Competition‐dependent incompatibility in Phormium tenax: Does self‐fertilisation provide reproductive assurance?

Abstract

New Zealand flax (Phormium tenax) has a reproductive system in which the early abscission of selfed flowers is determined by the degree of competition with crossed fruits on the same plant. It has been suggested that this functions as a form of reproductive assurance, allowing for high levels of outcrossing when pollinators are abundant, while ensuring seedset when pollinators are scarce. We performed experimental crosses and mating system estimation in natural populations to determine whether P. tenax can set seed in the absence of pollinators, and how pure and mixed pollen loads influence reproductive success and population‐level selfing rates. Phormium tenax can set seed autonomously, although if other flowers on the plant are available for outcrossing, resources are preferentially allocated to maturing those fruits. Experimental crosses suggest that flowers pollinated with mixtures of selfed and outcrossed pollen can result in fruitset, although fruitset is reduced if flowers pollinated with mixed pollen loads compete with cross‐pollinated flowers. Despite the potential for geitonogamy in P. tenax, outcrossing rates from five populations showed that these populations were universally outcrossing (t = 0.83–1.06). In P. tenax, self‐compatibility in the absence of competition from outcrossed pollen can provide some reproductive assurance, but in large populations, selfing and mixed mating rarely occur.