CONICET | Buscador de Institutos y Recursos Humanos

The incident light red/far red (R/FR) ratio is a key environmental signal that allows leaves to perceive the presence other leaves in the vicinity, either from the parent plant or from their near neighbours. When exposed to a low R/FR ratio leaves orchestrate changes that enable them to avoid shading by competitors. However, a low R/FR ratio also signals a lower risk of excessive irradiation and photo-oxidative stress, allowing the leaves to the shade environment. The following experiments were designed to determine if the R/FR ratio also controls the content of the low molecular weight soluble antioxidants ascorbate and glutathione in the unifoliate leaves of the common bean (P. vulgaris L) were cultivated under two R/FR ratios: (a) R/FR =1.0 (sun light) or (b) R/FR =0.2 (shade) under the same PPFD (300 µmol photon. m-2.s-1) for 7 days. At this point, some batches of plants were transferred from the higher (1) to the lower (0.2) R/FR growth conditions for a further 5 days. While photosynthesis rates expressed on a fresh weight basis were similar under both growth conditions, plants grown under far red enriched light (i.e. R/FR = 0.2), which showed a typical shaded plant phenotype (e.g. long petioles) had the lowest leaf ascorbic acid and glutathione contents. Dark respiration rates were also lower in leaves grown under a 0.2 R/FR ratio. Plants transferred from the 1 to 0.2 R/FR growth condition for 5 days showed a marked decrease in leaf ascorbic acid and glutathione contents, values falling to levels similar to those observed in plants grown at 0.2 R/FR. Furthermore, the low R/FR growth regime increased the proportion of the oxidised form of these antioxidants and decreased respiration rates. These results strongly suggest that the accumulation of ascorbic acid and glutathione is regulated by the environmental R/FR ratio in such a way as to increase the leaf antioxidants when the risk of high light stress is greatest.