Document Type

Article

Publication Date

9-6-2005

Publication Title

Proceedings of the National Academy of Sciences of the United States of America

ISSN

0027-8424

Volume

102

Issue

36

First Page

12968

Last Page

12973

DOI

10.1073/pnas.0503784102

PubMed ID

16120679

Abstract

Plants load sugars from photosynthesizing leaves into the phloem of exporting veins either "apoplastically" (by using H+/sucrose symporters) or "symplastically" (through plasmodesmata). The ability to regulate photosynthesis in response to the light environment was compared among apoplastic loaders (pea and spinach) and symplastic loaders (pumpkin and Verbascum phoeniceum). Plants were grown under low light (LL) or high light (HL) or transferred from LL to HL. Upon transfer, pea and spinach up-regulated photosynthesis to the level found in HL-acclimated plants, whereas up-regulation in pumpkin and V. phoeniceum was limited. The vein density of pea and spinach was the same in HL and LL. Although spinach did not exhibit anatomical or ultrastructural acclimation to the light environment, in pea, wall invaginations in minor vein companion (transfer) cells were more extensive in HL. Furthermore, upon transfer from LL to HL, these invaginations increased in mature pea leaves. Foliar starch levels in mature leaves of plants transferred from LL to HL were not greater than in HL-acclimated leaves of either apoplastically loading species. In the symplastic loaders, plasmodesmatal frequency per loading cell did not vary with treatment, but vein density and thus total plasmodesmatal frequency were higher in HL. Upon transfer of symplastic loaders, however, vein density remained low, and starch levels were higher than in HL; the incomplete acclimation of photosynthesis upon transfer is thus consistent with a carbon export capacity physically limited by an inability to increase vein and plasmodesmatal density in a mature leaf.

Comments

Publication of this article was funded by the University of Colorado Boulder Libraries Open Access Fund.

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