PHYTOCHROME SIGNALING IN PLANT CANOPIES - TESTING ITS POPULATION-LEVEL IMPLICATIONS WITH PHOTORECEPTOR MUTANTS OF ARABIDOPSIS

1. A number of signalling mechanisms are responsible for triggering pl astic morphological and physiological responses of plants to the proxi mity of neighbours. Among these mechanisms, the phytochrome-mediated c ontrol of branching and elongation in response to alterations in red:f ar-red ratio (R:FR) has been investigated in considerable detail. 2. W hile the role of phytochrome B in R:FR perception has been well establ ished, and the consequences of neighbour photo-detection on competitiv e ability are receiving attention, several important issues remain to be addressed regarding the ecology of plant-plant signalling in canopi es. In particular, the role of other photoreceptors in neighbour detec tion and the impact of neighbour detection on population-level attribu tes, such as size structure and productivity:density relationships are poorly characterized. 3. In the experiments reported here we addresse d these questions using wild-type (WT) plants and photomorphogenic mut ants of Arabidopsis thaliana that are specifically deficient in phytoc hrome A, phytochrome B, all phytochromes or the blue-light photorecept or cryptochrome. Plants were grown in monocultures of different densit ies (between similar to 300 and 2400 plants m(-2)) from planting to se ed set. Full competition among neighbouring plants was allowed both ab ove-and below-ground. 4. WT plants responded to crowding with the pred ictable increase in elongation growth and by producing more steeply or ientated leaves. Similar responses were observed in mutants deficient in phytochrome A or cryptochrome. Mutant plants lacking phytochrome B had an 'elongated' phenotype even when grown at low density and, in co mparison with the other genotypes, showed markedly reduced morphologic al responses to crowding. 5. All genotypes having functional phytochro me B had similar biomass production and fruit production, and fruit pr oduction per unit area was constant over the range of densities used. Stands of phytochrome-B-deficient plants were as productive as WT cano pies at intermediate densities, but had significantly reduced fruit pr oduction at low and, more strikingly, also at high densities, Size ine quality among neighbours, measured as the coefficient of variation of reproductive output per plant, increased with density in all genotypes , but significantly more in phytochrome-B-deficient stands than in WT crops. 6. Our results suggest that: (1) phytochrome B plays a unique r ole in neighbour photodetection, while other photoreceptors appear to be less important in this respect; (2) in WT stands, phytochrome-B-med iated neighbour detection leads to inverse rank-dependent morphologica l adjustments (i.e. greater fractional response in the small plants), which tend to buffer the plant population against size structuring; (3 ) the strong size-structuring in populations of R:FR 'blind' phytochro me-B mutants results in reduced stand fecundity at high densities.