Reproductive biology and pollination ecology of Feijoa Sellowiana

Abstract

The usefulness of pollinator syndromes as a predictor of pollination requirements, the factors that influence the effectiveness of pollinator and what determines fruit set and size were investigated in Feijoa sellowiana (Myrtaceae). Feijoa are grown commercially on a limited scale in New Zealand but development of the industry is hindered by low fruit set and a high degree of variability in size and shape of fruit. The plant was introduced from South America but little was known abut its reproductive biology or its pollination requirements and the development of current varieties has been largely by trial and error. This study aimed to redress this lack of knowledge. Investigation of floral architecture along with the use of controlled pollinations showed that feijoa flowers are hermaphroditic but exhibit spatial separation (herkogamy) and partial temporal separation (dichogamy) of female and male functions. Flowers are female first (protogynous) and receptivity declines soon after pollen dehiscence. As expected, outcrossed flowers are more likely to set fruit. Listing floral characteristics according to pollination syndromes led to the suggestion that wind was an unlikely pollen vector and that birds are more likely vectors than insects. The flowers are nectarless but offer two rewards in the form of pollen and large sugary petals. A wide range of insects and birds visit flowers and controlled pollination and the exclosure of trees were used to confirm the ineffectiveness of wind. Possession of sugary ,petals appears to be a unique animal attractant. Analysis of relative sugar levels in petals at different developmental stages showed that sugar levels are lowest in the bud stage but increase rapidly to peak at the time of pollen dehiscence when the flower is functionally both female and male. The sugar levels attained are comparable to those for nectar of bird visited flowers. A series of visual cues are available for birds to associate with reward differences. Birds do visit flowers and consume petals and petals were mostly eaten when sugar levels were highest but this depended on bird density. Moreover, sufficient petals were left after any visit to ensure revisitation. The effectiveness of the different floral visitors as pollinators was investigated by determining the number of pollen grains deposited and the time of deposition relative to receptivity. The behaviour of visitors at flowers, their, movements between flowers and trees and the use of partial exclosures were used to understand differences in effectiveness. Only large birds such as blackbirds (Turdus merula) and mynas (Acridotheres tristis) were effective pollinators of feijoa. Visits by insects and small birds resulted in the transferral of minimal pollen. Not only did large birds transfer more pollen per visit, they also moved larger distances and so were more likely to outcross flowers. Results of self and cross pollinations early in the study led to the questioning of the status of some of the common cultivars. "Triumph" and "Mammoth" appear to be of mixed genetic origins whereas "Apollo" and "Gemini" respond as clonal cultivars. Only 39.5% of flowers matured fruit even though most were receiving sufficient pollen. The source of pollen affected fruit set markedly. Cultivars and different individuals showed a range of compatibilities. Most did not set fruit when selfed, some did but the fruit were small while a minority set large selfed fruit. Cross pollinations resulted in a range of responses similar to selfing. Fruit set and the morphology of pollen tubes shoed that feijoa possesses a gametophytic self-incompatibility (SI) mechanism. Analysis of covariance was used to demonstrate that both these pre-zygotic effects and post=zygotic investment due to paternity have significant effects on fruit size. Current ideas of mechanisms affecting the quality of offspring are discussed. The need to consider paternity is stressed. Ratios of pollen tubes reaching the ovary, the presence or absence of the 10 different shapes of callose plugs along with the ratios of two groupings of plug shapes, and of the length of callose plugs are shown to predict independently the outcome of pollinations. These are proposed as quick methods for screening the likely complimentarity of a large number of individuals. Finally, results from all aspects of the study are used to suggest ways to improve both present management and future development of feijoa as a commercial crop.