Defoliation affects the root competitive balance for N between Poa annua plants grown in a split-root box

Background

Plants compete aboveground for light and belowground for patchily distributed nutrients. Defoliation causes an immediate loss of leaf area and photosynthetic capacity, leading to reduced root growth, with hypothesised implications for nutrient uptake and nitrogen use efficiency.

Methods

In Experiment 1, we grew single plants in split root boxes with N supplied equally or unequally, with half the plants subject to repeated defoliation to quantify the impact of each treatment. In Experiment 2, we grew pairs of Poa annua plants in three-chamber split root boxes, with N supplied either to the outer chambers (no competition) or to a shared centre chamber (competition), to quantify the influence of defoliation on root competition.

Results

In Experiment 1, defoliation caused a significant decrease in root mass but did not affect root distribution between chambers, while differential N supply led to an 11% increase in shoot mass. In Experiment 2, strong root competition was seen in undefoliated plants under competitive conditions. Where one plant was defoliated, the other exhibited increased shoot mass and N content in competitive, but not noncompetitive conditions.

Conclusions

Our data suggest that plant belowground competitive success following defoliation may be strongly influenced by the spatial distribution of soil resources.