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  • Author or Editor: Anwar A. Khan x
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A gibberellic acid (GA) biosynthesis inhibitor, tetcyclacis, induced dormancy in nondormant seeds of lettuce (Lactuca sativa L.), tomato (Lycopersicon esculentum Mill.), pepper (Capsicum annuum L.), carrot [Daucus carota var. sativus (Hoffn.)], onion (Allium cepa L.), celery (Apium graveolens L.), and impatiens (Impatiens novette), as most of the seeds failed to germinate after washing under conditions that permitted germination before dormancy induction. In lettuce seeds, tetcyclacis and paclobutrazol were more effective in inhibiting germination in light than in darkness. A 16- to 24-h soak treatment with tetcyclacis was sufficient to induce dormancy in nearly all seeds. Tetcyclacis failed to induce dormancy if applied after 6 h presoak in water. Dormancy induced by tetcyclacis was released by GA4+7 (a mixture of gibberellin A4 and A7), light, and moist-chilling treatments. When GA4+7 was applied with tetcyclacis, dormancy induction was prevented under both favorable, e.g., 25C, and unfavorable, e.g., 5C, or low water potential (Ψ), germination conditions. Unlike tetcyclacis, abscisic acid (ABA) failed to induce dormancy in lettuce seeds. Thermodormancy induction in lettuce seeds at 35C was prevented by fluridone. However, neither ABA nor tetcyclacis countered its effect. Dormancy was also induced in lettuce seeds by ancymidol, flurprimidol, or paclobutrazol. Dormancy induced by tetcyclacis in pepper, tomato, carrot, and onion seeds was released by GA4+7, but not by irradiation or moist-chilling. Chemical names used: 5-(4-chlorophenyl)-3, 4, 5, 9, 10-pentaazatetracyclo [5.4.102,6.08,11]-dodeca-3, 9-diene (tetcyclacis); 1-(4-chlorophenyl)-4, 4-dimethyl-2-(1H-1, 2, 4-triazole-1-yl)-3-pentanol (paclobutrazol); α-cyclopropyl-α-(4-methoxyphenyl)-5-pyrimidine methanol (ancymidol); α-(1-methyl)-α-[4-(trifluoromethoxy) phenyl]-5-pyrimidine-methanol (flurprimidol); 1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4 (1H)-pyridinone (fluridone).

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ACC-derived ethylene production was used as an index of seed vigor of lettuce (Lactuca sativa L.), cabbage [Brassica oleracea (Capitata Group)], tomato (Lycopersicon esculentum Mill.), snap bean (Phaseolus vulgaris L.), and sweet corn (Zea mays L.) seeds. Seeds were aged at 40C and 93% relative humidity over saturated solution of KH2PO4 for various times to obtain seeds of differing vigor. Naturally aged lettuce seeds, differing in vigor, were also used. Depending on the seed type, 0.25 to 2 mm ACC (saturating dose) was needed to produce maximal amounts of ethylene. Seeds in the presence of ACC produced a much larger amount of ethylene than those in the absence of ACC, the ACC-derived ethylene could be detected before germination, and ACC had no adverse effect on germination. ACC-derived ethylene production paralleled vigor loss as determined by a decrease in percentage germination over a soak period required for complete germination of nonaged seeds (16 hours for lettuce, 24 hours for cabbage, and 48 hours for tomato and sweet corn), an increase in mean germination time (determined for lettuce only), and a decrease in seedling growth (determined for snap bean only). Second degree polynomial and logarithmic equations generated for the relationship of ACC-derived ethylene production to germination or growth parameters following seed aging, provided good to excellent fit. As a vigor test, the ACC-ethylene procedure has several advantages over the non-ACC ethylene procedure: It improves the sensitivity of the test by enhancing ethylene production, permits detection of small differences in vigor, and allows detection of ethylene before germination within a few hours of soaking. Chemical name used: 1-aminocyclopropane-1-carboxylic acid (ACC).

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Quantification of seed dormancy has been achieved by measuring physiological, biochemical, and molecular changes accompanying dormancy release, as well as dormancy development. At the physiological level, dormancy is quantified in terms of stratification time, strength of embryo covering structures, embryo growth potential, responsiveness to light, and to temperatures and other changes. At the biochemical level, dormancy has been related to hormone (abscisic acid, gibberellin, etc.) levels, respiratory activity, and other metabolic functions. At the molecular and cellular level, dormancy has been associated with RNA and protein synthesizing ability and with gene expression. Our recent studies with lettuce seeds using gibberellin biosynthesis inhibitors indicate that the amount of gibberellin produced during seed soak may mediate dormancy release and is quantitatively related to the level of dormancy. Examples of quantifiable changes associated with dormancy will be described. Whether a quantifiable change reflects a causal relationship with dormancy release or development, or is a consequence thereof, will be discussed.

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Fluridone (FL), a catotenoid biosynthesis inhibitory herbicide, prevented supraoptimal temperatures from inducing thermodormancy in seeds and permitted seedling emergence in several lettuce cultivars. A 48-h dark soak at 35C with 23 μM FL completely prevented the induction of thermodormancy in `Mesa 659' and `Emperor' lettuce seeds as more than 90% of the seeds germinated on transfer to water in darkness at 25C. Abscisic acid (100 μM) applied with FL did not prevent FL from acting. Dormancy was released completely in the naturally dormant `Garnet' and `Grand Rapids' lettuce seeds at 25C in darkness by 1 μM FL. FL applied following thermodormancy induction at 35C in `Mesa 659' lettuce seeds had little effect on releasing the induced dormancy. A 2-h presoak of `Mesa 659' lettuce seeds with 0.47 μM FL followed by washing, drying, and sowing in a peatlite mix at 25C/35C (12 h/12 h), permitted 80% seedling emergence. Higher concentrations resulted in the emergence of albino seedlings. Similar results were obtained with seeds of other lettuce cultivars (`Prizehead', `Emperor', `Ithaca', and `Empress'). A 6-h matriconditioning (MC) (A.A. Khan, Hort. Rev. 13:131–181, 92) of `Mesa 659' lettuce seeds in 7.5 μM FL enhanced the emergence to a greater extent than by MC alone at 25C/35C. The FL procedure in alleviating high-temperature stress in lettuce and other seeds is being tested further.

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1-aminocyclopropane-1-carboxylic acid (ACC) was used to measure seed vigor of lettuce, cabbage, tomato, snap bean and sweet corn seeds. Accelerated aging at 40C and 93% RH over saturated solution of KH2PO4 and natural aging under ambient storage conditions (5-7C, 28 to 60% RH) were used to obtain seeds of differing vigor levels. Depending on the type of seeds, the amount of ACC needed for maximal ethylene production (saturating dose) ranged from 0.25 to 2mM. Seeds produced much larger amounts of ethylene in the presence than in absence of ACC, the ACC-derived ethylene was detected prior to germination, and ACC had no adverse effect on germination. ACC-derived ethylene production paralleled vigor loss as determined by percentage germination, mean germination time (for lettuce only) and seedling growth (for snap bean only). Second degree polynomial and logarithmic equations generated for the relationship of ACC-derived ethylene production to germination or growth parameters following seed aging, provided good to excellent fit. As a vigor test, the ACC-ethylene procedure has several advantages over the non-ACC ethylene procedure: it greatly improves the sensitivity of the test by enhancing ethylene production, it permits detection of small differences in vigor levels and it allows detection of ethylene prior to germination within a few hours of soaking.

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Seeds (intact or slit) of lettuce (Luctuca sativa L.) cultivars with greater ability to produce ethylene germinated better under stressful conditions. Highly significant correlations were found between ethylene production and germination in 0.1 m NaCI (- 0.49 MPa) solution at 25C (r = 0.95, intact seeds), in - 0.3 MPa PEG solution (r = 0.86, intact seeds; r = 0.81, slit seeds), and in water at 32C (r = 0.80, slit seeds) or 35C (r = 0.80, slit seeds). Slitting the seed coat increased the ethylene production and improved germination during osmotic restraint in most cultivars, particularly in `Mesa 659' and `Super 59'. The differing ability of cultivars to produce ethylene during stress generally corresponded with their ability to generate germination potential. Ethylene production and germination potential in untreated and ACC-treated `Mesa 659' seeds increased upon slitting under stressful conditions. Thus, the ability of seeds to produce ethylene and to generate high germination potential under stressful conditions may be used as criteria to select stress-tolerant lettuce cultivars. Chemical names used: polyethylene glycol 8000 (PEG), 1-aminocyclopropane-1-carboxylic acid (ACC), (2-chlorethyl) phosphoric acid (ethephon).

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Preconditioning `Mesa 659' lettuce (Lactuca sativa L.) seeds with the moist solid carrier Micro-Cel E at 15C for 20 hours removed thermoinhibition and allowed the seeds to germinate at 35C. Participation of 1-aminocyclopropane-1-carboxylic acid (ACC) and its use in the removal of thermoinhibition were indicated from the following: a) the ACC level in the preconditioned seeds peaked before germination at 35C; b) addition of aminoethoxyvinylglycine (AVG), an inhibitor of ACC synthesis, reduced the ACC level and inhibited germination at 35C; c) response of preconditioned seeds to ethylene at 35C preceded that of ACC; d) exogenous ACC, (2-chloroethyl) phosphonic acid (ETH, ethephon), or ethylene reversed the inhibitory effect of AVG and restored the capability of the seed to germinate at 35C; e) Co+2, an inhibitor of the ACC-to-ethylene step, partially inhibited germination of preconditioned seeds at 35C, and the effect was reversed by ETH. The major free polyamines in dry seed were spermidine (Spd) and putrescine (Put). An increase in polyamine level occurred following transfer of conditioned seeds to 35C. Of the various polyamine biosynthesis inhibitors, difluoromethylarginine and cyclohexylammonium sulfate inhibited an increase in Spd and Put levels only after germination, whereas difluoromethyorinthine had no effect on polyamine biosynthesis. None of these inhibitors affected the relief of thermoinhibition. Thus, the increase in polyamine level during germination at 35C does not appear to be associated with the alleviation of thermoinhibition.

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The effects of various drying conditions on seed quality and performance of matriconditioned `Bush Blue Lake 47' snap bean (Phaseolus vulgaris L.) seeds were studied. An exponential model based on the Page equation provided a good fit (R2 = 0.9) to changes in moisture content during drying. Drying matriconditioned seeds with high initial moisture content (47.2%) for 5 to 6 hours at 35C, 30% to 35% relative humidity, and 0.7 to 1.4 m·s-1 air velocity (v) retained, and in some cases augmented, the benefits derived from conditioning. Matriconditioning greatly reduced electrolyte leakage (34.3 vs. 94.7 μS·cm-1·g-1 for nontreated seeds); drying to 15% moisture content at 0.7 or 1.4 m·s-1 v moderately increased the leakage rate (59.1 to 60.9 vs. 34.3 μS·cm-1·g-1), while drying at 0.02 m·s-1 v (ambient) increased the rate to that of nontreated seeds. The leakage rate remained low (43.6 to 50.8 μS·cm-1·g-1) in matriconditioned seeds dried to 22% moisture content at all air velocities. In growth-chamber studies, rapidly drying matriconditioned seeds to 15% moisture content at 1.4 m·s-1 v improved the emergence percentage over that of nontreated seeds, increased the shoot fresh and dry weight over that of nontreated and nondried matriconditioned seeds, and increased the shoot fresh weight over that of seeds dried at 0.02 or 0.7 m·s-1 v. Drying matriconditioned seeds to 15% moisture content at 0.7 m·s-1 v improved plant fresh weight over that produced by nontreated seeds. Rapid drying to 22% moisture content at 1.4 or 0.7 m·s-1 v improved only shoot fresh weight over that of nontreated seeds. In a 1992 field planting, percent emergence of matriconditioned seeds dried at 0.7 or 1.4 m·s-1 v was similar to that of nondried matriconditioned seeds and higher than that of nontreated seeds. No significant differences were noted in plant yield among the treatments.

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