The effect of biostimulants on plant performance: an Australian urban horticultural perspective

Urban greening provides a range of ecosystem services that benefit human well-being. Although the benefits of urban greening are clear, they are often difficult to achieve due to urban areas being challenging environments for plants to survive and thrive. To overcome these challenging conditions, a range of management strategies can be used, including the application of biostimulants. Biostimulant is any substance or microorganism applied to plants to improve plant growth. To date, the majority of biostimulant research has been on edible crops, such as fruits and vegetables. In general, this research has found that biostimulant application enhances plant performance. However, this apparent pattern is yet to be confirmed by an extensive systematic review of the biostimulant literature. To address this knowledge gap, I performed a meta-analysis on the biostimulant literature published between 2012 to 2018 (chapter 2). The meta-analysis revealed that biostimulant application significantly increased plant growth, with biostimulant type, application method and growth form influencing this response. From the meta-analysis, it was clear that biostimulant research focusing on urban horticultural species is currently lacking. The first step in addressing this knowledge gap was to understand how horticultural practitioners are using biostimulants in Australia. To do this, I designed a questionnaire that was sent to a wide variety of horticultural practitioners across Australia (chapter 3). From the 44 qualified responses, I found that 82% of the practitioners had used biostimulants in the past and 74% of them were satisfied with the outcomes. Further, 89% of practitioners stated that they were interested in seeing more biostimulant-focused research being conducted. With this in mind, I conducted two manipulative experiments that tested the effect of biostimulants on mitigating the impacts of abiotic stresses that urban horticulture 26 species are commonly exposed to, namely low water availability (chapter 4) and root pruning pre-transplanting (chapter 5). Both experiments had a glasshouse component, while the root pruning experiment also had a field component. In both experiments, I found biostimulant application did not affect plant performance (growth and photosynthesis), irrespective of stress treatment. The inconsistent results between these experiments and the meta-analysis suggest that biostimulants do not universally improve plant performance and/or there is a publication bias in the biostimulant literature. The findings of the thesis have highlighted that we still know relatively little about the potential benefits of biostimulant use in an urban horticultural context. Therefore, it is crucial that future research efforts are directed to this research area in order to fill an important knowledge gap.