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AOBPreview published online on August 12, 2005
Annals of Botany, doi:10.1093/aob/mci246
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© The Author 2005. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oupjournals.org
Received May 3, 2005
Revised May 30, 2005
Accepted June 24, 2005

Article

Imaging and Quantifying Carbohydrate Transport to the Developing Ovaries of Maize

PIRJO MÄKELÄ 1, JOHN E. McLAUGHLIN 2, and JOHN S. BOYER 2*

1 College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, DE 19958, USA; Present address: Department of Applied Biology, Section of Crop Husbandry, PO Box 27, 00014 University of Helsinki, Finland
2 College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, DE 19958, USA

* To whom correspondence should be addressed.
JOHN S. BOYER, E-mail: boyer{at}cms.udel.edu


  Abstract

• Background and Aims Shade or inadequate water can inhibit photosynthesis and limit the development of maize (Zea mays) ovaries around the time of pollination, potentially reducing the number of kernels at harvest. This study investigated whether the decreased photosynthesis diminished only the sugar supply or also altered the transport path to the ovaries.

• Methods Photosynthesis and water potentials ({Psi}w) were measured in the leaves while dry matter delivery was monitored in the ovaries. Ovary glucose, starch and acid invertase activities were measured in situ. Stems were fed xylem-mobile safranin or phloem-mobile carboxyfluorescein (CF), and the dye transport to the ovaries was determined.

• Key Results Under normal conditions, the ovaries gained in dry mass, and starch accumulated in the pedicel and ovary wall. Glucose accumulated in the pedicel, apparently in the apoplast where insoluble (cell-wall-bound) acid invertase acted on the arriving sucrose. A glucose gradient developed from pedicel to nucellus. Safranin moved in the xylem and did not reach the ovary, but CF moved in the phloem and arrived at the ovary. CF also spread into the pedicel but unlike glucose it did not enter the nucellus. Low {Psi}w or shade decreased leaf photosynthesis, ovary dry mass accumulation, invertase activities, pedicel glucose, starch accumulation and CF delivery. Removal of these treatments reversed the effects.

• Conclusions The success of CF in tracing the general path and rate of carbohydrate transport gave visual evidence that phloem transport to the ovary decreased at low {Psi}w or in the shade but otherwise remained functional. The decreases indicated that losses in carbohydrate delivery are central features of failed ovary development under these conditions. The selectivity of transport into the nucellus resembled the situation later when embryo and endosperm are present and selective uptake occurs from the apoplast.

Keywords: Zea mays, carboxyfluorescein, shade, water deficiency, water potential, sucrose, glucose, invertase, pollination, ovary development, pedicel, starch, apoplast.
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