Within-population variation in the relation between node number and flowering time in Rhinanthus angustifolius (Orobanchaceae)

Background and aims - The annual hemiparasitic plant genus Rhinanthus displays large variation in the date of onset of flowering, and ecotypes have been described from populations with different flowering times. Much less is known, however, about the variation in flowering time within populations of an ecotype, which is important for the adaptive capacity of a population. The number of nodes produced before the first flower is an important trait linked to flowering time differences among populations, and this trait and its relation with flowering date were investigated. Methods - Seeds from a natural, early-flowering population of Rhinanthus angustifolius, mown in early July, were used to establish a new field population in 2003, mown after summer, and to cultivate plants in the greenhouse in 2001 and 2004. The onset of flowering, node number and plant size were recorded in the field population in 2005 and in 2008. In the greenhouse, germination date, node number and flowering date were recorded. Key results - Flowering time was strongly correlated with node number in all years: the production of more nodes before the first flower was associated with a delay in flowering. There was always considerable variation around the median flowering date among plants with the same number of nodes, even in the greenhouse. Removing variation in the timing of germination in the greenhouse experiment did not reduce variation in flowering date. Part of the remaining variation was due to variation in plant size: larger plants flowered earlier. After five years, the relaxed selection on flowering time by mowing later had increased node number and delayed flowering in the new field population. Conclusions - Both genetically determined (node number) and phenotypically plastic (plant size) traits contribute to variation in flowering time within populations, and even under strong selection against late flowering, wild populations may harbour enough variation to react to a decrease in this selection pressure by later mowing.