Germination of tropical forage seeds stored in ambient and controlled temperature and humidity conditions

Correct storage of tropical forage seeds in the humid tropics is critical in order to safeguard germination. Storing Mulato hybrid brachiaria (Brachiaria ruziziensis x B. brizantha) seed under ambient conditions could be safely done for 8−12 months in Thailand, but rapid deterioration in viability occurred with longer storage, with seed being totally non-viable after 20 months storage (Hare et al. 2008). However, Mulato seeds kept in cold storage (10 C and 40% RH) for 3 years still maintained 80% germination. Similar results were found in northern Australia (Hopkinson and English 2005), where loss of viability of Panicum maximum, Brachiaria decumbens, Brachiaria humidicola, Setaria sphacelata and Chloris gayana seeds was rapid under ambient conditions with total death at 3 years. Under cold storage, maximum seed viability was still maintained after 6 years of storage. Embryo dormancy is reduced with age (length of storage) and is usually short-lived (i.e. several months) in many Brachiaria species. Hare et al. (2008) found that dormancy was quickly lost in Mulato seed stored at ambient temperatures but still persisted strongly after 3 years in cold storage. Hopkinson and English (2005) found that dormancy persisted longer under cool storage than under ambient storage. Forage seeds produced by Ubon Forage Seeds at Ubon Ratchathani University (Hare et al. 2013) are stored in a large commercial cool room (18−20 C and 50% RH), but there are no data on how long seeds can be safely stored in this room. The objectives of this trial were to study: the effects of ambient storage and cool-room storage on germination of our range of tropical forage seeds; and the persistence of embryo dormancy with storage.


Introduction
Storage of tropical forage seeds in the humid tropics is critical in order to safeguard germination. Storing Mulato hybrid brachiaria (Brachiaria ruziziensis x B. brizantha) seed under ambient conditions could be safely done for 8-12 months in Thailand, but rapid deterioration in viability occurred with longer storage, with seed being totally nonviable after 20 months storage (Hare et al. 2008). However, Mulato seeds kept in cold storage (10°C and 40% RH) for 3 years still maintained 80% germination. Similar results were found in northern Australia (Hopkinson and English 2005) where loss of viability of Panicum maximum, Brachiaria decumbens, Brachiaria humidicola, Setaria sphacelata and Chloris gayana seeds was rapid under ambient conditions with total death at 3 years. Under cool storage (10°C and 40% RH), maximum seed viability was still maintained after 6 years of storage.
Embryo dormancy is reduced with age (length of storage) and is usually short-lived (i.e. several months) in many Brachiaria species. Hare et al. (2008) found that dormancy was quickly lost in Mulato seed stored at ambient temperatures but still persisted strongly after 3 years in cool-storage. Hopkinson and English (2005) found that dormancy persisted longer under cool storage compared to ambient storage.
Forage seeds produced by Ubon Forage Seeds at Ubon Ratchathani University (Hare et al. 2013) are stored in a large commercial cool room (18-20°C and 50% RH). We do not know for how long seeds can be safely stored in this room. The first objective of this trial was to compare ambient storage and cool-room storage on germination on our range of tropical forage seeds. The second objective was to study the persistence of embryo dormancy with storage.

Materials and methods
This study is an ongoing trial being conducted at the Faculty of Agriculture, Ubon Ratchathani University, Thai-land. It commenced in January 2011 and will continue for as long as seeds continue to germinate. This paper reports on the germination results for the first two years from January 2011 to January 2013. Seeds of the following species, Mulato II (Bracharia ruziziensis x B. decumbens x B. brizantha), Mombasa guinea (Panicum maximum), Tanzania guinea (Panicum maximum), Ubon paspalum (Paspalum atratum) and Ubon stylo (Stylosanthes guianen-sis var. vulgaris x var. pauciflora) were studied. Mulato II seeds were divided into lots harvested from ground sweeping the seed and from knocking the seed from seedheads and these lots were also further divided into lots scarified in sulphuric acid for 10 minutes and non-acid scarified lots. Ubon stylo seeds were divided into sulphuric acid scarified and non-acid scarified lots. Each lot consisted of 3 kg of seed placed into commercial polyethylene bags.
Seeds were placed in two storage rooms (ordinary ambient conditions and a cool room). The ambient seed room was a seed storage shed at Ubon Ratchathani University and average monthly temperatures in this room varied from 22°C (January) to 34°C (July) and average daily relative humidity from 80% (January) to 95% (August-September). The cool room was at the Ubon Rice seed station and was maintained at 18-20°C and 50% RH throughout the study.
Seed samples were drawn in August and January of each year. Three lots of 100 seeds for each cultivar test were placed on petri dishes on top of filter paper wet with a 0.2% potassium nitrate solution and placed in a germination cabinet at 25°C with 16 h dark and 35°C with 8 h light. Germination counts were taken at 7 and 14 days.

Results
Seed germination of all cultivars, except Ubon stylo, deteriorated rapidly under ambient conditions with almost total death after 1 year of storage (Table 1). After 2 years storage, cool room conditions maintained seed germination of Mulato II and Ubon stylo but germination of Mombasa, Tanzania and Ubon paspalum seeds were starting to decline. Under ambient storage, only seed of Ubon stylo that was not acid scarified still produced high germination (94%) after 2 years ( Table 1).
Seeds of Mombasa and Tanzania guinea grasses showed improved germination following 8 months cool-room storage (Table 1). Seeds of Mulato II and Ubon stylo not scarified in acid and stored in the cool room, maintained very low germination (0-23%) but once scarified with acid, germinations increased to 75-88% for Mulato II and 99-100% for Ubon stylo (Table 1).

Discussion
This study indicates that under ambient storage conditions seeds of Mulato II, Mombasa guinea, Tanzania guinea and Ubon paspalum can not be safely stored, even for short periods (8-12 months). Seeds of these species were almost totally non-viable after 12 months. The study also indicates that seeds of Mulato II and Ubon stylo can be safely stored in controlled cool room conditions for 2 years, but seeds of Ubon paspalum and Mombasa and Tanzania guinea can only be safely stored in a cool room for 20 months after which germination starts to decline. The study also shows that embryo dormancy in Mulato II and hardseededness in Ubon stylo could not be broken by storage and persisted strongly after 2 years storage in a cool room, but embryo dormancy in Mombasa andTanzania guinea grasses was quickly broken by cool room storage.