Maturity of surface structures in northern Pinus sylvestris L. seeds: A key to improved prediction of germination potential

Prediction of germination potential prior to cone harvesting is problematic due to great variation of maturity between and within northern Scots pine (Pinus sylvestris L.) seed lots. In forest seed centres maturity is traditionally evaluated on the basis of the embryo size using X-radiography, which is a relevant method for predicting germination potential of mature seeds. Still, except for embryo growth, maturation also includes morphological changes in the surface tissues, i.e. the seed coat, the nucellar layers, and the megaspore membrane, which we explored in this study. During two growing seasons 10–30 developing cones were harvested 11 times on two trees at each of three locations, at the boreal conifer forest line (68°48′ N), at the Arctic Circle (66°53′ N), and in central Finland (62°70′ N). Seeds were detached by hand for moisture content measurements, X-radiography, germination tests, light microscopic and field emission scanning electron microscopic studies. Our results confirmed the close relationship between temperature and the embryo growth, whereas maturation of the surface structures and the embryo were partly separate processes. Microscopy proved that biochemical maturation of the sheltering surface structures was partly light-reactive and continued as well as further development of the embryo for several weeks after the embryo had reached its full size. The reliability of radiography improved when the moisture content of seeds approached 30% and the temperature sum was about 800 degree days. The simple drying test of seeds from manually opened cones presented here can supply needed additional information for early prediction of the germination capacity particularly in the northern seed lots.