Phenotypic Plasticity of Pisum sativum in Response to Herbivory as Observed Through Root: Shoot Ratio
Organisms respond to certain environmental conditions by changing the distribution of energy among certain systems and processes through what is known as resource allocation. Plants often demonstrate variation in resource allocation through phenotypic plasticity, or the ability to change physical characteristics in order to adapt to biotic and abiotic factors in the environment. In this experiment, Pisum sativum (field peas) were used to demonstrate the effect of herbivory on resource allocation in plants. A control group of plants were grown in the greenhouse under controlled conditions, and the treatment group grew under exposure to herbivory, which was symbolized by the removal of leaves by the experimenter. It was hypothesized that the root: shoot ratio for the length and width of the treatment group would be greater than that of the control group. The root: shoot ratios of the plants were calculated and values compared. The results of a two sample T-test showed that exposure to herbivory did not have a significant effect on resource allocation in field pea plants, therefore rejecting the hypothesis
Fitness is a principle element to the theory of evolution. A species fitness to its environment is directly related to its ability to maximize environmental resources in order to survive and successfully reproduce [Johnson: 2018]. Scientists refer to the distribution of energy to processes within an organism which increases its fitness to an environment as resource allocation. Though resource allocation is genetically determined, species and individuals have the ability to modify their allocation pattern in order to adapt to abiotic and biotic conditions unique to their environment [Johnson: 2018]. This property is known as phenotypic plasticity, and can be easily observed in plants. Scientists often perform experiments to observe how plants adjust resource allocation based on the availability of resources above ground, at the shoot, and below ground, at the root. Thus, scientists commonly measure the influence of environmental factors on resource allocation by calculating the root: shoot ratio of biomass in plants growing under various conditions. In an experiment conducted Gedroc [1996] on two plant species growing in high and low nutrient soils, researchers found that species growing in high nutrient soil exhibited faster growth and a lower root: shoot ratio than plants growing in low nutrient soil [Gedroc: 1996]. Another study of plant plasticity focused on the effect of herbivory on plant fitness and found that, in some cases, herbivory increases plant fitness as a result of overcompensation of growth [McNaughton : 1983]. Similarly, in this experiment, Pisum sativum (field pea) plants were used to observe the effect of herbivory on resource allocation. The removal of plant leaves by experimenters was used to simulate herbivory in this experiment. It was hypothesized that field pea plants growing in the presence of herbivory would have a higher root: shoot ratio than plants growing in the absence of herbivory.
Six plant cartons were prepared with soil and Miracle Grow nutrients. 12 holes were poked in the soil of each of the cartons, providing space for two field pea seeds to be placed, after which the soil was redistributed to cover the seeds. Each carton was watered until saturation. Three cartons were labeled as control and three as treatment. All cartons with seeds were placed in the greenhouse. Every day, the plants were watered with 50 mL water. Every three days, starting on the third day, 1/2 of all of the leaves on the plants in the treatment group were removed by the experimenter to simulate herbivory. After two weeks, the plants were returned to the laboratory for analysis. 12 control plants and 12 treatment plants were selected, removed from the pots, roots washed and dried, and placed on labelled paper towels. The root of each plant was separated from the shoot by cutting just above the seed shell. The root and shoot length and weight were measured in cm and grams, respectively. From these values, the root: shoot ratio was calculated. Statistical analysis was performed using a two-sample unpaired T-test.
The data collected from this experiment showed that the root: shoot ratio for length of field pea plants in the treatment group was greater than that of the control group, and the root: shoot ratio for mass of plants in the treatment group was less than that of the control group. Figure 1 illustrates this data including error bars that show standard deviation of the results. There was a significant difference between the root: shoot ratio for length in the treatment and control group (t-stat=2.24, d.f.=22, p=0.018). There was not a significant difference between the root: shoot ratio for mass in the treatment and the control group (t-stat=1.13, d.f.=22, p=0.1360).
The results of this experiment lead to the rejection of the hypothesis that plants growing in the presence of herbivory would have a higher root: shoot ratio for length and mass than plants growing in the absence of herbivory. The data shows that the root: shoot ratio for length of the plants that were exposed to herbivory was significantly lower than that of the plants not exposed to herbivory. A possible explanation for this result is that the plants exposed to herbivory did not increase energy allocation to the roots because the nutrients acquired from soil at the roots is not equivalent to the sunlight energy supplied to the plant through the leaves during photosynthesis. Instead, the plant allocated more energy to the leaves and shoot in an effort to increase the capacity of sunlight absorption despite the effect of herbivory. The results of a study conducted on ecological limits to plant plasticity found that plants growing in the presence of herbivory experienced damage that inhibited expression of the optimal phenotype [Valladares: 2007]. This ecological limitation on growth while exposed to herbivory would hinder phenotypic plasticity, which could be a possible explanation for the non-significant results of this experiment.
Johnson, AD. Unit 1: Phenotypic Variation in Plants. Ecology and Evolution BIO 113 Laboratory Manual. Dept. Biology Wake Forest University, Winston-Salem, NC. Vers 18.2. (updated August 3, 2018), pp 15-24.
Gedroc J. J , McConnaughay KDM, Coleman JS. 1996. Plasticity in Root/Shoot Partitioning: Optimal, Ontogenetic, or Both? Functional Ecology. 10: 44-50.
Valladares F, Gianoli E, Gómez JM. November 2007. Ecological limits to plant phenotypic plasticity. New Phytologist. pp 743.
McNaughton, SJ. May 1983. Compensatory Plant Growth as a Response to Herbivory. Oikos, Herbivore-Plant Interactions at Northern Latitudes. 40: 329-336.
Figure 1. This graph shows the difference in root: shoot ratio for length and mass of P. sativum exposed to herbivory and not exposed to herbivory. Error bars show standard deviation of the results.
