| I review recent advances in photomorphogenic research and use information derived from physiological experiments to explain patterns of space occupation and competition among neighboring plants. I found compelling evidence that the presence of surrounding vegetation influences plant growth and development not only because it modifies the availability of light energy and materials, but also because it affects aspects of the environment that are specifically used by seeds and plants to obtain information about the proximity of neighbors (e.g. red:far-red ratio, thermal regime, photon-fluence-rate gradients). The significance of recent findings on signal perception by plants is discussed in connection with the development of mechanistic models of plant competition.; I used a photomorphogenic mutant of cucumber (Cucumis sativus L.) to explore the roles of the photoreceptor phytochrome B in morphological responses to UV-B radiation and in processes that influence morphological development in patchy light environments. The studies on UV-B responses provided strong circumstantial evidence that some morphological effects of UV-B on green plants can be separated from unspecific damage and involve the action of a UV-B photoreceptor. Experiments in natural and artificial light/shade mosaics showed that, when compared over periods of several weeks, wild-type (WT) plants are far more efficient than phytochrome-B-deficient mutants (lh) at invading and maintaining their shoots in light gaps. The reasons for this difference were multiple and appeared to result from several actions of phytochrome B on WT plants. These actions include: (i) establishment of a phototropic system that responds to lateral red:far-red gradients, (ii) modulation of gravitropic responses of stems, (iii) delay of tendril production and reduction of stem-bending responses to mechanical stresses exerted by tendrils, and (iv) switching plants grown under full light from the "shade-avoiding" (default) developmental program to the one that results in a phenotype with short internodes and reduced apical dominance.; In the final section of this thesis I discuss the agricultural significance of photomorphogenic research. Three areas are considered: photocontrol of seed germination, morphological and pigmentation responses to UV-B radiation, and photomodulation of plant morphology in canopies. |
