Seed Ontogeny and Nonstructural Carbohydrates of Cycas micronesica Megagametophyte Tissue

in HortScience

The profile of nonstructural carbohydrates (NSC) was quantified to determine sugar and starch relationships of megagametophyte tissue during Cycas micronesica K.D. Hill seed ontogeny. Field work occurred in northern Guam where megastrobili were marked and dated as they emerged from stem apices of plants in a natural population. Seeds were harvested beginning 6 months after megastrobili emergence and continuing until 28 months, and gametophyte tissue was separated from the remainder of each seed. Carbohydrates within lyophilized gametophyte tissue were quantified by high-pressure liquid chromatography. The levels of glucose and fructose declined from a high at 6 months to a homeostasis at 11 months, and the levels of sucrose similarly declined from 6 months to a homeostasis at ≈14 months. Starch content exceeded sugar content and increased from 6 months to reach a homeostasis at ≈18 months. Maltose was not detected in any sample. Stoichiometric quotients changed dramatically until ≈14 months, when they became fairly stable until 28 months. Starch concentration was ≈5-fold greater than sugar concentration at 6 months, and increased to ≈15-fold greater than sugar concentration by 28 months. Total NSC in mature megagametophytes was almost 70% on a dry weight basis. Our results are in agreement with the biological function of this haploid tissue, as copious carbohydrate resources are readily available to support embryo and seedling growth.

Contributor Notes

We thank Gil Cruz for help with field work. Support for analytical work provided by the U.S. Department of the Navy through N40192-12-2-8000 and N40192-13-2-8003, administered by Naval Facilities Engineering Command Marianas, Guam.

Corresponding author. E-mail: thomas.marler@gmail.com.

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Figures

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    General appearance of Cycas micronesica seeds from emergence to maturity. (A) Megastrobili phenotype at 6 weeks following emergence. (B) Seed at 6 months. Gametophyte (g) and flotation (f) tissues are labeled. (C) Seed at 11 months. (D) Seed at 14 months. (E) Seed at 20 months. (F) Seed at 28 months.

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    Cycas micronesica seed megagametophyte fructose, glucose, sucrose, and starch concentration as influenced by seed age. Markers are mean ± se, n = 6. Fructose: If x < 11.5, then y = 40.55 – 2.53x; r2 = 0.99; P = 0.05, glucose: If x < 11, then y = 30.71 – 1.61x; r2 = 0.99; P = 0.02, sucrose: If x < 14, then y = 61.98 – 3.07x; r2 = 0.96; P = 0.02, starch: If x < 18, then y = 253.75 + 22.60x; r2 = 0.99; P < 0.001.

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    Stoichiometric relations of pairs of nonstructural carbohydrate pools for Cycas micronesica megagametophytes, defined as quotients and as influenced by seed age. Markers are mean values, n = 6.

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