Seed priming positively affects Kentucky bluegrass

Seed vigor is a measure of seed's potential for rapid, uniform emergence under a range of field conditions. While genetics determines potential seed vigor, each seed lot's vigor can vary depending on environmental conditions influencing the mother plant, the maturity of the seed at harvest and any mechanical or disease-related injury to the seed. A seed lot's vigor can be physically altered by improving the purity of the seed or through size or density separation and physiologically by various methods including "seed priming."

Seed priming, or pre-hydration seed treatment, is a technique for enhancing seed vigor and, thus, improving overall germination and seedling development. Two seed-priming methods exist: osmotic priming (OP) and solid-matrix priming (SMP). In OP-the most common priming method-the seeds are soaked in aerated osmotic solutions containing either potassium nitrate (KNO3) or potassium phosphate (K3PO4) salts or polyethylene glycol. These solutes dissolve in water at a concentration diluted enough to permit seeds to imbibe water and initiate pre-germination metabolism but concentrated enough to prevent emergence of the radicle, or primary root.

Alternatively, in S MP-a more recently introduced method-the seeds are soaked in an aqueous suspension of such solid media as finely ground lignite, coal substances, vermiculite, hydrous silicate clay or calcined clay. With SMP, the chemical and physical characteristics of the solid materials, as well as the water potential (amount of available water), control the absorption of water by the seed.

Priming results in such improvements in seed performance as faster and more uniform germination, higher percent germination and more-vigorous seedling development in many species, especially where moisture stress and sub-optimal temperatures occur. Predicting field performance of primed seed is complicated, however, because the response to priming treatments can vary both within and among seed lots. Moreover, various environmental conditions may accelerate or diminish the priming effects. In field studies we conducted at The Pennsylvania State University with several cool-season turfgrass species, the greatest response to seed priming occurred with Kentucky bluegrass-the slowest germinating species. This response was especially pronounced under cooler temperatures, such as those typically encountered in early to mid-fall.

SMP effects on field emergence Table 1 (opposite page) compares percent seedling emergence of SMP-treated and untreated seeds of 11 Kentucky bluegrass cultivars under optimal planting conditions in late August. We calculated the percent seedling emergence as follows: (number of seedlings per unit of area) per (number of pure live seeds per unit of area) x 100. The average maximum and minimum air temperatures during this experiment were 71 degrees and 51 degreesF, respectively. We found the SMP effects to be especially prominent in the early stage of emergence (8 days after seeding). Ten days after seeding, the percent of seedling emergence from SMP-treated seed was higher than that from untreated seed for most of the cultivars we examined.

Under warmer conditions, the SMP treatment did not increase the final percent of seedling emergence of Kentucky bluegrass, but it did increase the speed of germination. The responses we observed under field conditions resulted from the combined effects of original seed vigor, priming treatments and environmental conditions. However, the effects of SMP varied among cultivars. Based on results of effectiveness of SMP in this experiment, we separated the 11 KBG cultivars into three groups: A. Most effective: Glade, Gnome, Marquis, Tendos and 229 B. Intermediate: Adelphi, Merit, Ram 1 and Rugby C. Essentially ineffective: Estate and Limousine.

We conducted a second seeding in mid-October, when the average maximum and minimum air temperatures during the experiment were 58 degrees and 38 degreesF, respectively. One week after seeding, seedlings began to emerge from SMP-treated seeds in 8 of 11 cultivars, while none emerged from the untreated seeds (see Table 2, below). At 2 weeks after planting, seedlings emerged from all of the SMP-treated cultivars except Gnome. The percent emergence of SMP-treated Glade was more than 73 percent, while that of untreated Glade seed was less than 1 percent. After 3 weeks, a substantial number of seedlings emerged from untreated seed. However, the percent emergence of SMP-treated seed was significantly greater in 9 of 11 cultivars. The SMP treatments enhanced percent seedling emergence and reduced the number of days required for emergence under cooler conditions. Based on these results, we reclassified the cultivars as follows (the letters in the brackets indicate the previously assigned groups): A. Most effective: Adelphi [B], Glade [A], Gnome [A], Marquis [A], Merit [B], Ram 1 [B], Rugby [B], Tendos [A] and 229 [A] B. Intermediate: Limousine [C] C. Essentially ineffective: Estate [C].

Cultivars in groups A and B in the previous experiment were now jointly classified as Group A (most effective). Limousine, a cultivar for which SMP treatment was essentially ineffective in the previous experiment, was now classified as Group B (intermediate), while Estate remained in Group C (essentially ineffective).

SMP effects under moisture stress In a separate study, we examined SMP's effects on the percentage germination of Limousine and Marquis Kentucky bluegrass at several water potentials (basically, the level of wetness) during germination. The initial water potentials ranged from 0.0 MPa (pure water) to -1.5 MPa (least-available water) at 0.3-MPa increments, and we incubated the seeds under a constant temperature of 15 degreesC (59 degreesF) in the dark for 4 weeks. In addition to percent germination, we estimated the speed of germination by calculating the mean time for germination (MTG).

The MTG correlated strongly with the amount of water the seed absorbed after 24 hours. We examined SMP effects on germination by plotting cumulative germination as a function of days after incubation (see Figure 1, page G 28). Cumulative germination varied among cultivars and between SMP treatments. For untreated seed, the germination of Limousine seed was faster and more uniform at 0.0 and -0.3 MPa than Marquis. However, Marquis achieved a higher percent germination at 0.0 and -0.3 MPa despite a 2-day lag in initial germination behind Limousine. SMP-treated Limousine initiated its germination on Day 3, 2 days prior to the germination of untreated seed. However, the final percent germination of SMP-treated Limousine seed was lower than for untreated seed, irrespective of water-potential levels. For Marquis, SMP treatment substantially enhanced the initiation of germination, germination uniformity and final percent germination at 0.0, -0.3, and -0.6 MPa. Furthermore, the percent germination of Marquis SMP seed at -0.6 MPa was nearly the same as that of untreated seed at -0.3 or even 0.0 MPa.

Initial vigor levels of seed lots and priming-treatment conditions can affect the response of seed. For example, the SMP-treated seed of Limousine we used in this study lost its vigor, as final percent GRM and uniformity figures indicate. At about 4,000 seeds per gram, Limousine is much smaller than Marquis, which has about 2,000 seeds per gram. Thus, Limousine seed might suffer from over-priming when the two cultivars are primed under similar conditions. Ideally then, you should know the best priming conditions for each cultivar or seed lot to obtain its maximum response. However, because that may not be practical, you should use the least-negative water-potential solution for most of the cultivars within a particular species and the minimum duration as a standard priming method. Standard methods may not provide the maximum response for every cultivar but can enhance seed performance of all cultivars without resulting in any negative effects. The degree of priming response variation due to environmental conditions suggests that we should further examine the priming response under various environmental conditions.

SMP effects under temperature stress Laboratory germination under sub-optimal conditions is one of the primary methods for examining the level of seed vigor. Therefore, we examined SMP effects on germination under various temperatures using four seed lots of Dawn Kentucky bluegrass. The alternative temperature regimes discussed below are 25 degrees/15 degreesC (77 degrees/59 degreesF) and 15 degrees/5 degreesC (59 degrees/41 degreesF) under light and darkness for 8 and 16 hours, respectively.

As Figure 2 (page G 30) shows, germination was faster and more uniform for SMP-treated seed lots than for untreated lots at 25 degrees/15 degreesC (an optimum condition). In addition, the germination responses of SMP-treated seed were similar among seed lots (except Lot 2, for which the distribution was only initially similar to the others). Unlike the SMP-treated seed, the germination distributions of untreated seed were not uniform among the seed lots. In contrast to the final percent germination, which was higher for all untreated lots, all SMP-treated seed lots germinated earlier than untreated lots, and MTG was significantly shorter at all temperature regimes. The SMP-treated Lot 2, for which the final percent germination after SMP treatment was significantly reduced, still germinated more quickly and uniformly, providing a shorter MTG than the untreated seed. SMP treatments minimized the variable germination responses of different seed lots, indicating that SMP may render vigor levels more uniform among individual seeds within a seed lots. The extremely low final percent GRM of SMP-treated Lot 2 might be due to the failure of SMP treatments or possible post-priming deterioration.

SMP treatments significantly increased the speed of germination for all seed lots at 15 degrees/5 degreesC (a sub-optimum condition). However, the magnitude of the effect differed among seed lots. For example, the difference in MTG between SMP-treated and untreated seed of Lot 1 was 5 days, while that of Lot 4 was only 2 days. Thus, the effects of SMP treatment were greatest in the slowly germinating (low-vigor) seed lots. This suggests that SMP treatments may bring the MTG to a constant that is specific for the cultivar (minimum MTG determined by genotypes).

Germination distributions after priming were more uniform among seed lots. The degree of response varied among seeds, depending on original levels of vigor. (SMP treatment cannot enhance the maximum genetic level of seed vigor.) However, seed performance within a cultivar was similar in all lots after priming. After optimum priming treatments, greater responses appeared in low- compared to high-vigor lots.

Seedling emergence is a critical stage in turfgrass establishment that can dramatically influence the subsequent management and use of the turf. Therefore, the use of SMP seed, especially under cool conditions and with selected cultivars of Kentucky bluegrass, could be a practical and economical choice.

Ikuko Yamamoto is a former graduate research assistant at The Pennsylvania State University-University Park (and former post-doctoral scholar at Texas A&M University). A. J. Turgeon is a professor of Turfgrass Management and director of the Educational Technologies Program in Agricultural Sciences at The Pennsylvania State University.

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