Sugar uptake into Allium cepa leaf tissue: an integrated approach

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Abstract
Allium cepa L. leaves were subjected to enzymatic (pectolyase) and mechanical manipulation in order to ascertain the contribution made by various leaf tissues to the total sugar uptake by the leaf. In order to develop an understanding of the basic anatomy and ultrastructure of the Allium leaf and assess the integrity of the tissue before and after enzymatic and mechanical manipulation, a light- and transmission-electron-microscopy study was performed. One outcome of this study was the discovery that the chloroplasts of the bundle-sheath cells contain starch. The function of these inclusions in relation to carbohydrate pools and translocation is discussed. Kinetic curves for sucrose and fructose uptake by leaf discs derived from control and modified leaves are presented. In addition, kinetic curves for the tissues removed by the enzymatic treatment (inner parenchyma, bundle sheath and some vascular parenchyma) and the vascular bundles were also obtained. All tissues exhibited the same linear plus saturable profile as the dicotyledon, Beta vulgaris, with the exception of fructose uptake into the inner parenchyma and bundle-sheath cells; in this case the response was linear. The effect of anoxia on uptake of exogenous sucrose was also investigated. Anaerobiosis inhibited both the linear and saturable component of sucrose influx. Adenine-nucleotide levels were obtained using high-performance liquid chromatography for control (air) and anoxia-treated (N2) leaf discs. A general loss of adenine nucleotides was observed. The results presented indicate that all tissues of the leaf retrieve exogenous sugar such that the kinetic curves derived from leaf discs cannot represent phloem loading, per se.