OVEREXPRESSION OF OAT PHYTOCHROME-A GENE DIFFERENTIALLY AFFECTS STEM GROWTH-RESPONSES TO RED FAR-RED RATIO SIGNALS CHARACTERISTIC OF SPARSEOR DENSE CANOPIES

The effects of overexpression of oat phytochrome A on neighbour detect ion and on stem-growth responses to changes in red light (R), far-red light (FR) and blue light (B) simulating neighbours were investigated in transgenic tobacco seedlings grown under natural radiation. In wild -type (WT) seedlings, stem extension growth was promoted: (1) by lower ing the R:FR by means of daytime supplementary FR, end-of-day FR, neig hbours reflecting FR, or selective light filters placed around the bas e of the shoot to reduce R without affecting FR; and (2) by lowering p hytochrome-absorbable radiation (R+FR) reaching the stem. Transgenic s eedlings only responded to reductions in R:FR involving no significant changes in FR irradiance, i.e. end-of-day FR and filters placed aroun d the stem to reduce R. Neither daytime supplementary R nor selective filters placed around the stem to reduce B affected stem growth in any genotype. In growing canopies, WT seedlings responded to the reductio n of R:FR caused by FR reflected in neighbour plants. Transgenic seedl ings responded to plant density about a week later, when mutual plant shading reduced R and (to a lesser extent) FR below sunlight levels. O verexpression of phytochrome A impaired early neighbour detection.