The impact of defoliation on the root foraging behaviour of sunflower (Heliathus annuus L.)
Creators: Barber-Cross, Tianna E
Publisher: name: University of Alberta Library
Year: 2024

<p>Plants have a remarkable ability to proliferate roots and increase nutrient uptake within nutrient patches in the soil. This behaviour, known as root foraging, describes this ability and what factors may influence or modify this response. It has been observed that plants integrate multiple environmental cues to inform their root foraging behaviour, resulting in non-additive responses to novel combinations of stimuli such as nutrient distribution and the presence and identity of neighbouring plants. These responses are highly species-specific and context- dependent. While the effect of nutrient distribution and competitors on foraging behaviour has been studied quite comprehensively in both plants and animals, the impact of injury on the foraging behaviour of plants remains largely unexplored.<br>
In plants, defoliation can be considered a form of injury. Although both plants and animals need to forage for nutrients, it is unclear if they respond to injury in a similar manner. The many physiological and ecological differences between plants and animals, as well as their distinct interactions with the environment, cannot be disregarded. Therefore, it is essential to experimentally test the assumption that they may behave in the same way.<br>
To experimentally evaluate the impact of injury and its severity on root foraging behaviour over time, we conducted a study using the model organism, common sunflower (Helianthus annuus L.). Individual sunflower seeds were planted in experimental arenas that allowed us to observe root growth over time. These arenas contained soil with low nutrient levels and a nutrient-rich patch, enabling us to measure root foraging responses to heterogenous nutrient distribution in the soil. Defoliation treatments were applied at two different severities: half clipped and fully clipped, alongside a control treatment in which no defoliation occurred.<br>
Defoliation was found to suppress the overall root length grown, but had no effect on root foraging precision. Over time, root foraging precision decreased from an initially high proliferation response to nutrients to an agnostic response to nutrient patch by the fourth week of growth. Other measures of root morphology, namely average root diameter and average root length, exhibited no changes over time or in response to defoliation. The average root diameter was consistently larger within the nutrient patch, while average root length remained equivalent in both the patch and background soil throughout the study.<br>
These findings generally align with assumptions based on optimal foraging and optimal defence theory. However, they also highlight the influence the ecological and physiological differences between plants and animals can have on the applications of optimality theories. Additionally, this study emphasizes the importance of considering the various different measures of root foraging behaviour and the timing of data collection. Different measures and their timing yielded distinct results, and only when all measures were compared could a comprehensive understanding be obtained. Ultimately, comparing the possible effects of timing and types of measures on a study’s findings and experimentally testing the applicability of established theories to novel organisms enhances our understanding of how past and future studies can be effectively compared. This includes considering the chosen measures and timing of data collection, as well as the theories themselves and their applicability in future research.</p>