Abstract
The growth of synchronized leaf flushes or male cones on Cycas trees is an ephemeral event, and non-structural carbohydrates (NSCs) are likely deployed from stem and root storage tissues to support their construction. The relationships among various stem NSCs and these rapid growth events have not been studied to date. Monosaccharides, disaccharides, and starch were quantified in Cycas micronesica stem tissue prior to and immediately after the growth of leaf flushes or male cones to determine the influences on the concentration of these carbohydrates. The pre-existing leaves were removed from half of the plants to determine if the elimination of this carbon source would influence the NSC behaviors. Starch and sucrose dominated the NSC profiles, and these two NSCs declined following cone or new leaf growth. Removal of pre-existing leaves generated a greater decline in starch and sucrose for cone growth, and a greater decline in sucrose, but not starch following new leaf growth than in control trees with no leaf removal. The initial differences in starch and sucrose among cortex, vascular, and pith tissues disappeared as the concentrations declined in all three tissue categories to reach similar post-growth concentrations among the stem tissue categories. The fructose, glucose, and maltose behaviors were not consistent, and their concentrations were low such that their influence on the total NSC behaviors was minimal. These results provided indirect evidence that stem NSCs were mobilized to support ephemeral male cone and new leaf growth for this arborescent cycad. Growth of female strobili is slow and lengthy, so we did not include female trees in this study. The contributions of stem NSCs to female strobili growth remain to be studied with alternative methods.
Highlights
Cycads are long-lived perennial gymnosperm plants that exhibit discontinuous primary growth characterized by long periods of apparent quiescence between ephemeral bursts of leaf or reproductive structure growth from stem apices [1]
The stem carbohydrate concentrations ranked in the order starch > sucrose > glucose > fructose > maltose
The patterns of declines of these two non-structural carbohydrates (NSCs) in stem tissue were similar for rapid new leaf or male cone growth
Summary
Cycads are long-lived perennial gymnosperm plants that exhibit discontinuous primary growth characterized by long periods of apparent quiescence between ephemeral bursts of leaf or reproductive structure growth from stem apices [1]. The cycad pachycaulous stems are manoxylic and lack the bifacial secondary cambium present in lignophyte tree species for radial growth [1,2]. Persistent pith and cortex are separated radially by concentric vascular cylinders that contain copious parenchyma tissue [3,4,5]. Living parenchyma tissue persists across the entire radial area of the cycad stem. Abundant non-structural carbohydrates are stored in cycad stems. Carbohydrate resources using contemporary analytical approaches have been quantified for Cycas micronesica K.D. Hill [6,7], Cycas revoluta Thunb. Hill [6,7], Cycas revoluta Thunb. [8], and Zamia muricata Willd. [9]
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