Reproduction in Heteropogon contortus. I. Photoperiodic effects on flowering and sex expression

The flowering behaviour of selected Australian and African populations of Heteropogon contortus in response to a range of photoperiods was assessed in the Canberra phytotron (CERES) under a 30/25°C day/night temperature regime. Photoperiods comprised 8 hr of natural light supplemented with low intensity incandescent illumination to give a range between 10 and 18 hr in each photocycle. The patterns of response showed H. contortus to be a short-day plant. Lines from both Australian and African populations originating from lower latitudes, and late-flowering when grown in a grass garden, flowered only in photoperiods shorter than 12 hr, remaining vegetative at longer day lengths. These were qualitative short-day plants. In contrast, the early-flowering lines showed either a qualitative or a quantitative short-day response; the latter flowered in all photoperiods but more rapidly at shorter day lengths. The nature of the early- and late-flowering behaviour is explained on the basis of the short-day responses and climatic adaptation. Since the inflorescences are polygamous, a sex ratio was calculated for all inflorescences formed by using the ratio of number of male+male spikelet pairs v. the number of female+male spikelet pairs. Increasing the photoperiod resulted in inflorescences containing more male spikelets and fewer awned female spikelets. From this it appears that day length exerts some influence over the determination of sex, and thus of seed production, in H. contortus. Versatility of this kind may well have adaptive significance depending on the breeding system in that it can partially control population heterogeneity and thus be of phytogeographic significance.