Reproductive biology of Sorghum halepense (L.) Pers. (Poaceae; Panicoideae; Andropogoneae) in relation to invasibility

- A comprehensive account of the reproductive biology of S. halepense (L.) Pers.
- Amphimictic fruit set along with apomictic propagation create high propagule pressure.
- Normal meiosis and regular embryo sac development account for genetic stability.
- A first report on the role of insects as pollinators of the species.
- The diaspores are predisposed to dispersal through autochory and endozoochory.

Sorghum halepense (L.) Pers. is the most aggressive grass weed of the world resulting from an efficient system of its reproduction and propagation through a combination of vegetative and sexual means. However, reproductive biology of the species is not understood in sufficient details and had serious gaps of information on its pollen and pollination system, breeding behaviour, fruit and dispersal biology. The species was found to be chasmogamous with an andromonoecious sexual system of well-developed bisexual and male spikelets in pairs and terminal triads. Precocious extrusion of receptive stigmas preceding anther dehiscence, abundant pollen of high fertility and longevity and a high pollen-ovule ratio strongly indicate xenogamy in the species. Laser Confocal Microscopy revealed rapid germination of pollen and growth of pollen tubes down the pistil to the ovule which caused high fruit set and facilitated the amphimictic cycle. The present study is presumably the first report on the role of several insect pollinators mainly from the order Diptera (Isomyia paurogonita Fang & Fan.) and Hymenoptera (Hylaeus sp., Sphecodes ephippius Linnaeus , Megachile sp.). The visitation time of insect pollinators closely synchronized with peak anthesis resulting in high pollen loads. Breeding experiments revealed significantly higher fruit set in open pollinated in comparison to bagged synflorescences. The diaspore enclosed a well-developed caryopsis surrounded by accessory bracts along with persistent pedicels and a rachis segment preparing them for dispersal through autochory and endozoochory. Thus, cosmopolitan and invasive nature of S. halepense could be attributed to a combination of xenogamous breeding system, high seed production and efficient fruit dispersal through resistant diaspores. This study on the reproductive system of S. halepense shall provide a framework for developing management strategies both for the control of this invasive grass species as also for its propagation as a fodder crop in vacant rangelands.