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Collection and Analysis of Arabidopsis Phloem Exudates Using the EDTA-facilitated Method
Published on: October 23, 2013
The origin and composition of cucurbit "phloem" exudate
Cankui Zhang1, Xiyan Yu, Brian G Ayre
1Department of Plant Biology, Cornell University, Ithaca, New York 14853, USA.
Plant Physiology
|February 15, 2012
Summary
Cucurbit plants like pumpkin and cucumber release significant exudate when cut. This study reveals phloem sap is present but diluted by other plant fluids, with P-proteins potentially preventing nutrient and water loss.
Area of Science:
- Plant Physiology
- Plant Anatomy
Background:
- Cucurbits exhibit profuse exudation from cut stems and petioles.
- The origin and composition of this exudate require detailed investigation.
Purpose of the Study:
- To determine the source and chemical makeup of exudate in pumpkin (Cucurbita maxima) and cucumber (Cucumis sativus).
- To differentiate between phloem sap and other sources contributing to the total exudate volume.
Main Methods:
- Morphometric analysis to assess exudate volume relative to phloem capacity.
- Investigating the effect of temperature and root pressure on exudation rates.
- Microscopic examination to identify the primary sites of sap flow.
- Radiolabeling experiments using 14CO2 to trace sugar movement and composition in the exudate.
Main Results:
- Exudate volume exceeds what can be solely attributed to phloem transport.
- Root pressure significantly influences exudation rate, while cold temperatures do not inhibit it.
- Microscopy showed sap primarily originates from fascicular phloem in cucumber and extrafascicular phloem in pumpkin.
- Radiolabeled sugars (stachyose) were detected in exudate, confirming phloem origin, but were dilute and released rapidly.
- Exudate from extrafascicular phloem in pumpkin stems showed distinct sugar profiles (high hexose, low stachyose).
Conclusions:
- Wounding releases cucurbit phloem sap, but it constitutes a minor fraction of the total exudate.
- The majority of exudate volume consists of water from cut cells, apoplast, and xylem, diluting the phloem sap.
- Obtaining pure phloem sap is challenging due to dilution and potential contamination from other cell types.
- Phloem P-proteins may play a role in preventing water loss from xylem and nutrient loss from phloem.
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