PHOTOMORPHOGENESIS AND CANOPY DYNAMICS - PHYTOCHROMEMEDIATED PROXIMITY PERCEPTION ACCOUNTS FOR THE GROWTH DYNAMICS OF CANOPIES OF POPULUS-TRICHOCARPA X DELTOIDES BEAUPRE

The phytochrome family of signal-transducing photoreceptors provides plants with the capacity to perceive variations in the relative fluxes of red (R) and far-red (FR) radiation, This capacity has been proposed to be of ecological value in the perception of the proximity of neighbouring plants and the consequent induction of shade avoidance responses, The work reported here has evaluated this potential by determining quantitatively the effect of neighbour proximity on the growth of canopies of Populus trichocarpa x deltoides 'Beaupre' trees, and relating the measured variables to the long-term vectoral radiation quality inside each canopy. Tbe spectral distribution of radiation inside four canopies of Populus trichocarpa x del toides 'Beaupre' of different densities was monitored throughout the growing season. Spectral distributions inside the canopies were measured in 10 degrees wedges at different heights and angles, The results are presented as PFD over 400-700 nm (PFD400-700) and PFD over 400-800 nm (PFD400-800). Results are also presented for the calculated phytochrome photoequilibrium (Pfr/P) and red:far-red ratio (R:FR). Data are presented as in-canopy angular and height profiles, and as diurnal and seasonal variations, PFD400-700 and Pfr/P were found to be reduced inside each canopy, the reduction being greatest in the most dense canopy, and least in the most open canopy, At any height within each canopy, calculated Pfr/P decreased linearly with time throughout the growing season, until leaf senescence began, The reduction was greater in the denser canopies and was found to be similar for three consecutive field seasons. Linear relationships were found between plant stem growth rate, plant spacing and Pfr/P calculated from radiation propagated approximately horizontally within the canopies, The findings support the role of phytochrome in proximity perception in the natural environment and provide a quantitative basis for investigating the competitive interactions between plants growing in dense stands, The hypothesis is proposed that the dynamics of developing or regenerating canopies can be accounted for on the basis of phytochrome-mediated perception of the proximity of neighbouring plants.