Specific Functions of Red, Far Red, and Blue Light in Flowering and Stem Extension of Long-day Plants

Abstract

For many long-day plants (LDP), adding far red light (FR, 700 to 800 nm) to red light (R, 600 to 700 nm) to extend the day or interrupt the night promotes extension growth and flowering. Blue light (B, 400 to 500 nm) independently inhibits extension growth, but its effect on flowering is not well described. Here, we determined how R-, FR-, or B-deficient (R d , FR d , or B d , respectively) photoperiods influenced stem extension and flowering in five LDP species: Campanula carpatica Jacq., Coreopsi × grandiflora Hogg ex Sweet, Lobelia × speciosa Sweet, Pisum sativum L., and Viola × wittrockiana Gams. Plants were exposed to R d , FR d , B d , or normal (control) 16-hour photoperiods, each of which had a similar photosynthetic (400 to 700 nm) photon flux. Compared with that of the control, the R d environment promoted extension growth in C. carpatica (by 65%), C. × grandiflora (by 26%), P. sativum (by 23%), and V. × wittrockiana (by 31%). The FR d environment suppressed extension growth in C. × grandiflora (by 21%), P. sativum (by 17%), and V. × wittrockiana (by 14%). Independent of the R: FR ratio, the B d environment promoted stem extension (by 10% to 100%) in all species, but there was little or no effect on flowering percentage and time to flower. Extension growth was generally linearly related to the incident wide band (100 nm) R: FR ratio or estimated phytochrome photoequilibrium except when B light was specifically reduced. A high R: FR ratio (i.e., under the FR d filter) delayed flower initiation (but not development) in C. carpatica and C. × grandiflora and inhibited flower development (but not initiation) in V .× wittrockiana . Therefore, B light and the R: FR ratio independently regulate extension growth by varying magnitudes in LDP, and in some species, an FR d environment can suppress flower initiation or development.