LAUSR

Abstract Floral longevity can vary substantially with temperature in a given plant species. Thermal time has been widely used to measure developmental age of plants and poikilothermic organisms. We tested whether thermal time was suitable to express flower age to characterise the duration of pistil receptivity and that of nectar secretion in undisturbed flowers of oilseed rape (OSR), and whether these two functional processes were synchronous. Flower age was measured in a male fertile hybrid F1 line and in its male sterile (MS) parental line to determine if the presence of pollen affected the nectar secretion period. Hand pollinations and nectar samplings were conducted throughout the lifespan of OSR flowers on several flower cohorts from plants grown under tunnels under various temperature conditions. The number of seeds produced per flower was used as a proxy to assess the level of pistil receptivity at the time of hand-pollination. Thermal time was more suitable than calendar time to express OSR flower age. A phase of maturity followed by a phase of senescence took place in the two processes, and both proceeded faster as temperature increased. Nectar was secreted during the phase of full pistil receptivity, and, in the MS line, was then reabsorbed during the gradual decline of pistil receptivity. However, the nectar secretion period was extended beyond the onset of pistil senescence in the F1 line. OSR flowers invest in nectar secretion to attract insect pollinators at least to cover the whole receptivity period of their female part. Nectar secretion may be prolonged in pollen-laden male fertile flowers to attract insect pollinators longer to ensure as long as possible the probability of their pollen being disseminated. Flower senescence is triggered by pistil senescence, while the presence of pollen in anthers acts as an inhibitor and therefore delays flower senescence. Thermal time should henceforth be used to express more universally the age of plant organs and to model the duration of processes in relation to temperature, especially in the advent of global warming.