Dual allelopathic effects of subtropical slash pine (Pinus elliottii Engelm.) needles: Leads for using a large biomass reservoir

Pinus elliottii Engelm. (slash pine) is distributed along the maritime coast of Southern Brazil, where it shows invasive pattern and typical allelopathic features. Large quantities of needle litter are produced by pine trees, a biomass that is little explored in areas where this species is alien. Little is known about the dynamics of needle and litter phytochemical interactions, particularly in subtropical environments. To elucidate the full range of needle and litter allelopathic potential, the effects of litter (superficial and deep) and seasonally harvested fresh slash pine needles stored for different times were evaluated against lettuce, tomato and cucumber seeds and seedlings. Increasing concentrations (0%, 1%, 2%, 4%, and 8% w/v) of hot and cold aqueous extracts of needles and litter affected in different ways target plant development. Growth and germination inhibition were directly related to the highest extract concentrations (regardless of the season and mainly in hot water extracts) of needles. On the other hand, stimulatory effects of litter extracts on lettuce growth were observed. Growth and germination of cucumber and tomato were not affected by pine litter as substrate when compared to rice husk. The presumable high polarity and thermal stability of slash pine leaf biomass allelochemicals and their transient toxic effect or growth promoting impact suggest potential applications of this largely available biomass both as a biological herbicide and growth substrate in plant propagation.