An evaluation of osmotic pre-sowing seed treatments as a potential method for improving the germination performance of Pinus radiata D. Don seeds : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Seed Technology at Massey University
This study was conducted lo characterise optimum conditions for osmotic pre-sowing treatment as an effective means of improving the germination and/or emergence performance of Pinus radiata from different seed grades.
The results indicated that osmotic treatment could reduce the germination and/or emergence times of Pinus radiata seeds by 40% of controls, if treated seeds were not subsequently dried back to original moisture contents. Osmotic treatment did not alter both the uniformity and final percentage germination. Rapid germination at this rate was only obtained if seeds were treated in optimum treatment conditions, i.e. with a -1.0 MPa solution for 10 d at 20° C. The correct choice of water potential and treatment duration is crucial in determining the level of treatment benefits. At high water potential, seeds were lost due to pre-germination during treatment, while at low water potential treatment benefits were less. Treatment with sail solutions (KNO3 + KH2PO4) was better than with polyethylene glycol, an effect which seemed to be a result of differing seed moisture content attained during treatment as no pre-germination occurred during PEG treatment while the moisture content of PEG-treated seeds attained following drying was less than that attained by salt-treated seeds,
Since seeds are kept in the imbibed state during treatment, the prevention of microbial proliferation is of prime importance. The use of Thiram at 1% seed weight and applied before osmotic treatment gave good protection against microbial attacks without losing treatment benefits in terms of rapid germination. Application of Thiram beyond its optimum rate should be avoided as it can delay seed germination.
Treated seed should not be dried back to low moisture contents rapidly, even at ambient temperatures (22-27° C, 50-6-% RH) as drying for 4 d in these conditions resulted in a complete loss of treatment benefit. However, slow drying of osmotically treated seeds at high relative humidity (20°C, 80-85% RH) prevented the adverse effects of desiccation on germination performance. Seed dried back in this way had 30% less in median germination times relative to untreated controls.
The response of osmotic treatment applied to different seed grades gave consistent results. Rapid germination due to osmotic treatment occurred in all seed grades at similar rates and was reflected in a significant increase in seedling dry weight. As the increases in seedling dry weight were more evident in larger or heavier seeds than in smaller or lighter seeds, it is suggested that osmotic treatment seems to influence relative growth at this stage of seedling development.
Osmotic treatment reduced the storability of seed, although applications of this treatment after storage restored the level of vigour of aged seeds which had just begun lo decline. Although total dehydrogenase activity in osmotically treated seeds was higher than in untreated controls, there was no difference in oxygen uptake between treated and untreated controls prior to radicle emergence. It was suggested that factors other than energy production are perhaps responsible for ensuring rapid germination of treated seeds.
The commercial implications of this study are potentially good. Osmotic treatment in tree seeds is no longer restricted lo using only polyethylene glycol. Improved seedling growth as a result of early emergence can help low vigour /moderate vigour seedlings become more vigorous and meet standard specifications required for outplanting.