Reproductive biology and nectary structure of Lythrum in central Saskatchewan

This project examined multiple aspects of the reproductive biology of the wetland invasive species, purple loosestrife (Lythrum salicaria L.), in central Saskatchewan. An examination of insect taxa visiting the three floral morphs of Tristylous L. salicaria, as well as a ranking of the pollination efficiency of individual insect species, an apparent first for L salicaria, was undertaken. Surface features of the floral nectary of L. salicaria, as well as floral nectar secretion dynamics, were also investigated. This project also re-visited some of the previous work done on this invasive species, including various floral organ morphometrics in relation to heterostyly, and aspects of the tristylous breeding system including self-fertilization, and fertilization potential of both “illegitimate” pollination and “legitimate” pollination.The trimorphic nature of the sexual floral organs of L. salicaria were well defined in Saskatchewan. Significant differences in length (long-, intermediate- and short-style lengths) exist between all three floral morphs. Lengths of the staminal filaments (long, intermediate, and short) were also significantly different. Also the floral nectary in L. salicaria is located in a depression formed at the interface of the hypanthium and the gynoecium. Several stomata are located at regular intervals along the nectary surface, and may constitute the escape route for floral nectar. No morphological differences in nectary structure were apparent among the three floral morphs.Nectar secretion dynamics of L. salicaria were examined between the three floral morphs throughout two summer days in 2006. Peak average nectar volumes and nectar sugar quantities were detected at 3:00 pm, and, interestingly, no significant differences were detected between floral morphs, in accordance with nectary morphology. The estimated secretion rates for L. salicaria ranged from 61 – 83 µg of nectar sugar per flower per hour.Hand-pollination experiments carried out over the summers of 2006 and 2007 at three field sites in and around Saskatoon have verified the strong self-incompatibility in the breeding system of this tristylous species. Intramorph pollination, using illegitimate pollen, did not result in fertilisation, whereas legitimate hand-pollination experiments yielded multiple pollen tubes at the style base, without exception.Lythrum salicaria in central Saskatchewan was visited by several bee taxa including honeybees (Apis mellifera L.), bumblebees (Bombus spp.), leafcutter bees (Megachile spp.), and sweat bees (Lasioglossum spp.). A single visit by Anthophora furcata (Panzer) was also recorded in 2007. Generally, bee visits led to high levels of pollination success as determined by fluorescence microscopy of pollen tubes following single insect visits to previously-unvisited flowers. However, most visits by hoverflies (Syrphidae) were non-pollinating. Visits by Pieris rapae (L.), yellowjacket wasps (Vespidae) and some non-syrphid flies (Diptera) also yielded no pollen tubes at the style base.A study of the ultrastructure and development of the floral nectary of the purple loosestrife cultivar ‘Morden Gleam’ (Lythrum virgatum L. x L. alatum Pursh.) showed that starch build up in pre-secretory nectary tissues declined throughout secretion, and is virtually absent in post-secretory nectary tissues. The lack of a direct vascular supply to the floral nectary suggests that the starch breakdown products likely make up most of the floral nectar carbohydrates. Surface features of the floral nectary in ‘Morden Gleam’ closely resembled those of L. salicaria, located in the valley formed between the hypanthium and gynoecium. Nectary stomata, occasionally in pairs, likely serve as outlets for nectar in this cultivar