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WHOLE PLANT AND ECOPHYSIOLOGY

Vascular Function in Grape Berries across Development and Its Relevance to Apparent Hydraulic Isolation^1,[C]

Department of Viticulture and Enology (B.C., G.A.G., M.A.M.) and Department of Plant Sciences (K.A.S.), University of California, Davis, California 95616

During the latter stages of development in fleshy fruit, water flow through the xylem declines markedly and the requirements of transpiration and further expansion are fulfilled primarily by the phloem. We evaluated the hypothesis that cessation of water transport through the xylem results from disruption or occlusion of pedicel and berry xylem conduits (hydraulic isolation). Xylem hydraulic resistance (R_h) was measured in developing fruit of grape (Vitis vinifera ‘Chardonnay’) 20 to 100 d after anthesis (DAA) and compared with observations of xylem anatomy by light and cryo-scanning electron microscopy and expression of six plasma membrane intrinsic protein (PIP) aquaporin genes (VvPIP1;1, VvPIP1;2, VvPIP1;3, VvPIP2;1, VvPIP2;2, VvPIP2;3). There was a significant increase in whole berry R_h and receptacle R_h in the latter stages of ripening (80–100 DAA), which was associated with deposition of gels or solutes in many receptacle xylem conduits. Peaks in the expression of some aquaporin isoforms corresponded to lower whole berry R_h 60 to 80 DAA, and the increase in R_h beginning at 80 DAA correlated with decreases in the expression of the two most predominantly expressed PIP genes. Although significant, the increase in berry R_h was not great enough, and occurred too late in development, to explain the decline in xylem flow that occurs at 60 to 75 DAA. The evidence suggests that the fruit is not hydraulically isolated from the parent plant by xylem occlusion but, rather, is "hydraulically buffered" by water delivered via the phloem.

² Present address: Research School of Biology, Sullivans Creek Road, Australian National University, Acton, Australian Capital Territory 0200, Australia.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Brendan Choat (brendan.choat{at}anu.edu.au).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.143172

^* Corresponding author; e-mail brendan.choat{at}anu.edu.au.

Received June 21, 2009; accepted September 4, 2009; published September 9, 2009.