Text and pics by Susanna Vain
The tiny weedy species Arabidopsis thaliana can teach us quite a bit about agriculture, even though the species itself is not agriculturally important. Arabidopsis has played an immense role in helping us understand the molecular and/or genetic mechanisms behind plant functioning. Many such parallels have also been found in crops, hence the importance of Arabidopsis for agriculture. In this particular study we were interested in using Arabidopsis to study competition on a population-level and its effects on seed yield, which is also something that could help us advance agriculture.
Plant breeding so far has focused on individual performance of plants, i.e. the best-performing individuals are selected. Farmers, however, do not pick out the best individuals, they are interested in the yield from the whole field where there are tens of thousands of plants, all growing and interacting together. This has brought discussion to alternatives – perhaps plant breeding should use group selection in which not best individuals but best groups are selected for further trials. This method centers around the collective performance of plants and puts much greater emphasis on plant-plant interactions. Still, there are great gaps in our knowledge about plant-plant interactions, especially on a population-level scale.
In this paper, the effect of plant density in two different photoperiod conditions was studied to learn how it affects the collective performance of plants. Plants were sown at five densities (17.6, 8.8, 4.4, 2.2 and 1.1 cm2 per plant) and grown either in 16 h or 12 h day length conditions. What is also relatively novel about the methodology of this experiment was that instead of small pots, quite large trays were used (44×25 cm). This ensured truer monoculture conditions – one sown population constituted minimum of 64 plants in the sparsest and 1000 plants in the densest treatment.
Surprisingly, populations across all sowing densities attained constant seed yield, which was greater for plants that grew in 16 h photoperiod treatment. This means that regardless of whether there were 64 plants, 500 or 1000 plants in a tray within a photoperiod treatment, all populations produced the same amount of seeds. Furthermore, no main effect of photoperiod treatment was found for vegetative biomass production. So, seemingly, differences in biotic and abiotic conditions did not trigger competitive responses which is usually indicated by differential biomass production (and that could have led to reduced yield as well).
Did plants just make use of resources available to them, not minding their co-competitors at all? Not quite. Even though we found no evidence of differential vegetative biomass production, closer inspection revealed a sowing density and photoperiod treatment interaction for the average mass of 100 seeds. Plants that grew sparsely, produced seeds of similar weights in both photoperiod conditions. When sowing density was increased, the weight of individual seeds diverged in opposite directions – the average seed produced in 16 h conditions was heavier than that produced in 12 h photoperiod conditions. So, instead of varying vegetative biomass production, plants’ focus seemed to be on the seeds, which is not unexpected since Arabidopsis is a species with relatively short life-cycle (approx. 6 weeks).
In conclusion, this study showed that vegetative biomass and total seed yield are not the only things to consider when assessing the performance of plants. When plants produce the same amount of vegetative or generative biomass, then this does not automatically mean that plants have no reactions to different conditions at all. There is still a lot to be learned.
Citation: Vain, S., Gielen, I., Liira, J., & Zobel, K. (2020). Population-level performance of Arabidopsis thaliana (L.) Heynh in dense monocultures. Journal of Plant Ecology, rtaa006, https://doi.org/10.1093/jpe/rtaa006 (link to full text)
Very little is known about the performance of non-agricultural plant species in monocultures, even though nearly all agricultural species have experienced the transition from multi-species environments to dense monospecific stands during the breeding process. In the light of recent work that highlighted the possibility that the weedy species Arabidopsis thaliana can offer novel insight into crop breeding, we aimed to test the effect of sowing density on group and individual performance in different photoperiod environments in A. thaliana.
We studied the performance of A. thaliana Cvi-0 ecotype. The choice of Cvi-0 was based on a preliminary experiment in which plants of Cvi-0 ecotype exhibited high competitive performance. Sowing densities used were 17.6, 8.8, 4.4, 2.2 and 1.1 cm2 per plant and photoperiod environments 12 h or 16 h of day light.
In this experiment, populations attained constant total seed yield for all densities. Some interaction effect occurred, as at high sowing density and at longer day length plants produced heavier seeds, whereas at shorter day length seed weight was negatively related to plant density. These results shed light on different strategies that annual plants can adopt when they face intense intraspecific competition, and could help to offer new perspectives for breeding crops with enhanced group performance.