Maturation of Sweetgum and American Sycamore Seeds

Over three consecutive years in central Mississippi, sweetgum (Liquidambar styraci/lua L.) and sycamore (Platanus occidentalis L.) fruits had nearly reached fullsize by late June. Sweetgum seeds were physiologically mature by mid-August, but dry weight increased until late September. As sweetgum seeds matured, the crude fat level rose to 27 percent of seed dry weight. During maturation, concentrations of soluble nitrogen, soluble carbohydrates, and magnesium decreased, while those of protein nitrogen and phosphorus increased. Sycamore seeds became physiologically mature by early September. Chemical changes in sycamore were similar to those in sweetgum, except that the major food reserves were carbohydrates and not fats. Forest Sci. 18:223-231. Additional key words. Liquidambar styraci/lua, Platanus occidentalis, chemical analysis. KNOWLEDGE of the morphological, physiological, and chemical changes that take place as seeds grow and mature is necessary to improve seed technology. This paper reports results of 3 years' study on fruits and seeds of sweetgum (Liquidambar styraciflua L.) and American sycamore (Platanus occidentalis L.), two small-seeded hardwoods of great economic importance. Sweetgum's unisexual flowers appear in March to May, depending on latitude and weather (Martindale 1965). The pistillate flowers are borne in axillary, globose heads which form the 1to 1 %-inch diameter multiple heads of small, two-celled fruits (Sargent 1965). As they mature in early fall, the beak-like capsules open to disperse the small, winged seeds. Average yields of 52 to 58 good seeds per fruit have been reported for central Mississippi sources Bonner 1967, Kearney and Bonner 1968). Sweetgum seeds are slightly dormant and normally germinate rapidly after 2 to 4 weeks of cold, moist stratification (Bonner 1967). Seeds from the southern part of the range require less stratification than northern seeds (Wilcox 1968). Sycamore's unisexual flowers also appear in March to Mayas the leaves unfold (Merz 1965, Sargent 1965). The pistillate head matures into a globose fruit 1 to 1 % inches in diameter. The seed is an elongated, single-seeded achene, about % inch long, with a hairy tuft at the base (Sargent 1965). The fruits remain on the trees throughout winter and gradually break up, dispersing the seeds. Average yields of 1700 seeds per fruit have been reported for Mississippi and Louisiana sources, but germination averaged only 46 percent (Briscoe 1969). Isolated trees may produce fruits with practically no filled seeds. Sycamore seeds are not dormant, and stratification is not normally required for rapid germination (Bonner 1970b, Webb and Farmer 1968).