Biostimulants do not affect the performance of urban plant species grown under drought stress

Urban areas often have low soil water availability due to their impervious surfaces reducing rainfall infiltration. These water-limited conditions may be exacerbated by the projected increases in drought events caused by climate change. As a result, plants that grow in urban areas are vulnerable to drought stress. There are a range of practices that can be used to help mitigate drought stress, including the use of biostimulants. This study aimed to determine whether biostimulant application (1) improves plant performance and (2) mitigates the drought stress on urban plant species. To address these aims, we selected six woody and three graminoid plant species that are commonly planted in Australian urban areas and exposed them to different watering (drought-stressed, well-watered) and biostimulant (control, humic acid, protein hydrolysate, seaweed extract) treatments. We then measured their assimilation rate, growth metrics and biomass allocation. We found that drought stress reduced the assimilation rates and shoot growth of the study species. However, this did not translate into a biomass reduction because the drought-stressed plants reallocated resources towards root biomass. We found no evidence to suggest biostimulant application mitigated the impacts of drought stress on plant performance. Further, the only effect biostimulant application had on plant performance irrespective of the watering treatment was that the seaweed biostimulant increased the plant height growth of the woody species. These results show that the biostimulants used in this study will have a limited effect on the performance of plant species commonly planted in Australian urban areas.