Effect of Priming Temperature on Storability of Primed sh-2 Sweet Corn Seed K. Y. Chiu, C. L. Chen, and J. M. Sung* ABSTRACT specific, depending on the nature of the storage and Priming offers a means to raise seed performance in many crop priming conditions. species, but the longevity of primed seed is generally decreased. The Van Pijlen et al. (1996) demonstrated that the adverse exact causes of the more rapid deterioration of primed seed are still not effects of priming during storage were caused by de- established. This study evaluated the effects of priming and storage creased DNA repair activity resulting from progression temperatures on germination and antioxidative activities of sweet corn in the cell cycle. Increased lipid peroxidation, mediated seed (Zea mays L.) carrying the shrunken-2 (sh-2) gene. Seed were by AOS attack during desiccation in the absence of solid-matrix primed in moistened vermiculite at 10, 15 or 20 C for protection mechanisms (e.g., superoxide dismutase and 36 h, then air-dried to near original moisture level. Primed seed were stored at 25, 10, or 80 C for up to 12 mo. Solid-matrix priming im- catalase) was also involved in reducing the longevity of proved germination, reduced lipid peroxidation, enhanced antioxida- primed seed (Bruggink et al., 1999). tive activities, and increased seedling growth. Seed longevity was The environment during priming also influences the decreased when 20 C-primed seeds were stored at 25 C for 12 mo. seed response to priming. Bradford (1986) reported that Seed primed at 10 and 15 C had superior viability and vigor responses priming at 15 C resulted in enhanced seed performance compared with nonprimed control seeds when they were stored at for beet (Beta vulgaris L.), carrot, celery (Apium graveo- 25 C for 12 mo. Reduced storability of the 20 C-primed seeds was lens L.), lettuce, onion, corn, and soybean [Glycine max attributable to enhanced peroxidation and decreased antioxidative (L.) Merr.]. Ozbingol et al. (1998) found that the opti- activities. Storage at 10 or 80 C extended the storability of matrix mum temperature (27–28 C) for priming was the same primed sh-2 seed for at least 12 mo. Enhanced antioxidative activity as the optimum temperature for germination of tomato plays a role in maintaining the viability and vigor responses of solid matrix primed seed stored at cool (10 C) or subzero ( 80 C) tempera- seed. Haigh et al. (1986) concluded that temperatures tures. Moreover, 10 or 15 C-primed sh-2 seed can retain viability for during priming (15, 20, or 25 C) had little effect on 12 mo, provided that the primed seed is stored at 10 C. subsequent emergence responses of onion seeds. These findings suggest that different temperatures must be evaluated for each crop species to determine which pro- vides the best priming result. S eed deterioration is associated with loss of mem- brane integrity, changes in enzymatic activities, de- cline in protein and nucleic acid synthesis, and lesions Accumulated evidence shows that 20 C-primed sh-2 seed deteriorated more rapidly than nonprimed sh-2 in DNA (McDonald, 1999). These deteriorative changes seed when they were stored at 10 or 25 C for 12 mo have frequently been related to activated oxygen species (Chang and Sung, 1998). Since it is known that subzero (AOS)-induced oxidative injury (Hendry, 1993; Bernal- storage temperature preserves seed better do than cool Lugo and Leopold, 1998; McDonald, 1999). Priming can (10 C) and room (25 C) temperatures (Roos, 1989), it reverse some of the aging-induced deteriorative events, is reasonable to hypothesize that the longevity of primed and thus improve seed performance (Taylor et al., 1998). seed stored at 80 C may be greater. Moreover, longev- One crucial determinant of primed seed performance ity may vary among the seed primed and stored at differ- is postpriming storage environment. Long-term priming ent temperatures. Thus, to test these hypotheses, we (10 d) of leek (Allium porrum L.) and onion (Allium investigated the germination, seedling growth, and lipid cepa L.) seeds stored at 10 C retained viability after 1 yr peroxidation of sh-2 sweet corn seed primed at 10, 15, (Drew et al., 1997). Carrot (Daucus carota L.) seed or 20 C stored at 25, 10, and 80 C for 12 mo. The primed for 17 d lost some viability after 12 mo of 10 C changes in activity of several peroxide-scavenging en- storage (Dearman et al., 1987). Primed tomato (Lyco- zymes in relation to priming, storage duration, and stor- persicon lycopersicum L.) seed (7 d priming) stored at age temperature are also measured. Collectively, these 10 C retained viability for 12 mo, but at 30 C viability data provide more insights about how priming and stor- was reduced (Alvarado and Bradford, 1988). A higher age temperatures affect sh-2 sweet corn seed longevity. viability was reported for sweet pepper (Capsicum an- nuum L.) seeds primed for 4 d, following 3 yr of storage MATERIALS AND METHODS at 25 C (Thanos et al., 1989). In contrast, 1-d, short- Seed Materials term priming decreased the storage life of lettuce (Lac- tuca sativa L.) seed (Tarquis and Bradford, 1992). To- A commercially produced seed lot of sh-2 sweet corn hybrid mato seed receiving 1 d of priming, however, had better cultivar Honey 236 (92 g kg 1 on fresh weight basis) was ob- tained from a local vendor. For priming treatment, 135 repli- storability (Van Pijlen et al., 1996). It appears that the cates each of 150 g of seed were mixed with 300 g of vermiculite effects of priming on seed longevity may be species Dep. of Agronomy, National Chung Hsing Univ., No. 250, Kuokuang Abbreviations: AOS, activated oxygen species; APX, ascorbate perox- Rd., Taichung, Taiwan 402, ROC. Received July 31, 2001. *Corre- idase; ASC, ascorbate; CAT, catalase; DHA, dehydroascorbate; GR, sponding author (email@example.com). glutathione reductase; GSH, reduced glutathione; GSSG, oxidized glutathione; MDA, malondialdehyde; MGT, mean germination time; Published in Crop Sci. 42:1996–2003 (2002). SOD, superoxide dismutase. 1996 CHIU ET AL.: STORABILITY OF PRIMED SWEET CORN SEED 1997 Fig. 1. Germination, mean germination time (MGT) and seed moisture level of sh-2 sweet corn seed primed under different temperatures and nonprimed control stored at 25, 10, and 80 C for up to 12 mo. Vertical bars represent the mean and SE of three replications. LSD (P 0.05) for germination, MGT and seed moisture level, across all the treatments, are 5.99, 0.45, and 0.04 respectively. No. 3 to which 375 mL of distilled water were added (Sung replicates that were soaked in 20 mL of deionized water at and Chang, 1993), sealed in a plastic bag, mixed to provide 25 C for 24 h. Leachates were measured with a conductiv- uniform seed-substrate contact, and incubated at 10, 15, or ity meter (Suntex, model 17A, Taipei, Taiwan; cell constant 20 C for 36 h to test temperature effects. Chiu (2000) reported 1.217). that, under 10, 15 or 20 C priming temperature, 36 h of hydra- tion duration was optimum for the tested seed lot. The partially Determinations of Peroxidative Products hydrated seeds were then separated from the vermiculite, and air-dried at 25 C for 48 h to near original moisture level. After Malondialdehyde (MDA) and total peroxide were deter- drying, all the seeds were sealed in aluminum foil bags coated mined on three replicates of 5 seeds. The seeds were hand- with polyethylene and stored at 25, 10, or 80 C (45 bags per ground in a mortar and pestle with 4 mL 50 g L 1 trichloroace- storage treatment) for up to 12 mo. tic acid at 4 C and then centrifuged at 14 000 g for 20 min. The supernatants were used for MDA (Heath and Packer, Germination Test 1968) and total peroxide (Sagisaka, 1976) determinations. For Maillard reaction product (as indicated by browning intensity) Seed germination and moisture content were determined determinations, three replicates of 5 seeds were hand-ground at 0, 3, 6, 9, or 12 mo storage. Laboratory germination tests with 4 mL of deionized water at 4 C in motar and pestle and were conducted in three replicates of 50 seeds. Seed was then centrifuged at 14 000 g for 20 min. The supernatants were planted 1.5 cm deep in a plastic tray filled with 100 g No. 3 used for browning intensity measurements (Yen and Lai, 1987) vermiculite and watered with 200 mL of distilled water. Seed was incubated in controlled chambers with 12-h photoperiod and 300 W m 2 light intensity at 25 C and watered as necessary. Determinations of Enzymes Activities Germination was defined as the point coleoptiles were visible For enzyme activity measurements, three replicates of 5 above the vermiculite surface and counted daily for 10 d for seeds were hand-ground at 4 C in a mortar and pestle with mean germination time (MGT) calculation (Chang and Sung, 4 mL of 0.1 M potassium phosphate buffer (pH 7.0), followed 1998). Seed moisture contents (fresh weight basis) were deter- by centrifugation at 20 000 g for 20 min. The supernatants mined by sampling three replicates (20 seeds per sample) and were used for determination of enzyme activity. Superoxide weighing seed before and after heating in a forced-air oven dismutase (SOD; EC. 188.8.131.52) activity was assayed according at 103 C for 24 h. Seedling dry weights were determined 14 d to the method of Stewart and Bewley (1980). Catalase (CAT; after sowing. Leaching tests were conducted in three 10-seeds EC. 184.108.40.206) activity was assayed according to the method of 1998 CROP SCIENCE, VOL. 42, NOVEMBER–DECEMBER 2002 Fig. 2. Seedling dry weight, seed leakage and soluble protein content of sh-2 sweet corn seed primed under different temperatures and nonprimed control stored at 25, 10, and 80 C for up to 12 mo. Vertical bars represent the mean and SE of three replications. LSD (P 0.05) for seedling dry weight, seed leakage, and soluble protein content, across all the treatments, are 1.85, 1.23, and 0.05, respectively. Kato and Shimizu (1987). Glutathione reductase (GR; EC droascorbate (DHA) content was deduced as the difference 220.127.116.11) activity was determined according to the method of between total ascorbate and ascorbate contents. Foster and Hess (1980). Ascorbate peroxidase (APX; EC 18.104.22.168) and dehydroascorbate reductase (DHAR; EC 22.214.171.124) activities were determined by the methods of Nakano Statistics and Asada (1981). The SOD activity was expressed as the The experimental design for emergence test was a random- unit of enzyme activity needed to inhibit the reaction by half ized complete block design with three replicates. Percentage per g fresh weight per min. The activities of CAT, DHAR, data were arcsin-transformed before analysis. All data were GR, and APX were expressed per gram fresh weight, and one subjected to an analysis of variance, and a LSD was calculated unit represented 1 mol of substrate undergoing reaction per when a significant (P 0.05) F ratio occurred for treatment minute. Portions of enzyme extract were used for determina- effects. tion of soluble protein (Sung and Chang, 1993). RESULTS Determinations of Glutathione and Ascorbate Contents Germination and Seedling Dry Weight Glutathione was determined on three replicates of 5 seeds. Unstored nonprimed seeds used in the study had 88% The seeds were hand-ground in a cold mortar and pestle with germination and 4.1 d MGT (Fig. 1A). No changes in 4 mL ice-cold 50 g L 1 sulfosalicyclic acid and centrifuged at seed moisture content occurred during storage (Fig. 1G). 20 000 g for 20 min. The supernatants were used for total Nonprimed seeds stored at 25 C for up to 6 mo showed glutathione, reduced glutathione (GSH), and oxidized gluta- no evident decline in germination percentage, but thione (GSSG) determinations (Smith, 1985). For ascorbate marked reductions in germination percentage and ex- and dehydroascorbate determinations, a modification of the method of Law et al. (1983) was used. Three replicates of 5 tensions in MGT (Fig. 1D) were observed for non- seeds were homogenized in a cold mortar and pestle with 4 mL primed seeds stored for 9 and 12 mo. Germination per- ice-cold 50 g L 1 trichloracetic acid solution and centrifuged at centage and MGT of nonprimed seeds were decreased 16 000 g for 20 min, and 5 L of supernatant was used for slightly by 12 mo of 10 C storage (Fig. 1B and E). Elec- total ascorbate and ascorbate (ASC) determinations. Dehy- trolyte leakage increased and seedling dry weight de- CHIU ET AL.: STORABILITY OF PRIMED SWEET CORN SEED 1999 Fig. 3. Malondialdehyde, total peroxide and browning intensity of sh-2 sweet corn seed primed under different temperatures and nonprimed control stored at 25, 10, and 80 C for up to 12 mo. Vertical bars represent the mean and SE of three replications. LSD (P 0.05) for malondialdehyde, total peroxide, and browning intensity, across all the treatments, are 0.05, 2.56, and 0.05, respectively. creased for sh-2 seeds stored at 10 C (Fig. 2C), whereas seed stored at 25 C for 6 to 12 mo accumulated more germination percentage and seedling dry weight of non- MDA, total peroxide, and browning products (Fig. 3A, D primed seeds stored at 80 C for 12 mo were unchanged and G). Nonprimed seed stored at 10 and 80 C for 6, (Fig. 1C and 2C). 9, and 12 mo accumulated less MDA, total peroxides, Nonstored primed seeds performed better than non- and browning products than those stored at 25 C (Fig. 3). primed seeds (Fig. 1A). Primed seeds also had reduced Nonstored primed seed showed lower levels of MDA electrolyte leakage and produced heavier seedling and total peroxide accumulation and lower browning in- (Fig. 2A and D). Priming temperature decreased stora- tensity than nonprimed seed; however, levels increased bility of primed sh-2 seeds. Germination decreased 70% gradually with up to 12 mo of 25 C storage (Fig. 3A, D, for seeds primed at 20 C and stored at 25 C for 12 mo. and G). Seed primed at 20 C showed steep increases in Seeds primed at 10 or 15 C and nonprimed seeds had MDA, total peroxide, and protein browning after 3 mo similar germination percentage and MGT (Fig. 1A and of storage at 25 C, and gradual increases with longer D). The MGT of seeds primed at 20 C and stored at storage (6 to 12 mo), and these accumulations were 25 C for 12 mo was 6 d, whereas it was 4 to 4.5 d for greater than those for nonprimed seed. The levels of seeds primed at 15 and 10 C (Fig. 1D). At 10 and 80 C MDA, total peroxide, and browning products for 10 and 15 C-primed seed also increased following 12 mo storage, primed seeds had 91 to 97% emergence after storage, but the extent of accumulation were lower than 12 mo storage, respectively, and MGT averaged about nonprimed seeds. Primed seed stored at 10 C for 6 to 4.1 d (Fig. 1C and F). Storing the primed seeds at 10 C 12 mo accumulated less MDA, total peroxides, and brown- resulted in decreased seedling dry weight after 12 mo ing products than 25C-stored primed seed (Fig. 3B, E, storage, particularly for 20 C-primed seeds (Fig. 2B). and H). The MDA, peroxides, and browning products Only slight declines in seedling dry weight were ob- in 80 C-stored primed seeds were lower than those of served for primed seeds stored at 80 C (Fig. 2C). 10 C-stored primed seeds (Fig. 3C, F, and I). Lipid Peroxidation and Protein Browning Antioxidative Systems Unstored control seeds had measurable levels of MDA, The activities of antioxidative enzymes in both 25 and total peroxides and browning products (Fig. 3A). The 10 C-stored nonprimed seeds decreased during 12 mo 2000 CROP SCIENCE, VOL. 42, NOVEMBER–DECEMBER 2002 Fig. 4. Activities of superoxide dismutase, catalase, and ascorbate peroxidase of sh-2 sweet corn seed primed under different temperatures and nonprimed control stored at 25, 10, and 80 C for up to 12 mo. Vertical bars represent the mean and SE of three replications. LSD (P 0.05) for superoxide dismutase, catalase, and ascorbate peroxidase activities, across all the treatments, are 0.39, 1.98, and 3.54, respectively. storage (Fig. 4 and 5). Under 80 C condition, only SOD, found for primed seed stored at 10 and 80 C, but the APX, and GR showed a decrease in enzyme activity dur- changes were less pronounced than for primed seed ing 12 mo storage. Primed seed generally exhibited stored at 25 C. greater activities of antioxidative enzyme (except dehy- Both ASC and DHA for nonprimed seed stored at droascorbate reductase, DHAR) than nonprimed con- 25, 10, or 80 C decreased linearly, with 80 C-stored trol seed before storage. Under 25 and 10 C storage seed having the lowest rate of decline (Fig. 7). Primed conditions, the activities of antioxidative enzymes de- seed had more ASC and less DHA than nonprimed creased with increasing storage duration, with the rates seed (Fig. 7). Both ASC and DHA in primed seed stored of decline for 10 and 15 C-primed seed lower than for at 25, 10, or 80 C decreased with increasing storage 20 C-primed seed. At 80 C storage, only slight de- duration, but the decline was much less for 80 C- creases in SOD, APX, and GR activities occurred with stored seed. After 12 mo storage, 10 or 15 C-primed increasing storage duration for primed seeds (Fig. 4 seed had greater ASC and less DHA (except for the and 5). Marked decreases in soluble protein content primed seed stored at 25 C) levels than 20 C-primed occurred for the nonprimed and primed seed during seed. storage (Fig. 2); the decreases were less with 10 and 80 C storage (Fig. 2H and I) compared with 25 C- stored seeds (Fig. 2G). DISCUSSION Marked decreases in GSH content were accompanied In the presence of oxygen, activated oxygen species by increases in GSSG for nonprimed seed stored at 25 (AOS) often are generated through autooxidation within or 10 C (Fig. 6B and E). The GSH and GSSG contents dry seed (Hendry, 1993; McDonald, 1999). In fresh sh-2 of nonprimed seed were unchanged at 80 C storage sweet corn seed receiving priming treatments, AOS is (Fig. 6C and F). Primed seed had greater GSH content kept at low levels by the priming-enhanced cooperative than nonprimed seed during storage. GSH decreased catalysis of AOS-scavenging enzymes (e.g., SOD and rapidly during 25 C storage, particularly for 20 C-primed CAT) and antioxidants (e.g., GSH and ASC) Sung and seed. GSSG increased markedly for 20 C-primed seed Chiu, 2001). Chang and Sung (1998) reported that long- stored at 25 C. Similar GSH and GSSG responses were term 25 C storage resulted in impairment of AOS scav- CHIU ET AL.: STORABILITY OF PRIMED SWEET CORN SEED 2001 Fig. 6. Reduced glutathione (GSH) and oxidized glutathione (GSSG) Fig. 5. Activities of glutathione reductase and dehydroascorbate re- of sh-2 sweet corn seed primed under different temperatures and ductase of sh-2 sweet corn seed primed under different tempera- nonprimed control stored at 25, 10, and 80 C for up to 12 mo. tures and nonprimed control stored at 25, 10, and 80 C for up Vertical bars represent the mean and SE of three replications. LSD to 12 mo. Vertical bars represent the mean and SE of three replica- (P 0.05) for GSH and GSSG, across all the treatments, are 16.57 tions. LSD (P 0.05) for glutathione reductase and dehydroascor- and 0.93, respectively. bate reductase, across all the treatments, are 7.32 and 0.61, respec- tively. peroxides were lower in 15 and 10 C-primed seed than in 20 C-primed seed (Fig. 3), suggesting that lipid perox- enging enzymes in primed sh-2 seeds. Similar responses idation also was decreaseing. Enhanced antioxidative were also found in the present study (Fig. 4 and 5). activities are probable reasons for the reduced lipid Reduction of enzyme activity in stored seed receiving peroxidation in 10 and 15 C-primed seed (Fig. 4 and 5). priming is probably due to aging-enhanced protein deg- The higher levels of soluble and lower levels of browning radation (Fig. 2) and protein modification (browning observed in 10 and 15 C-primed seed compared with products) (Fig. 3). Bernal-Lugo and Leopold (1998) re- 20 C-primed seed support this argument (Fig. 2). Thus, ported that, in dry seed, protein modification was taking decreased lipid peroxidation, under the enhanced pro- place through nonenzymatic glycation with reducing tection by antioxidative mechanisms, might extend the sugars and/or by AOS oxidations. As a result, the perox- longevity of 10 and 15 C-primed seed stored at 25 C. idative injuries are intensified during storage, and seed Decreased longevity of 20 C-primed seed stored at germination is limited. 25 C might also result from a combination of hydration Many environmental factors are known to influence and temperature effects. The final water gain for 20 C the success of priming, priming temperature being criti- primed-seed, at the end of hydration, was higher than cal (McDonald, 1999). In this study, we found a rapid 10 and 15 C-primed seed (Chiu, 2000). Under higher decline in viability for 20 C-primed sh-2 seed during 25 C seed moisture conditions, 20 C-primed seed might pass storage. Rates of germination decline were less pro- the threshold of desiccation tolerance (i.e., irresistible nounced when the seed was primed at 15 or 10 C (Fig. 1). cell division), thus injuring membranes in the seed be- Reduced electrolyte leakage and improved seedling yond the point of recovery, even though they dehy- growth confirmed the superior quality of 10 and 15 C- drated faster than 10 or 15 C-hydrated seeds during primed seed (Fig. 2). In addition, both MDA and total redrying period (Chiu, 2000). 2002 CROP SCIENCE, VOL. 42, NOVEMBER–DECEMBER 2002 at least 12 mo. This would appear not to be commercially practical, however, except for germplasm conservation of inherently short-lived species. In conclusion, our results confirm that 3 mo storage at 25 C of 20 C-primed sh-2 sweet corn seed reduced seed quality and inhibited antioxidative activity. The quality of 10 or 15 C-primed seed could be maintained for 6 mo even when they were stored at 25 C. However, 10 or 80 C storage extended the storability of solid- matrix primed sh-2 seeds to 12 mo. The improved anti- oxidative activity seems to play a role in maintaining the viability and vigor of solid-matrix primed seed stored at low temperature. ACKNOWLEDGMENTS This work was supported by the National Science Council of ROC (NSC87-2313-B-005-056). REFERENCES Alvarado, A.D., and K.J. Bradford. 1988. Priming and storage of tomato (Lycopersicon lycopersium) seeds. I. effects of storage tem- perature on germination rate and viability. Seed Sci. Technol. 16: 601–612. Bernal-Lugo, I., and A.C. Leopold. 1998. The dynamics of seed mortal- ity. J. Exp. Bot. 49:1455–1461. Bradford, K.J. 1986 Manipulation of seed water relations via osmotic priming to improve germination under stress conditions. Hort- Science 21:1105–1112 Bruggink, G.T., J.J.J. Ooms, and P. van der Toorn. 1999. Induction of longevity in primed seeds. Seed Sci. Res. 9:49–53. Chang, S.M., and J.M. Sung. 1998. 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