The Effects of Short-term Drying on Seed Germination in Rosa

Abstract

Seed propagation of roses is used for breeding new cultivars, native plant restoration, selected rootstocks, and some roses used for hip production (Gill and Pogge, 1974; Krussmann, 1981; Weremark et al., 1995). Rose achenes (seeds) are notorious for having low viability and erratic germination (Buckley, 1985). Factors such as genetics, temperature during both seed maturation and stratifi cation, scarifi cation, pericarp removal, and plant growth regulators can affect germination frequency and uniformity (Buckley, 1985; Gudin et al., 1990; VonAbrams and Hand, 1956). Satisfactory viability has been retained for multiple years using dry longterm seed storage of Rosa multifl ora Thunb. and other Rosaceous species (Crocker, 1926; Crocker and Barton, 1931). Short-term drying ( 500 seeds of each cultivar or species were allowed to air dry for four days, and at the same time >500 seeds of each cultivar or species were kept continually moist wrapped in damp paper towels within sealed polyethylene bags. After treatment, all seeds were submerged in distilled water for 24 h. Five replications of 100 seeds were counted for each moisture treatment per cultivar or species per year. Each replication of seeds was mixed with 80 cm of moistened Canadian sphagnum peatmoss (Hyponex Corporation, Marysville, Ohio) and sealed in a 16.8 × 14.9-cm polyethylene bag (Glad zipper bags, Oakland, Calif.). Seeds were cold stratifi ed for 12 weeks at 4 °C and then moved to 14 °C for the duration over which germination was observed (11 weeks in year 1; 16 weeks in year 2). Bags were monitored once per week at which time germinating seeds (seeds with an emerged radicle) were removed and counted. The 2 weeks (X and X +1 ) bracketing the 50% total germination value and their cumulative germination values (C 1 and C 2 , respectively) were used to calculate T 50 for each replication with the following formula. Student’s t tests were calculated using SPSS software (SPSS 11.0 for Windows, Chicago, Ill.): T 50 = X + [(50% total germination – C 1 )/(C 2 – C 1 )]. Short-term drying reduced total percent germination and altered T 50 for R. rubiginosa and R. rugosa, but did not signifi cantly affect germination of the other cultivars or species relative to constant moisture (Table 1, Fig. 1). A signifi cantly longer T 50 in response to shortterm drying was found for R. rubiginosa and shorter T 50 for R. rugosa relative to constant moisture (Table 1). Some may consider the year 1 reduction in total percent germination (6.2% reduction) for ‘Bucbi’ due to short-term drying to be slightly signifi cant (P = 0.10) depending on where one sets their signifi cance threshold, however, in year 2 a trend towards divergence between moisture treatments for ‘Bucbi’ (0.2% reduction; P = 0.96) was not observed (Table 1, Fig. 1). The total percent germination was greater in year 2 at week 11 for both ‘Bucbi’ moisture treatments and the continuous moisture treatment for R. rubiginosa (Fig. 1). Different degrees of physiological dormancy across seed lots collected from the same parents in different years is common for many plant species and can be due to differing environmental conditions (Baskin and Baskin, 1998). Depending on cultivar or species, germination improved or stayed the same when rose seeds were kept moist from harvest through germination relative to short-term drying after harvest. Roses and other Rosaceous species are not generally thought of as having recalcitrant seeds, since viability is generally satisfactory after drying (Crocker and Barton, 1931). In breeding programs where there is a high investment in hand-pollinated seed, not allowing seeds to dry out before stratifi cation treatments could maximize both rapid and a high fi nal percent of germination. Since commercial rose breeding programs generally try to obtain rapid germination after achene harvest and then raise seedlings in greenhouses (Krussmann, 1981), keeping seeds continually moist should not interfere with established protocols and could easily be adopted. On the other hand, if seed is abundant, limited resources may be best allocated using dry storage to delay stratifi cation treatments and germination, a situation which may occur when raising open-pollinated