Effect of storage duration, storage room and bag type on seed germination of brachiaria hybrid cv. Mulato

Transcription

1 Tropical Grasslands (2008) Volume 42, Effect of storage duration, storage room and bag type on seed germination of brachiaria hybrid cv. Mulato M.D. HARE, P. TATSAPONG AND S. PHENGPHET Faculty of Agriculture, Ubon Ratchathani University, Ubon Ratchathani, Thailand Abstract We determined germination of fresh seed of brachiaria hybrid cv. Mulato (Brachiaria ruziziensis B. brizantha) stored under ambient conditions at Ubon Ratchathani, Thailand (range in mean monthly temperature: C; range in mean monthly relative humidity: 80 94%), or in a cool room (10 C and 40% RH), and in three bag types (nylon, plastic and paper). Germination of seed stored under ambient conditions reached a peak (78%) at 4 months storage, and thereafter declined progressively until no seed germinated at 20 months. Germination of seed stored in nylon and paper bags under ambient conditions deteriorated more rapidly after 12 months storage than that of seed in plastic bags. Seed stored in the cool room reached peak germination (80%) at 14 months storage and maintained this level until the end of the experiment after 3 years. Bag type did not affect germination of seed stored in the cool room. Embryo dormancy was lost quickly in seed stored at ambient temperatures but persisted for several more months in cool-stored seed. Physical dormancy imposed by the tightly bound lemma and palea husks was still very strong in seed stored for 3 years in a cool room, with germination of only 28.6%. Scarification with sulphuric acid (96.1% concentration for 10 min duration) significantly increased germination to 80%. Correspondence: M.D. Hare, Faculty of Agriculture, Ubon Ratchathani University, Warin Chamrab, Ubon Ratchathani 34190, Thailand. michaelhareubon@hotmail.com Introduction In Thailand, seed of brachiaria hybrid cv. Mulato (Brachiaria ruziziensis B. brizantha) has been produced in small quantities since 2003 (Hare et al. 2007; Nakamanee et al. 2007; Phunphiphat et al. 2007). When harvested by hand in Thailand, seed is strongly dormant, requiring both seed storage and acid scarification to improve germination. The Mexican seed company, Grupo Papalotla, the head licensee for Mulato, requires that all Thailand-produced Mulato seed be stored for at least 6 months and then acid-scarified before sale to hasten and increase germination. Dormancy in Mulato and other Brachiaria species is imposed physically by tightly bound lemma and palea husks that cover the caryopsis, and physiologically by embryo dormancy. Scarification by soaking in concentrated sulphuric acid is the most common method of breaking physical dormancy by removing or breaching the husks. Embryo dormancy is reduced with age (length of storage) and is short-lived (i.e., several months) in many Brachiaria species. However, in some species (e.g. B. decumbens and B. dictyoneura), embryo and husk dormancy may last for between 10 months (Grof 1968) and 2 years (Hopkinson et al. 1996). Our hypothesis was that embryo dormancy would be short-lived and would be broken with storage, which made seed-storage conditions important, because Brachiaria seeds, in general, can deteriorate rapidly in storage. Sylvester- Bradley and Ferguson (1993) showed that B. dictyoneura seeds stored under room conditions (conditions not stated) for 6 months showed poor germination (0 10%), while seed stored in a cool room (conditions not stated) for a similar length of time showed 70 90% germination in 7 days. Therefore, care must be taken in storing Brachiaria seed to break embryo dormancy, as seed germination could be substantially diminished rather than improved.

2 Effect of storage conditions on seed germination 225 Our objective was to determine how storage of fresh Mulato seed for different lengths of time, under different storage conditions and in different types of bags, would affect germination, in an effort to define optimal storage conditions for Mulato seed. Materials and methods The experiment was conducted in the Faculty of Agriculture, Ubon Ratchathani University, Ubon Ratchathani, Thailand, for 3 years from January 2004 to January Mulato seed was handharvested from field plots in November 2003, dried to 8% seed moisture content, cleaned and stored in an open room at ambient temperatures in a cloth bag until the experiment commenced on January 23, Seed germination and seed moisture content on January 23, 2004 were 30% and 8.5%, respectively. The experiment was a factorial experiment with 2 storage rooms (ordinary ambient conditions and a cool room), 3 bag types (nylon, plastic and paper) and 3 replications. Mulato seed was divided into 20 g lots, placed in the bags and stored. For each germination test period, 18 bags of 20 g were drawn. Average monthly temperatures in the ordinary ambient room varied from 22 C (January) to 30 C (June) and average daily relative humidity from 80% (February) to 94% (September). The cool room was maintained at 10 C and 40% relative humidity throughout the study. The three bag types were close-weave nylon that readily allowed passage of water vapour, plastic of 9 µm thickness and brown paper of 70 µm thickness. Seed samples were drawn at 2 month intervals from bags in both rooms through 20 months storage and thereafter at 4 6 month intervals from the cool room until 36 months storage. At each sampling date, 18 bags were drawn and from each bag 2 lots of 120 seeds for each test were soaked for 10 minutes in concentrated sulphuric acid (96.1%), then washed in running cold water and dried. Two lots of 100 seeds for each test were drawn from the treated seed and placed in petri dishes on top of filter paper wet with a 0.2% potassium nitrate solution. Petri dishes were placed in a germination cabinet at 25 C with 16 h dark and 35 C with 8 h light. Distilled water was added as needed. Germination counts were taken at 7 and 14 days. At 24, 30 and 36 months storage, seed from the cool room was further tested with 3 bag types (as above) and 4 methods of germination testing [1. control (distilled water only); 2. potassium nitrate and distilled water; 3. sulphuric acid and distilled water; and 4. sulphuric acid, potassium nitrate and distilled water] and 3 replications. At each sampling date, 2 lots of 120 seeds for each test were taken and then two 100 seed lots tested for germination. Cabinet conditions and counts were the same as for earlier tests. Data were analysed by conventional analysis of variance, using the IRRISTAT program from the International Rice Research Institute (IRRI). Treatment means were compared by LSD at P = 0.05 probability level. Results Germination percentage of seed increased initially in both storage rooms, reaching similar peaks (about 80%), which occurred much earlier at ambient temperature (4 vs 14 months) (Figure 1). Germination then declined under ambient conditions, while germination levels were maintained in the cool room. The germination of seed stored under ambient conditions was higher (P<0.05) than that of seed stored in the cool room after 2, 4 and 6 months storage, with no difference (P>0.05) after 8 months storage. By this time, germination of seed stored under ambient conditions had decreased by 13.5 percentage points from the peak 2 months earlier. From 10 months storage onwards, seed stored in the cool room had higher germination than seed stored under ambient conditions. Germination of seed stored under ambient conditions was below 50% after 14 months storage and continued to decrease until no seed germinated after 20 months of storage (Figure 1). Germination of seed stored in nylon and paper bags under ambient conditions deteriorated more rapidly from months of storage than that of seed in plastic bags (Table 1). When stored in a cool room, germination of seed was largely unaffected by the type of storage bag, although some significant differences between bag types did occur on 2 occasions during the study (Table 2). Bag type had no effect on germination of seed using 4 different methods of germination (P>0.05), so the germination means of the bag types were pooled and the

3 226 M.D. Hare, P. Tatsapong and S. Phengphet Figure 1. Effects of seed storage room on seed germination of Mulato. LSD (P<0.05) values are presented as a bar. Each column is pooled across bag types C and 40% RH. 2 Range in mean monthly temperature: C; range in mean monthly relative humidity: 80 94%. results presented in Table 3. Mulato seed soaked in acid had 75% and 80% germination at 7 days and 14 days, respectively. This was times the germination rate of seed not soaked in acid (Table 3). Potassium nitrate did not affect germination of seed either with or without acid scarification (P>0.05). Discussion This study has provided information on which recommendations for the storage of Mulato seed can be determined. It indicates that Mulato seed can be safely stored under ambient conditions for short periods (8 12 months), but that rapid deterioration in viability occurs with longer storage, with seed being totally non-viable after 20 months storage. While the rate of decline in germination rate appeared slower in plastic bags than nylon and paper bags during this intervening period (12 20 months), this is of little consequence as storage under ambient conditions for this time period is unacceptable. For medium-term storage (longer than 12 months), cool storage is recommended. After 3 years of cool storage, germination of Mulato seed still exceeded 80% regardless of the type of storage bag. Based on these findings, if Mulato seed is harvested in the dry season (November in north-east Thailand) and then sown in the following wet season (6 months later in May), storing under ambient conditions is adequate. However, if seed is to be stored for periods of Table 1. Effects of bag type on seed germination of Mulato seed stored in an ambient room (Range in mean monthly temperature: C; range in mean monthly relative humidity: 80 94%). Nylon bag Paper bag (%) Plastic bag LSD (P<0.05) ns ns ns ns LSD (P<0.05)

4 Effect of storage conditions on seed germination 227 Table 2. Effects of bag type on seed germination of Mulato seed stored in a cool room (10 C and 40% RH). Nylon bag Paper bag (%) Plastic bag LSD (P<0.05) ns ns ns ns ns ns ns ns ns ns ns LSD (P<0.05) years, it should be stored in a cool room to maintain viability. Our data suggest that, under conditions like those for our study, seed stored under ambient conditions would be non-viable after 2 years. Hopkinson et al. (1996) demonstrated that dormancy diminishes progressively with age. Our results support this finding and show that embryo dormancy in Mulato seed is broken more quickly at ambient temperatures than at lower temperatures. While peak germination was reached at 7 months of age (3 months storage before the experiment plus 4 months storage during the experiment) under ambient conditions, embryo dormancy persisted longer in cool storage, as found in other tropical grass seeds (Harty et al. 1983; Hacker 1984; Hopkinson and English 2005a). Seed was 17 months of age (3 months storage before the experiment plus 14 months cool storage) before peak germination was reached. The rapid decline in germination of seed stored under ambient conditions after reaching peak germination shows that embryo death occurred quite rapidly once dormancy was broken under these conditions. By contrast, embryos remained viable until the end of the study for seed stored in the cool room. Physical dormancy, however, persisted in cool-stored seed as demonstrated by the low germination percentages of untreated seed (8.3 and 31.3% after 7 and 14 days, respectively) after 3 years storage. Hopkinson and English (2005b) discussed the persistence and even deepening of dormancy in tropical grass seed held in cold storage (10 C and 50% RH). They suggested the removal of seed from cold storage at least a year before its planned sowing. Hopkinson (1993) advised against use of acid treatment of seed in Australia as a means of breaking physical dormancy because it was dangerous and produced no consistent benefit in the field. However, we have found that sulphuric acid was quite successful in breaking dormancy and have developed a safe and practical method of applying it. Acid treatment increased Mulato seed Table 3. Effects of seed germination method on germination performance of Mulato seed stored in a cool room (10 C and 40% RH), pooled across bag types. Control (water only) KNO 3 Acid Acid + KNO 3 LSD (P<0.05) 7 days (%) LSD (P<0.05) ns ns 14 days (%) LSD (P<0.05) ns ns ns ns

5 228 M.D. Hare, P. Tatsapong and S. Phengphet germination to over 70% at 7 days and 80% at 14 days after 6 months storage, which are the standards at which Grupo Papalotla sells brachiaria hybrid seed (Eduardo Stern, Grupo Papalotla, personal communication). Farmers in Central and South America demand high 7-day germination in order to get rapid pasture establishment to compete with weeds. Mulato seed ground-harvested in Brazil will achieve 70% germination in 7 days without acid scarification (Eduardo Stern, Grupo Papalotla, personal communication). The lemma and palea husks closely cover the seed embryo and prevent the root and hypocotyls from emerging. While it appears that the husks of Mulato seed in contact with the ground deteriorate naturally before ground-harvesting, aging in cool storage does not soften these husks. We have tried other softening techniques such as rubbing, mechanical scarification and soaking in various detergents and none of these methods increased germination (M.D. Hare, unpublished data). Only acid scarification was effective. Acid scarification is now a streamlined method and can be carried out on a commercial scale both quickly and safely. In Thailand, we scarify Mulato seed in a concrete mixer for minutes with commercial grade sulphuric acid (98% concentration). In addition to hastening and increasing germination, acid scarification is a quarantine requirement of nearly all importing countries in Central and South America, especially for ground-harvested seed. Acknowledgements We thank the Thailand Research Fund (TRF) for providing financial support to this research program and the Faculty of Agriculture, Ubon Ratchathani University for research facilities. References GROF, B. (1968) Viability of seed of Brachiaria decumbens. Queensland Journal of Agricultural Science, 25, HACKER, J.B. (1984) Genetic variation in seed dormancy in Digitaria milanjiana in relation to rainfall at the collection site. Journal of Applied Ecology, 21, HARE, M.D., TATSAPONG, P. and SAIPRASET, K (2007) Seed production of two brachiaria hybrid cultivars in north-east Thailand. 3. Harvesting method. Tropical Grasslands, 41, HARTY, R.L., HOPKINSON, J.M., ENGLISH, B. and ALDER, J. (1983) Germination, dormancy and longevity in stored seed of Panicum maxmum. Seed Science and Technology, 11, HOPKINSON, J.M. (1993) Tropical pasture establishment. 2. Seed characteristics and field establishment. Tropical Grasslands, 27, HOPKINSON, J.M., SOUZA, F.H.D. DE, DIULGHEROFF, S., ORTIZ, A. and SÁNCHEZ, M. (1996) Reproductive physiology, seed production, and seed quality of Brachiaria. In: Miles, J.W., Maass, B.L. and Valle, C.B. do (eds) Brachiaria: Biology, Agronomy and Improvement. pp (CIAT: Cali, Colombia). HOPKINSON, J.M. and ENGLISH, B. (2005a) Influence of storage conditions on survival and sowing value of seed of tropical pasture grasses. 1. Longevity. Tropical Grasslands, 39, HOPKINSON, J.M. and ENGLISH, B. (2005b) Influence of storage conditions on survival and sowing value of seed of tropical pasture grasses. 2. Sowing value and storage strategies. Tropical Grasslands, 39, NAKAMANEE, G., PHAIKAEW, C., THINNAKHON, S. and KRUEMANGKORN, P. (2007) Brachiaria hybrids regional adaptability test. 1. Yield and chemical composition of herbage, seed yield and seed quality of Mulato, Mulato II and ruzi grass at Pakchong, Nakhonratchasima. In: Hare, M.D. and Wongpichet, K. (eds) Forages: A Pathway to Prosperity for Smallholder Farmers. Proceedings of an International Forage Symposium, Faculty of Agriculture, Ubon Ratchathani University, Thailand. p (Faculty of Agriculture, Ubon Ratchathani University: Thailand). PHUNPHIPHAT, R., PHAIKAEW, C., PHUNPHIPHAT, W. and NAKAMANEE, G. (2007) Brachiaria hybrids regional adaptability test. 2. Yield and chemical composition of herbage, seed yield and seed quality of Mulato, Mulato II and ruzi grass at Lampang. In: Hare, M.D. and Wongpichet, K. (eds) Forages: A Pathway to Prosperity for Smallholder Farmers. Proceedings of an International Forage Symposium, Faculty of Agriculture, Ubon Ratchathani University, Thailand. p (Faculty of Agriculture, Ubon Ratchathani University: Thailand). SYLVESTER-BRADLEY, R. and FERGUSON, J.E. (1993) Commercial seed production of tropical forage grasses and legumes in Costa Rica. Proceedings of the XVII International Grassland Congress, Palmerston North and Rockhampton, pp (Received for publication October 4, 2007; accepted April 23, 2008)