Morphological, Molecular and Physiological Mechanisms of Early Season Weed Competition in Soybean (Glycine max (L.) Merr.)
The presence of early emerging weeds alters light quality perceived by crop plants through reducing the red (670 nm) to far-red (730 nm) ratio of light (R/FR). This change in light quality has long-lasting effects on crop plants, which may lead to yield loss. The objective of this thesis was to investigate the morphological, molecular and physiological responses of soybean (Glycine max (L.) Merr.) to far-red-enriched (FR-E) light reflected from neighbouring weeds in a controlled environment, in order to gain insight into the mechanisms of yield loss. Based on the existing knowledge on the effect of FR-E light on photosystem stoichiometry, it was hypothesised that if soybean seedlings perceive FR-E light reflected from neighbouring weeds at early growth stages, then an initial oxidative stress response will trigger a cascade of down-stream morphological and physiological events that may potentially contribute to yield loss. Morphological analyses revealed rapid changes in the root/shoot ratio of soybean seedlings upon emergence in response to FR-E light. This was accompanied by rapid induction of reactive oxygen species (ROS) scavenging genes. Analyses of enzymatic and non-enzymatic scavengers of ROS demonstrated that a primary and fundamental impact of FR-E light on soybean unifoliate leaves was increased generation of singlet oxygen and its conversion to hydrogen peroxide via ascorbate. This led to reductions in activity of a thiol-modulated Calvin cycle enzyme and CO2 assimilation as well as altered carbon partitioning. Chemical inhibition of plastid terminal oxidase and mitochondrial alternative oxidase as well as measurements of malate/oxaloacetate shuttle enzyme activities demonstrated the involvement of alternative electron pathways in early responses of soybean seedlings to FR-E light. Furthermore, investigation of the impact of FR-E light on plastic responsiveness of soybean to subsequent drought stress under field conditions revealed that the frequency, type and severity of stress influenced the ability of a soybean to express plasticity. The findings of this research not only shed light on the mechanisms of resource independent crop-weed competition that may contribute to yield loss, but also provide an important foundation for studies aimed towards improving crop competitive ability and reducing the negative impact of weeds on yield.