Testing the Geometric Framework on the Root Foraging Behaviour of Poa annua

In the process towards understanding plant root foraging behaviour, much focus has been on root proliferation responses to nutrient availability and on using animal foraging theories such as the marginal value theorem to understand plant root foraging behaviour. A common problem is the lack of generality of these theories and variation is often explained by life history.
The geometric framework was developed as a general animal foraging concept and has been tested on a variety of organisms such as locusts and slime mould. I adapted this framework by replacing the carbohydrate and protein axes with nitrogen and phosphorus. I used Poa annua as the model species for the first test on a plant, and I defined the intake target by applying nitrogen and phosphorus at 6 different concentrations and measuring nutrient intake levels. I hypothesised that the plants would defend an optimal intake target and I was able to define the intake target from the data. However, the results showed that there was a phosphorus limitation to reach this target and I found that the plants tend to defend an optimal intake ratio for suboptimal nutrient availability within the boundaries of regulative ability. This provided the optimal nutrient intake ratio which defines the optimal feeding rail. This rail separates the zones in the fitness landscape that determine which nutrient the plant should take up exclusively to reach the optimal feeding rail.
I tested the hypothesis of exclusive feeding and the plant’s ability for nutrient self selection with two split root experiments, one to measure root biomass and another to measure carbon allocation using 11C. I found a stronger ability for the plant to regulate phosphorus intake than to regulate nitrogen intake and the labelled carbon experiment showed a direct foraging response of a nitrogen starved plant when provided with a high nitrogen patch. The geometric framework is an invaluable tool for plant research with respect to nutrient cycling and behaviour, and nutrient uptake and regulation mechanisms need to be considered for developing a general plant foraging concept.