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Phloem metabolism and function have to cope with low internal oxygen
Joost T van Dongen1, Ulrich Schurr, Michelle Pfister
1Max Planck Institute of Molecular Plant Physiology, Am Mühlenberg 1, 14476 Golm, Germany.
Plant Physiology
|April 15, 2003
Summary
Low oxygen levels within plant phloem impair sucrose transport and energy metabolism. This study reveals how oxygen limitation triggers adaptive metabolic changes in phloem function.
Area of Science:
- Plant Physiology
- Plant Metabolism
- Transport Phenomena
Background:
- Phloem transport is crucial for distributing sugars throughout plants.
- Oxygen availability is a critical factor influencing cellular respiration and metabolic processes.
- Endogenous oxygen limitations within plant tissues can impact physiological functions.
Purpose of the Study:
- To investigate the effects of endogenous oxygen limitations on phloem transport and energy metabolism in Ricinus communis.
- To quantify oxygen levels within the phloem under various conditions.
- To understand the metabolic adaptations of phloem to hypoxic environments.
Main Methods:
- In situ measurement of oxygen profiles in plant stems using microsensors.
- Analysis of phloem sap composition, including ATP/ADP ratios, energy charge, sucrose concentration, and amino acid profiles.
- Experimental manipulation of external oxygen levels to assess impacts on phloem transport and metabolism.
- Measurement of NADH/NAD+ ratios, lactate, ethanol, and organic acid ratios under varying oxygen tensions.
Main Results:
- Oxygen levels in the vascular region of Ricinus communis stems were found to be significantly lower than ambient air, with phloem sap being hypoxic.
- Reduced oxygen availability led to decreased phloem sucrose concentration and transport rates, linked to inhibited retrieval processes.
- Stepwise oxygen reduction progressively lowered phloem energy state, indicating inhibited respiration.
- Under severe hypoxia, increased lactate and ethanol production, altered NADH/NAD+ ratios, and changes in amino acid and organic acid profiles were observed, indicating anaerobic metabolism.
Conclusions:
- Oxygen concentration within the transport phloem in planta is inherently low.
- Low oxygen levels induce adaptive changes in phloem metabolism and function to cope with hypoxic conditions.
- These findings highlight the importance of oxygen availability for maintaining efficient phloem transport and energy homeostasis in plants.