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Understanding Phloem Function in Plants: Carbon Balance and Signal Dynamics

Explore the intricate mechanisms of phloem function in plant physiology and ecology, focusing on carbon balance, sugar transport, pressure flow, and signaling dynamics. Learn about sieve elements, companion cells, Münch hypothesis, and environmental controls influencing phloem functions. Discover how carbohydrates, hormones, and RNA play key roles in regulating photosynthesis and respiration in plants.

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Understanding Phloem Function in Plants: Carbon Balance and Signal Dynamics

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  1. Feb. 17th, 2011B4730/5730Plant Physiological Ecology Whole Plant Carbon Balance II

  2. Fagus sylvatica; Ferrio et al PCE 2009

  3. Phloem Properties • Sieve elements retain minimal metabolic function to facilitate transport • Increasing loss of metabolic function with evolution • No nucleus • Surrounding parenchyma have increased metabolic function • Companion cells in angiosperms • Rapid plugging of wounded sieve elements precluded functional studies until recently • Phloem specific protein

  4. Sources and Sinks • Munch hypothesis that osmotic forces drive sugar transport • Active loading of phloem at sources • Moves water from xylem to phloem • Active unloading of phloem at sinks • Moves water from phloem to xylem • Pressure gradient maintained between sources and sinks • Environmental controls largely unknown • Sugar signals • mRNA transport • Girdling studies • pressure waves and osmotica

  5. Van Bel PCE 2003

  6. Münch Pressure Flow Sevanto et al. PCE 2003

  7. Sevanto et al. PCE 2003

  8. Signaling and Phloem Flow • Carbohydrate source/sinks control photosynthesis • Lack of sink causes buildup and feedback inhibition of A; starch granules • symplastic vs apoplastic phloem loading • Hormones and RNA provide direct signals • Type of RNA and exact mechanisms not fully understood • Many studies document time lags between photosynthesis and respiration -isotopic vs time lag evidence

  9. Tournier et al Pl. J. 06

  10. Pinus taeda; Johnsen et al PCE 2007

  11. Iglesias et al Phys Plant 02 Citrus unshiu

  12. Mencuccini & Holta; New Phyt 2010

  13. grassland; Bahn et al New Phyt 2009

  14. Gough et al Biosci 2008

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