Phytochrome signalling in plant canopies: testing its population‐level implications with photoreceptor mutants of Arabidopsis

• Published in: Functional ecology Vol. 11; no. 4; pp. 441 - 450
• Main Authors: BALLARÉ, C. L., SCOPEL, A. L.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01.08.1997; Blackwell Science
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Arabidopsis thaliana; Biological and medical sciences; Demecology; Fundamental and applied biological sciences. Psychology; Phytochrome; plant competition; Plants and fungi; red:far‐red ratio; sensory ecology; size inequality; Branching; Morphological variation; Photoreceptor; Red light; Biomass; Crowding; Arabidopsis thaliana; Cruciferae; Dicotyledones; Angiospermae; Phytochrome; Intraspecific competition; Herbaceous plant; Spermatophyta; Density dependence; Experimental plant; Fecundity; Population density; Elongation; Canopy(vegetation)
• ISSN: 0269-8463; 1365-2435
• DOI: 10.1046/j.1365-2435.1997.00108.x

1. A number of signalling mechanisms are responsible for triggering plastic morphological and physiological responses of plants to the proximity of neighbours. Among these mechanisms, the phytochrome‐mediated control of branching and elongation in response to alterations in red:far‐red ratio (R:FR) has been investigated in considerable detail. 2. While the role of phytochrome B in R:FR perception has been well established, and the consequences of neighbour photo‐detection on competitive ability are receiving attention, several important issues remain to be addressed regarding the ecology of plant–plant signalling in canopies. In particular, the role of other photoreceptors in neighbour detection and the impact of neighbour detection on population‐level attributes, such as size structure and productivity:density relationships are poorly characterized. 3. In the experiments reported here we addressed these questions using wild‐type (WT) plants and photomorphogenic mutants of Arabidopsis thaliana that are specifically deficient in phytochrome A, phytochrome B, all phytochromes or the blue‐light photoreceptor cryptochrome. Plants were grown in monocultures of different densities (between ~300 and 2400 plants m–2) from planting to seed set. Full competition among neighbouring plants was allowed both above‐and below‐ground. 4. WT plants responded to crowding with the predictable increase in elongation growth and by producing more steeply orientated 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 comparison with the other genotypes, showed markedly reduced morphological responses to crowding. 5. All genotypes having functional phytochrome B had similar biomass production and fruit production, and fruit production per unit area was constant over the range of densities used. Stands of phytochrome‐B‐deficient plants were as productive as WT canopies at intermediate densities, but had significantly reduced fruit production at low and, more strikingly, also at high densities. Size inequality 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 role in neighbour photo‐detection, while other photoreceptors appear to be less important in this respect; (2) in WT stands, phytochrome‐B‐mediated neighbour detection leads to inverse rank‐dependent morphological 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’ phytochrome‐B mutants results in reduced stand fecundity at high densities.