The water relations of seed pretreatments and their effects on the germination of radiata pine seeds

Seeds sown in Pinus radiata D. Don (radiata pine, Monterey pine) nurseries are generally the culmination of generations of breeding and selection. Seed germination characteristics have no generally been of high significance to the breeding programme and seedling emergence from valuable seeds is often low and variable. Seed pretreatrnents are sometimes used in an attempt to overcome variation in seed quality and to achieve best possible germination. The present study investigated commonly used seed pretreatments and their effect on the germination of Pinus radiata seeds in laboratory, glasshouse and field conditions. Current nursery practice is to soak seeds at or near room temperature prior to sowing. In this study it was found that soaking decreased the time to emergence in the laboratory and field but damage occurred to some seedlots when soaked. Dissolved oxygen in soak water was depleted rapidly during soaking and damage was prevented when soak water was aerated throughout soaking. Stratification (moist chilling) was once widely used to pretreat radiata pine seeds in Australia, but use of the practice has declined recently. Twelve weeks of stratification at 5 ° C decreased the time to emergence in the laboratory by up to 12 days. This effect decreased with shorter duration of stratification. After 12 weeks of treatment seeds began to germinate at low temperature and were susceptible to damage and desiccation during sowing. A third pretreatment not presently in commercial use examined the effect of stratifying while controlling seed water potential. A treatment was developed that enabled seeds to attain the benefits of stratification without risking damage. This was achieved by limiting the moisture available during treatment, by controlling the relative humidity of the environment in which the seeds were chilled. The treatment was as effective as conventional stratification in overcoming barriers to rapid germination. The seeds were fully hydrated by 4 weeks of treatment, but extending the treatment to 12 weeks further improved germination. This suggests that processes occur in fully hydrated seeds held at low temperature which overcome barriers to germination. In view of the interrelationship between metabolic activity and water uptake, aspects of water relations and gas exchange were investigated in more detail. An examination of seed water relations during imbibition at 200 C revealed that the embryo was apparently fully hydrated within 24 hours. Water potential of intact seeds rose to near -8 MPa within 24 hours. Final water potential achieved 6 days later before splitting and germination was also -8 MPa. After initial water uptake there appears to have been a readjustment of both water content and seed water potential over the next 4 days, as both declined following the rapid initial rise. The decline was also associated with increasing respiration, which suggests there may be active movement of water out of the seed after imbibition. After this period of readjustment potential increased rapidly up until embryo expansion and radicle emergence. During the period of readjustment of seed water potential there remained a potential difference of 7 -8 MPa between the extracted embryos and the remainder of the seed, in spite of available free water. Results are discussed in terms of the physiology of water uptake and seedling emergence and practical implications of commercial management of high value P. radiata seeds.