How Pruning Affects Carbon Use and Growth in Citrus Trees

Jim Crocker
1st May, 2024

How Pruning Affects Carbon Use and Growth in Citrus Trees

Orange (Citrus x sinensis)

Photo adapted from: David J. Stang / CC BY SA (Source)

Key Findings

  • In a University of Florida study, defoliated sweet orange trees increased their photosynthesis rates per leaf
  • Despite higher photosynthesis in remaining leaves, overall plant growth and carbon fixation were reduced
  • Less carbon was allocated to fine roots, suggesting a survival strategy focusing on aboveground regrowth
Understanding how plants distribute and use their energy is crucial for improving crop yields and managing agricultural resources. A recent study by researchers at the University of Florida[1] has shed light on this process by examining how sweet orange trees (Citrus x sinensis) adjust their carbon fixation, transport, and distribution when faced with different levels of leaf removal, a condition that alters the balance between the sources of carbon (leaves) and the sinks (areas where carbon is used, such as roots and fruits). In plants, leaves act as factories where sunlight is converted into chemical energy through photosynthesis. This energy, in the form of carbon compounds, is then transported to other parts of the plant where it is either used immediately or stored for later use. The balance between the production of these compounds (source) and their utilization or storage (sink) is known as the source-sink balance. Disruptions to this balance can have significant impacts on plant growth and productivity. The study focused on 'Valencia' sweet orange trees and involved varying degrees of defoliation, which means removing leaves to simulate different source-sink ratios. They removed 0%, 33%, 66%, and 83% of leaves and then measured several key indicators of photosynthetic efficiency and carbon transport in the plants. One of these indicators is the maximum rate of carboxylation (Vcmax), which is a measure of the plant's capacity to fix carbon dioxide during photosynthesis. Another is the electron transport rate (J1200), which is related to the energy available for carbon fixation. Lastly, the triose-phosphate utilization rate (TPU) reflects the plant's ability to use the products of photosynthesis to synthesize sugars. Interestingly, the researchers found that more defoliation led to increases in Vcmax, J1200, and TPU. This suggests that even with fewer leaves, the remaining leaves increased their photosynthetic activity, possibly to compensate for the loss of leaf area. However, despite these increases, the overall carbon fixation per plant was likely reduced due to the smaller leaf area. Moreover, the study used radioisotope tracer techniques to track how carbon compounds were transported within the plants after defoliation. They discovered that carbon export from individual leaves was not affected by the amount of defoliation. However, leaves closer to the shoot apex were more active in exporting carbon, indicating a prioritization of resources to support new growth. Despite the increased photosynthetic rates in individual leaves, the overall growth of the plants was affected. Defoliated plants started more new shoots but accumulated less biomass over time. This finding suggests that while the plants may try to compensate for leaf loss by increasing photosynthesis, the reduced leaf area ultimately limits growth. The study also found that defoliation led to a decrease in carbon allocation to fine roots. This implies that the plants were retaining more carbohydrates in their aboveground parts, possibly as a survival strategy to support rapid regrowth of leaves. These findings are consistent with earlier research, such as a study on hemp[2], which showed that leaves' photosynthetic capacity decreases with age, and that incorporating leaf age is important when estimating a plant's primary production. Additionally, the 'plantecophys' R package[3] could be used to further analyze the gas exchange data from the sweet orange trees, applying models of photosynthesis and stomatal conductance to better understand the physiological changes observed. In the case of rice[4], the study highlighted the importance of triose phosphate utilization in regulating photosynthesis, particularly under conditions of elevated CO2. This ties into the current research by demonstrating how plants can adjust their internal processes in response to changes in carbon supply and demand. Overall, the University of Florida's study offers valuable insights into how sweet orange trees, and potentially other crops, manage their resources when faced with challenges such as defoliation. It also points to the importance of considering intermediate sinks, like the trunk and older shoots, in understanding how plants regulate the flow of carbon. These insights could inform agricultural practices aimed at optimizing yield and managing plant stress. Further research in this area will continue to unlock the complexities of plant growth and resource allocation.

AgricultureEcologyPlant Science

References

Main Study

1) Response of carbon fixation, allocation, and growth to source-sink manipulation by defoliation in vegetative citrus trees.

Published 30th April, 2024

https://doi.org/10.1111/ppl.14304

Related Studies

2) Leaf Age and Position Effects on Quantum Yield and Photosynthetic Capacity in Hemp Crowns.

https://doi.org/10.3390/plants9020271

3) Plantecophys--An R Package for Analysing and Modelling Leaf Gas Exchange Data.

https://doi.org/10.1371/journal.pone.0143346

4) Is triose phosphate utilization involved in the feedback inhibition of photosynthesis in rice under conditions of sink limitation?

https://doi.org/10.1093/jxb/erz318


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