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- Author or Editor: Anwar A. Khan x
- HortScience x
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.
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.
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.
Emergence and stand establishment of pepper and tomato seeds often are slow and erratic, particularly under stress conditions. Field emergence trials sometimes have not responded to priming in pepper. In this study, we examined the combining effect of matriconditioning with GA4+7 to hasten germination and improve stand establishment of pepper and tomato seeds. The results showed that, in all cases, even under stressful conditions, the combined matriconditioning plus GA treatment was effective in improving germination and emergence of pepper and tomato seeds. Emergence was improved in 20% when seeds were treated with GA4+7 up to 200 mM. Thus, matriconditioning during which germination is suspended, provides a unique means to rapidly and efficiently digest the endosperm by GA-induced enzymes and reduce not only the mechanical restraints but also provide the energy for embryo growth.
Papaya (Carica papaya L.) seeds germinated poorly at 25C in the presence of gibberellin (GA4+7) or following matriconditioning at 25C for 4 days. However, a combined treatment of matriconditioning and GA4+7 for 4 days synergistically promoted germination and seedling emergence. Drying the seeds after conditioning reduced the percentage of seedling emergence in the combined treatment involving 400 μm GA4+7 only. Combining matriconditioning with 100 or 200 μm GA4+7could effectively reduce germination time and improve seedling emergence and is recommended as a standard procedure for testing papaya seed germination.
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.
The level of 1-aminocyclopropane-1-carboxylic acid (ACC) was 0.55 nmol.g-1 in dry lettuce (Lactuca sativa cv. Emperor) seeds. After 4h soak at 25°, 35° and 35°C+ KIN (kinetin, 50μM), the levels were 0, 0.2 and 1.14 nmol.g-1 seeds, respectively. The level of ACC was higher at 35°+KIN than at 35°C for up to 16h soak. No ACC was detectable at 25°C during 2 to 16h soak. In the presence of 50μM ABA, ACC level decreased to 0.2 nmol.g-1 at 4h soak and to zero level during 8 to 16h soak. The level of l-(malonylamino) cyclopropane-1-carboxylic acid (MACC), in dry seeds was 14 nmol.g-1. Exposure to 35°C in the presence or absence of KIN increased the level to 40-42 nmol.g-1 within 2h soaking, while at 25° only a slight increase (23 nmol.g-1) occurred. As in the case of ACC, the level of MACC was higher at 35°C+ KIN than at 35° or 25° for up to 16h soak.When seeds were soaked in ABA, the pattern of MACC produced was similar to that produced at 35°C. The results indicate that ACC synthase activity is enhanced by the addition of KIN at 35°C resulting in increased synthesis and/or accumulation of ACC and MACC. The relationship of ethylene biosynthesis to changes during stress imposition and alleviation by various factors will be discussed.
Dormancy was induced in nondormant (germinate readily in light or darkness) seeds of several lettuce cultivars (Mesa 659, Emperor, Empress, Montello, Ithaca) by soaking in the dark in 5-100μM tetcyclacis (TCY) for 24h at 25°C as the seeds failed to germinate in the dark upon removal of TCY by washing. Higher concentrations of TCY was needed to induce dormancy in the light Paclobutrazol (PP 333) was relatively less effective. No dormancy was induced in nondormant lettuce seeds soaked for 24h in 100μM ABA as the seeds germinated readily in the dark upon removal of the inhibitor by washing. Thus, contrary to popular belief ABA does not appear to be a dormancy factor. Dormancy induced by TCY was released by soaking seeds in petri plates in water at 25°C in the light, in the presence of 0.001-1mM GA4+7, or by moist-chilling for 4-15d at 5°C. Dormancy was also released when dried dormant (dormancy induced by TCY) Mesa 659 or Emperor lettuce seeds were planted in a moist peat-lite mix in plastic containers and kept moist for 30d at 5°C, as indicated by emergence of normal, healthy seedlings upon transfer of the containers to 25°C. The significance of TCY induced dormancy in altering planting strategy in field plantings of lettuce and other crops will be discussed.