The Origin and Composition of Cucurbit "Phloem" Exudate

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This article discusses the origin and composition of cucurbit phloem exudate.

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Zhang, Cankui; Yu, Xiyan; Ayre, Brian G. & Turgeon, Robert April 2012.


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This article discusses the origin and composition of cucurbit phloem exudate.

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10 p.


Plant Physiology, April 2012, Vol. 158, pp. 1873-1882,; © American Society of Plant Biologists

Abstract: Cucurbits exude profusely when stems or petioles are cut. The authors conducted studies on pumpkin (Cucurbita maxima) and cucumber (Cucumis sativus) to determine the origin and composition of the exudate. Morphometric analysis indicated that the exudate is too voluminous to derive exclusively from the phloem. Cold, which inhibits phloem transport, did not interfere with exudation. However, ice water applied to the roots, which reduces root pressure, rapidly diminished exudation rate. Sap was seen by microscopic examination to flow primarily from the fascicular phloem in cucumber, and several other cucurbit species, but primarily from the extrafascicular phloem in pumpkin. Following exposure of leaves to 14CO2, radiolabeled stachyose and other sugars were detected in the exudate in proportions expected of authentic phloem sap. Most of this radiolabel was released during the first 20 s. Sugars in exudate were dilute. The sugar composition of exudate from extrafascicular phloem near the edge of the stem differed from that of other sources in that it was high in hexose and low in stachyose. The authors conclude that sap is released from cucurbit phloem upon wounding but contributes negligibly to total exudate volume. The sap is diluted by water from cut cells, the apoplast, and the xylem. Small amounts of dilute, mobile sap from sieve elements can be obtained, although there is evidence that it is contaminated by the contents of other cell types. The function of P-proteins may be to prevent water loss from the xylem as well as nutrient loss from the phloem.


  • Plant Physiology, 2012, Rockville: American Society of Plant Biologists, pp. 1873-1882


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  • Publication Title: Plant Physiology
  • Volume: 158
  • Page Start: 1873
  • Page End: 1882
  • Peer Reviewed: Yes


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  • April 2012

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  • Sept. 11, 2012, 10:10 a.m.

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  • July 22, 2013, 2:54 p.m.

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Zhang, Cankui; Yu, Xiyan; Ayre, Brian G. & Turgeon, Robert. The Origin and Composition of Cucurbit "Phloem" Exudate, article, April 2012; [Rockville, Maryland]. ( accessed April 30, 2017), University of North Texas Libraries, Digital Library,; crediting UNT College of Arts and Sciences.