Red:far red and photosynthetically active radiation filtering by leaves differs with species

Plant leaves filter light. Specifically, leaves allow more far red light (720-740 nm) to pass through than red light (660-680 nm), thus altering the red:far red ratio below them. By altering light quality, plants perceive canopy shading via alterations in phytochrome photoequilibria that can result in increased stem elongation and reduced branching among other responses. Although we know filtering occurs and species can respond differently to an alteration of red:far red light, no data demonstrate variation in light filtering between species. In contrast to red:far red light filtering, leaves absorb much of the photosynthetically active radiation (PAR) incident on the leaf surface, but allow some to pass through. Differences in red:far red filtering by leaves and PAR transmission through leaves of 42 herbaceous greenhouse and vegetable crop species were documented. Large differences existed in PAR transmission and red:far red leaf filtering among species studied (based on quantum, not energy). Percent PAR transmission varied from 5.0-12.1%. Lowest PAR transmission was observed on Brassica spp, Celosia, Coleus, Heliotropum, Ocimum, Salvia, Thunbergia, Torenia and Tropaeolum spp studied. Highest PAR transmission was observed on Celosia, Cleome, and Coleus spp. Red:far red light ratio below a leaf varied from 0.16-0.58. Lowest red:far red light ratio below a leaf was observed on Ageratum, Brassica, Catharanthus, Coleus, Melampodium, Nicotiana, Ocimum, Salvia, Thunbergia and Viola. Highest red:far red light ratio below a leaf was observed on Gomphrena, Coleus, Dianthus, Gazania, Hypoestes, Mathiola and Rudbeckia spp.