Photoperiod, light quality, and irradiance effects on flowering in the alpine and prairie ecotypes of Stellaria longipes

Abstract

Plants growing in vegetative shade are subjected to reductions in both red to far-red (R/FR) light ratio and in irradiance levels of photosynthetically active radiation (PAR). In this report we have attempted to uncouple the reduced R/FR ratio- and low PAR-mediated effects of shade on flowering in the ramets of Stellaria longipes Goldie s.l. (Caryophyllaceae) ecotypes collected from alpine “sun” and lower elevation prairie “shade” habitats. Both ecotypes were also tested for their flowering response (defined as the number of open flowers per ramet) to daylength. The alpine ecotype plants can best be classified as day-neutral, whereas prairie ecotype plants require long-days (LD). Under a low PAR of 115 μmol m–2·s–1 given under LD conditions at a reduced (0.7) R/FR ratio, alpine ecotype plants flowered significantly later relative to plants grown under the low PAR at a normal (1.22) R/FR ratio. In contrast, plants of the prairie ecotype flowered earlier under the reduced R/FR ratio combined with the same low PAR. Flower number per ramet differed significantly between the two ecotypes, with alpine ecotype plants developing fewer flowers under a low PAR (109 μmol m–2·s–1 irradiance) relative to a high PAR of 611 μmol m–2·s–1 (both given at a normal R/FR ratio). The prairie ecotype plants responded differently and had similar flower numbers under both low and high PARs at the normal (1.22) R/FR ratio. However, growing the prairie ecotype plants under a reduced R/FR ratio at a low PAR showed a significant increase in number of flowers. In contrast, plants grown under high (2.7) and normal (1.9) R/FR ratios combined with low PAR produced many more flowers than the alpine ecotype. Thus, the two components of shade, reduced R/FR ratio and low PAR can cause distinctly different flowering responses in sun and shade plants of S. longipes.