Kelting 1997). A positive effect of products containing humic acids on nitrogen uptake has been shown by other authors (Tattini et al. 1991), who stated that they are especially positive at low concentration, while a negative effect can be observed at high rates of application. The same authors also stated that humic concentrations stimulated root development that probably represents the most important effect of humic acids on plant growth, even if some contrasting experience are reported in the literature (Kelting et al. 1998a). As a matter of fact, the higher chlorophyll content, leaf area, and leaf gas exchange found in our research resulted in a less than expected enhancement in shoot growth and stem diameter. We hypothesize that in the first two years after transplanting, the biomass partitioning is more to increase root growth than top growth, though, in the long term, once a vigorous growth resumes, it may also result in a greater top growth.

The results of this research support the utility of biostimulants application as a potential alternative to fertilizers to stimulate fine root and ectomycorrhizae development, above all where natural mycorrhizae are suppressed due to poor soil conditions and where little water is available, according to Smiley et al. (1997). These products seem to be more effective in pedoclimatic conditions than in the controlled environment of the nursery or where the plants are well treated after transplanting. However, since their effects were primarily on leaf gas exchange and leaf characteristics, further research is needed to fully demonstrate their advantages both in terms of plant growth and in economic terms. In fact, the cost versus benefits analysis has shown a high ratio for Root Grow WP (US$1.2/tree/year), while Mycobacter DP seemed to be too expensive for a large-scale treatment (US$7.15/tree/year).

Moreover, it has to be pointed out that, consistent with what was found in a study carried out on container-grown plants of Acer platanoides and Liquidambar styraciflua (Nicese and Ferrini, unpublished data), no differences emerged between the two products used. English oak generally forms ectomycorrhizae, and we expected that Mycobacter DP, made of a mix of these fungi, could be more effective. Thus, is difficult to make specific recommendations on the choice of the best biostimulant product for Quercus robur in terms of growth response to the treatment in our conditions.