The pollination biology of Papaw (Carica papaya L.) in Central Queensland

Project aims to evaluate the pollination biology of C. papaya in regards to anemophily, entomophily and agamospermy. The variation in seasonal fruit set typical of subtropical climates is also investigated. The research aims to improve crop management strategies.. Papaws are one of the most significant fruits in central Queensland, yet there are specific problems associated with consistent fruit production in this subtropical region. This is the first time that a comprehensive analysis into the pollination mechanism of papaw has been conducted, in a country other than that of its origin. Additionally, research into factors influencing fruit set and seed set under subtropical climates has been performed. Earlier studies on the pollination biology of C. papaya have suggested that pollen transfer is wind and/or insect mediated. Whilst papaws grown in Australia are believed to be wind-pollinated, a diverse number of insect species have been proposed to fulfil pollinator function in other areas of cultivation, which reflects that the pollination mechanism in papaw is unknown. Contrary to all previously published research in the area, pollination of C. papaya is carried out by hawkmoths (Lepidoptera: Sphingidae). Seven pollinator species and a further four suspected pollinator species have been identified for the central Queensland region. All species belong to the same subfamily, the Macroglossinae. Contrary to anecdotes from growers, neither native or European bees, nor wind was of significance in the pollination of dioecious papaw cultivars and incidences of apomictic and parthenocarpic fruit set occurred though were rare. Sphingid pollinators showed marked patterns of seasonal occurrence. Weekly observations and light trap results conducted over two years in the vicinity of Rockhampton, indicated that adult sphingids were absent from the central Queensland region during winter from the middle of June until the middle of August. Their absence correlated with relatively low average weekly minimum temperatures of 10.4 °C and below. Overwintering of sphingids in the pupa stage in order to overcome adverse climatic effects7 was observed for two species. Seasonally occurring fruit set and periods of low seed set of C. papaya, under the subtropical central Queensland climate were due to three key factors - the availability and viability of pollen and the absence of sphingid pollinators. The overall stagnation of tree growth including a decrease in open flower numbers during winter also played a role. Individual papaw lines (particularly Hybrid 29) showed better adaptations to heat and cold stress in respect to pollen quantity and viability. Seed set, irrespective of viability status, increaSea fruit size by an average of 0.89 g/seed. The pollination mechanism by which nectarless pistillate flowers attract pollinator visitation has been identified and involves a multitude of stimuli, including those of olfaction, gustation, tactility and vision. Petals of both flower types show the same visual properties, of absorbing wavelengths of the UV-spectrum (<405 nm), selectively reflecting only those of longer wavelengths. Additionally, pattern fanning trichomes attached to scent glands are present on both pistillate and staminate C. papaya flowers. Mechanosensory receptors on the sphingids probosces exactly match trichome patterns and the presence of contact chemoreceptors suggests their involvement for foodsource recognition.. The results of this study provide an information base which can now be integrated into new horticultural practices with regard to pollination requirements of papaw grown under subtropical climates. In particular seasonal trends of pollinator abundance, pollen quantity and pollen viability could translate directly into strategies for improving fruit yield. Plantings of larval host plants in vicinity to papaw orchards could provide the frrst step towards the management of hawkmoth pollinators. The existence of apomictic seed formation in papaw establishes a basis for further studies concerning sex determination prior to anthesis (as it is only possible at this point of time), making use of the effects of heterosis yet circumventing the constant breeding programmes providing the Fl-crosses.. Further investigations into varietal differences of pollen quality and quantity during the subtropical growing season as well as the influence of seedlike structures on fruit set and flesh thickness may also prove useful in the selection of more suitable papaw cultivars grown under subtropical climates. Apart from objectives focusing on the fruit production of papaw, results obtained on the mechanisms of pollinator constancy, in particular the sensory recognition of floral parts by hawkmoth pollinators, offer a far more universal approach into plant-pollinator relationships, which presently are known to function by 'deceit'.