Scheme optimization for seed dormancy breaking, germination and rapid seedling establishment in blueberry

Abstract:【Objective】Blueberry is a small fruit native to North America and cultivated worldwide. For decades, both the consumption and cultivation acreage of blueberries in China have been continually increasing. In the meantime, the industry faces great challenges. More than 80% of the new varieties were bred in America and have been released since the 1990 s, and now American takes the leading position of world blueberry industry. The situation greatly limits the introduction and use of excellent varieties in China. Therefore, to breed blueberry varieties with independent intellectual property rights is important to China's blueberry industry. In the past ten years, there has been a small number of new blueberry varieties generated from the local breeding programs in China. As a major route in breeding, the conventional artificial crossing is always a time-consuming process to obtain sufficient quantity of offspring for evaluation. It is essential to ensure germination and well development of every precious hybrid seed, which cannot be neglected during seed production. However, low germination rate of seeds has become a serious limiting factor retarding the efficiency in blueberry breeding. Generally, there are a number of measures to improve seed germination, such as suitable temperature range, relative humidity and so on.Actually, most of breeders pursue improvement of cultivation efficiency in order to shorten process under natural environmental conditions. Advanced measures should be applied natural and artificially-added seed germination. Consequently, we focused on several low cost operational measures in seedlingcultivation, and attempted to acquire a simple and acceptable scheme, which may substantially increase the germination rate and improve the efficiency in blueberry breeding process.【Methods】In this study, we investigated the germination rate and seedling growth during the past four years in cultivars'Bluecrop''Duke'and'Northland', and several offspring populations generated from different crossings. Our experiment included three main stages. Gibberellin (GA3) at eight concentration levels (0, 12.5, 25, 50, 100, 200, 400, and 800 mg· L-1) was tested to find the optimal concentration for seed germination and subsequent seedling growth. Once the optimal concentration of GA3 was chosen, 'Northland 'seeds from six storage treatments (fresh seeds stored at room temperature, fresh seeds at 4 ℃, fresh seeds at-20 ℃, dried seeds stored at room temperature, dried seeds at 4 ℃ and dried seeds at-20 ℃) were used for the evaluation and observation of germination and seedling after treatment with GA3. Finally, five different types of materials including sphagnum moss, peat, sand, soil and nutrient soil were used as the growth substrates for seed cultivation.【Results】1) GA3 treatment promoted dormancy break of the seeds. Blueberry varieties were sensitive to GA3 in terms of the germination rate.During a period of 60 d, the highest germination rate was observed under the treatment of 400 mg· L-1 of GA3. 2) The fresh seeds had a highest germination rate of 88%, followed by dried seeds stored at 4 ℃ (80%) , non-dried seeds stored at 4 ℃ (79%) and dried seeds stored at ordinary temperature (78%) . However, treatments of freezing seeds at-20 ℃ severely damaged the vitality of seeds (19%-23%) . 3) Our study showed that successful cultivation required suitable substrate materials, which might greatly affect seed germination rate and seedling growth. The'Northland'seeds had the highest germination rate of 88% (fresh seeds, 60 d) in sphagnum moss due to its loose texture as well as strong water retention and temperature buffering capacity, which was higher than in other substrate materials. Consistent with our expectations, the germination rate in the sand treatment was 10% and the lowest. Furthermore, we also explored the utilization of mixed nutrient soil throughout the whole growing stage. As a result, at the end of the season, the young seedlings reached a height between 31.2 and 60.3 cm, with an average value of 40.1 cm.【Conclusion】Pretreatment with 400 mg· L-1 GA3 by soaking seeds for 20 h was effective to remove the dormancy of blueberry seed. The fresh seeds at ordinary temperature had the highest germination rate. The substrate consisting of sphagnum moss in the upper layer and nutrient soil at the lower layer (Vpeat∶Vsoil∶Vcow dung/organic fertilizer=2∶1∶0.5, 1-1.5 kg sulphur per m3) could not only provide the requisite oxygen, moisture and stable temperature, but also ensure the demand for nutrients and acid environment for the growth of blueberry seedlings. Seedlings in such substrate emerged at days 35-40, then developed well with sturdy and complete branches. In this study, our scheme significantly increased the germination rate of hybrid seeds with sound development of the seedlings, and was low-cost and convenient for management. It meets the needs for large-scale breeding of blueberry.