Low red to far-red (R:FR) ratios of light act as a competition cue prior to direct competition for resources. Upon detection of low R:FR light by phytochromes, a suite of signalling events occur that lead to changes in the morphology and physiology of the plant. Many of the molecular mechanisms and signalling events that lead to these changes remain to be elucidated. The main objective of this research was to further understand the molecular and physiological mechanisms underpinning the response to resource-independent weed competition from light detection to altered plant growth. This research provides important insights into the biological function of singlet oxygen (1O2) in Arabidopsis under resource-independent weed competition. The biological function of 1O2 as a signalling molecule under this condition appears to be dependent on the repression of superoxide (O2-) and hydrogen peroxide (H2O2) production, which occurs through dramatic alterations in activities of O2- and H2O2 scavenging enzymes (superoxide dismutase and glutathione peroxidase) and the redox status of ascorbate and glutathione that balance the redox poise. The specificity of 1O2 signaling is demonstrated by minimal overlap and differences in whole transcriptomic responses between Arabidopsis under low R:FR light and other plant 1O2 generating systems. Upregulation of the 1O2-responsive ILL6 along with ST2a, which act as negative regulators of jasmonate (JA) accumulation and response, connect 1O2 signalling to the shade avoidance response observed under low R:FR. This connection provides important clues about how the growth inhibitory effect of 1O2 through the attenuation of JA-mediated responses may be exploited by plants under low R:FR light to control rapid elongation growth in anticipation of impending competition.