Effects of desiccation on temperate recalcitrant seeds: differential scanning calorimetry, gas chromatography, electron microscopy, and moisture studies on Quercus nigra and Quercus alba

Investigations into the nature of desiccation-sensitive, or recalcitant, seed behavior have as yet failed to identify exact causes of this phenomenon. Experiments with Quercus nigra L. and Quercus alba L. were conducted to examine physiological and biochemical changes brought about by seed desiccation and to determine if there were predictable changes in seed moisture content, in enthalpy (heat content) of seed moisture, in the lipid fraction, or in seed ultrastructure as viability declined. Quercus nigra intact acorn moisture contents at 50% and 5% viability were 15% and less than 14%, respectively; those of intact Q. alba at 50% and 0% viability were much higher, 32% and 22%, respectively. Generally, it was found that as the seeds of both species dried, the moisture content of the axes remained high (26-27%), even after 9 days of drying. In Q. nigra acorns, there was little difference in average percent moisture lost per day among axes, proximal cotyledon tissue, and distal cotyledon tissue. Quercus alba acorns, however, lost moisture more rapidly from the axes than from the cotyledons. This was probably caused by the longitudinal splitting of the pericarp during the drying process. Lipids composed 28.4% of the dry weight of Q. nigra and 5.7% of Q. alba dry weight. Neither individual fatty acids nor total fatty acid content exhibited definite patterns of change over the course of the experiment. The most prevalent saturated fatty acid in both species was palmitic acid, and the most common unsaturated fatty acid was generally oleic acid. Electron microscopy studies of Q. nigra showed cell wall trauma after 3 days of drying (moisture content 23%); by day 7, when moisture content had dropped to 15.6%, there was a definite dissolution of cytoplasmic density and a reduction of spherosome concentration. Quercus alba exhibited similar responses to drying, but cell wall integrity was maintained. Differential scanning calorimetry studies revealed strong relationships between onset and enthalpy values of all acorn tissues and percent germination, as did regressions involving moisture content and seed germination.