Experimental Analysis of the Cost and Benefits of Carbon Allocation to Shoots and Roots

Over five decades since Mooney's (Mooney, 1972) submission about the pivotal role of quantitative data in predicting plant success in different environments, the understanding of the regulation of carbon (C) allocation and its trade-off remains deficient, hindered by measurement complexities in crucial components such as respiration and root exudation. This PhD capitalizes on technological advancements on plant gas exchange measurements to experimentally explore the C costs and benefits of plant roots and shoots across different soil penetration resistances, aiming to contribute empirical data to improve the robustness of predictive models and accuracy in long-term plant responses.

This study organizes it's findings into three chapters focusing on the development and testing of a growth chamber to monitor plant gas fluxes and presenting the results on studies on C assimilation, transpiration and above and below-ground respiration and the associated cost-benefit relationships in terms of water and N acquisition.

In the first chapter, a novel plant growth chamber is introduced, enabling concurrent monitoring of above and below-ground gas exchange. Testing the growth chamber with chemicals and plants highlights its potential for advancing research involving the measurement of plants' fluxes. Challenges in accurately quantifying root C allocation are addressed and minimized using a custom-developed potting substrate.

Utilizing the growth chamber developed in Chapter 1, the second study investigates the cost and benefit ratios of C allocation in response to different soil penetration resistance in maize plants. The findings highlight elevated root respiration costs under high soil penetration resistance. However, increased below-ground respiration costs were not associated with a decrease in water and nutrient uptake per root length under high bulk density conditions. Due to higher root respiratory cost incurred by the plants grown under the high soil bulk density, the cost/benefit ratio was higher for this group.

The third chapter extends the investigation to a comparative analysis between maize and barley plants, emphasizing species-specific traits and bulk density effects on the studied cost-benefit relationships. Maize demonstrated a higher water use efficiency (WUE) compared to barley, while the root respiration cost per water uptake for maize was twice that of barley. However, N uptake per root length in maize was higher than those of barley, providing evidence of increased N demand and heightened respiration cost in maize. Comparing the respiratory cost of N uptake between the two species revealed similarities in this relationship for both species. This finding highlights that while different species vary in their different uptake strategies, the environment imposes the cost for resources acquisition. The similarity in the cost/benefit ratio of N acquisition in maize and barley highlights the fundamental costs associated with resource uptake, and suggests that they are species-independent.

Furthermore, both species exhibited similar respiration to assimilation suggesting that the energy investment in below-ground respiration is proportionate to C assimilated above-ground across both species. Similar to the second chapter, high bulk densities were associated with higher root respiration per root length across both species, while high bulk density treatments in barley corresponded to increased water uptake rates. Connecting this finding with those of Chapter 2 where higher soil penetration resistance did not diminish benefits in terms of water and N acquisition, the findings in this study correspond with the well-documented adaptive responses of root systems to optimize resource acquisition under constrained conditions and highlights the complexity of cost-benefit relationships under adverse below-ground conditions.

While this research provided valuable insights into C allocation patterns and their cost/benefit ratios, unanswered questions involve isolating the cost of root penetration and investigating the cost/benefit ratios associated with rhizosphere interactions in field conditions. Scaling these findings to larger plants necessitate longer-term experiments, to measure these ratios under various plant developmental stages.

Recommendations for future studies include adopting an integrated approach that combines dynamic root imaging with continuous respiration measurements for a detailed understanding of C costs and benefits across different plant physiological processes and a wide range of environmental conditions.