Effects of biostimulants on ornamental plants grown in solid soil less cultural systems : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

With raised public awareness of potential negative effects caused by synthetic fertilisers and pesticides, there is interest amongst nursery growers for environmental friendly ways to improve plant performance. Application of plant biostimulants is one potential approach to increase plant growth with little environmental impact. The present study aimed to investigate the effectiveness of biostimulants on ornamental plants under solid soil less cultural systems, and to develop a methodology to effectively apply biostimulants that would be practicable in the ornamental nursery industry. A preliminary experiment showed variable responses to a range of recognised biostimulant products. The effectiveness of biostimulants on plants were then systematically studied to evaluate application rates, application methods, chemical nutrient status of growth media and plant species variability, with a focus on three main categories of biostimulants: humic acids (HA), protein hydrolysates (PH) and seaweed extract (SW). Optimal application rates in potting mix were 1.5 g L 1 for HA, 0.24 ml L 1 for PH and 0.25 g L 1 for SW, about half of the recommended application rates to field grown plants. Drenching was found to be the best application method. Complex interactions were found between biostimulant effects and nutrient status of the growth medium. Seaweed extracts had a better growth--stimulating effect than other biostimulants. However, the effects of all biostimulants differed greatly between plant species, and the results indicated that single dose and common prescriptive applications are unlikely to be suitable for different species of plants. In vitro assays were use to standardize variables, to investigate the combined effects of biostimulants and to evaluate the potential of plant extracts as novel biostimulants. Two combinations (0.001%HA+0.001%SW and 0.0001%HA+0.005%PH) showed positive effects, but most biostimulant combinations led to detrimental effects on plant growth. It was also found that optimal application rates under in vitro conditions are about five times less than under solid soil--less cultural conditions for SW and PH, but 150 times less for HA, indicating particular sensitivity to the latter. Although biostimulants showed positive effects in pot experiments, the effects under in vitro conditions were only marginal. Complex interactions between characteristics of the growth medium and the effects of biostimulants were observed, which suggested an association with microbial activity in the growth medium. Developing these findings, the final experiment using Griselinia littoralis was designed to test the two most promising combinations with additional amendments of vermicompost (VC), to modify the characteristics of the growth medium (based on optimal rates and methods from previous studies). Combined use of HA (0.1 g L--1) and PH (0.24 ml L--1) resulted in 58% and 77% increases in shoot and root biomass respectively. Shoot biomass was further enhanced up to 69% when using a combination of HA (0.5 g L--1), PH (0.24 ml L--1) and VC (20%), despite a decrease in root biomass. Of the 31 combinations, 27 did not significantly increase plant yield. However, substantial interactions were evident between the three biostimulants and vermicompost that may be additive, synergistic or detrimental. The results are interpreted to conclude that it is feasible to apply biostimulants alone or in combination to ornamental plants under solid soil--less cultural systems to achieve major improvements of growth rates and yield. However, biostimulants need to be applied with caution since detrimental effects were commonly observed when the application rates are out of optimal ranges, particular for humic acids. It is recommended that small-scale trials are always carried out prior to large-scale application of biostimulants to particular species and under specific nursery conditions.