Journal Article

Photosynthesis down-regulation precedes carbohydrate accumulation under sink limitation in <i>Citrus</i>

Sergio G. Nebauer, Begoña Renau-Morata, José Luis Guardiola and Rosa-Victoria Molina

Edited by Joao Pereira

in Tree Physiology

Volume 31, issue 2, pages 169-177
Published in print February 2011 | ISSN: 0829-318X
Published online March 2011 | e-ISSN: 1758-4469 | DOI: http://dx.doi.org/10.1093/treephys/tpq103
Photosynthesis down-regulation precedes carbohydrate accumulation under sink limitation in Citrus

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Photosynthesis down-regulation due to an imbalance between sources and sinks in Citrus leaves could be mediated by excessive accumulation of carbohydrates. However, there is limited understanding of the physiological role of soluble and insoluble carbohydrates in photosynthesis regulation and the elements triggering the down-regulation process. In this work, the role of non-structural carbohydrates in the regulation of photosynthesis under a broad spectrum of source–sink relationships has been investigated in the Salustiana sweet orange. Soluble sugar and starch accumulation in leaves, induced by girdling experiments, did not induce down-regulation of the photosynthetic rate in the presence of sinks (fruits). The leaf-to-fruit ratio did not modulate photosynthesis but allocation of photoassimilates to the fruits. The lack of strong sink activity led to a decrease in the photosynthetic rate and starch accumulation in leaves. However, photosynthesis down-regulation due to an excess of total soluble sugars or starch was discarded because photosynthesis and stomatal conductance reduction occurred prior to any significant accumulation of these carbohydrates. Gas exchange and fluorescence parameters suggested biochemical limitations to photosynthesis. In addition, the expression of carbon metabolism-related genes was altered within 24 h when strong sinks were removed. Sucrose synthesis and export genes were inhibited, whereas the expression of ADP-glucose pyrophosphorylase was increased to cope with the excess of assimilates. In conclusion, changes in starch and soluble sugar turnover, but not sugar content per se, could provide the signal for photosynthesis regulation. In these conditions, non-stomatal limitations strongly inhibited the photosynthetic rate prior to any significant increase in carbohydrate levels.

Keywords: Citrus sinensis L. ‘Salustiana’; gene expression; girdling; leaf-to-fruit ratio; soluble sugars; starch

Journal Article.  6183 words.  Illustrated.

Subjects: Plant Sciences and Forestry

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