Lettuce (Lactuca sativa L.) seeds are inhibited from germinating above an upper temperature limit that is dependent upon cultivar, growing conditions and seed treatments. Thermoinhibition is accompanied by an increasing sensitivity of germination to reduced water potentials (ψ). We have employed a water relations analysis (hydrotime model) of seed germination rates to investigate the basis of thermoinhibition. Germination rates can be characterized by the distribution of base water potentials among seeds in the population (ψb(g)) and a hydrotime constant indicating the total accumulated hydrotime (MPa · h) above the base required for radicle emergence. The hydrotime model adequately described germination time courses across a range of ψ at both high and low temperatures. Increasing temperature caused the ψb(g) distributions to become more positive, accounting for the greater sensitivity to ψ and inhibition of germination. Increases in embryo osmotic potential and in the turgor yield thresholds of both the radicle and the endosperm/pericarp envelope contributed to this change. Seed priming (prehydration and drying) treatments speeded germination by reducing the hydrotime requirement. Priming also resulted in smaller increases in ψb(g) at high temperature, alleviating thermoinhibition by lowering the embryo yield threshold sufficiently to compensate for the increased endosperm resistance. The beneficial effects of priming in lettuce appear to occur primarily in the embryo, rather than in the surrounding envelope tissues.

A water relations (hydrotime) model was employed to characterize the responses of lettuce (Lactuca sativa L.) seed germination to temperature, water potential (ψ), endosperm disruption, and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). The base water potentials (ψb), standard deviations of base water potential and hydrotime constants (θH) estimated by the model were used to predict the germination rates at reduced ψ and supraoptimal temperatures in the presence or absence of ACC. The distribution of ψb values among seeds in the population (ψb(g)) increased (became more positive) as the imbibition temperature increased, indicating a greater sensitivity of seeds to reduced ψ at thermoinhibitory temperatures. Slitting the seeds at the cotyledonary end to disrupt the integrity of the endosperm extended the high temperature range for germination and maintained lower ψb(g) distributions compared with intact seeds as temperature increased. ACC (10 mm) in the imbibition solution raised the upper temperature limit for germination by about 2°C, and also lowered ψb(g) values by 0.1 to 0.2 MPa. The effect of ACC on ψb(g) was more pronounced at higher imbibition temperatures and in slit seeds. ACC (via conversion to ethylene) extends the high temperature limit for lettuce seed germination by acting in the embryo to maintain a lower ψ threshold for the initiation of growth as temperature increases. Evidence from other species as well suggests that this may be a general mechanism for the action of ethylene in promoting seed germination.

The effect of temperature on the Pfr level required to induce germination in lettuce cv. Grand Rapids was investigated. Various Pfr levels were established in seeds, by irradiation with different fluences of red light, or by saturating irradiations with narrow bandwidth light. Both approaches indicated that increasing Pfr levels caused a progressive increase in the upper temperature limit for germination. The Pfr level required to induce 50% germination after a single light pulse increased from about 11% at 15° and 20°C to 86% at 30.5°C. Repeated red light irradiations raised the GT50 (temperature giving 50% germination) by about 2°C, suggesting that Pfr reversion limited germination at high temperatures. However, a large increase in Pfr level required to induce 50% germination was still apparent at high temperatures, even when relatively constant levels of Pfr were maintained by repeated irradiations with narrow bandwidth light. These results indicate that following a single light pulse, germination at high temperatures is limited solely by the availability of Pfr. The precise position of the upper temperature limit for germination is influenced by reversion; however, the underlying cause of thermoinhibition relates to an increasing requirement for Pfr at elevated temperatures.

The cytoplasm of soybean seed axes is known to undergo a glass transition as moisture content decreases. In this study we have investigated whether the glass transition is reflected in the macroscopic mechanical properties of the seeds. Constant-deformation-rate compression tests and constant-deformation relaxation tests were performed with a cylindrical indenter on soybean cotyledons equilibrated to moisture contents ranging from 2% to 16% (dry basis). The apparent modulus of elasticity was found to rise sharply as moisture content declined below 10%, showing an inflection point at 9.8%, and a relatively constant elasticity below 7%. Other measures of mechanical behaviour were also constant in the drier ranges, including linear deformation, and plastic deformation. Each of these viscoelastic characteristics showed an inflection point or a departure from a linear function in the region of 7.5–9.8% moisture content, the hydration region below which the tissue is in a glassy state. It is suggested that the viscoelastic qualities of dry soybeans are principally a consequence of the tissues being in the glassy state.

The effect of accelerated aging (AA) and seed priming on specific seed enzymes was studied in soybeans. Aging seeds for 48 and 96 h at 41°C and 100% humidity reduced germination from 92% to 68% and 0%, respectively. Malate dehydrogenase (MDH) glutamate dehydrogenase (GDH), and esterase were assayed in extracts from dry and imbibing seeds. Malate dehydrogenase activity was the least affected by AA treatments. Certain GDH isozyme activity was reduced by 17% after 48 h of AA and almost completely lost after 96 h. Imbibing for 24 h increased GDH activity in 48 h-accelerated-aged seeds but not the 96 h-aged seeds. Two esterase isozymes in dry seeds showed a 77% loss of activity after 48 h of AA. This treatment increased the total esterase activity, with the greatest activity observed after 96 h of AA. This was due to the appearance of a diffuse high molecular weight (HMW) region of activity observed on native polyacrylamide gels and specifically induced in the embryonic axes. Imbibing 48-h-accelerated-aged seeds for 48 h caused a reappearance of the two esterase enzymes lost during aging. The HMW esterase staining region also appeared after imbibing unaged seeds for 24 h. Seed priming, caused a response similar to imbibition for each enzyme except the HMW esterase activity. Priming 48-h-accelerated-aged seeds reduced the HMW esterase activity. This was not observed during imbibition of aged seeds and indicates the first identification of a specific enzymatic response unique to a seed priming treatment.

DNA synthesis in organelles seems to be an early event during maize germination. Mitochondria and plastids from maize embryo axes have been separated using differential and gradient centrifugations and DNA polymerase activity has been studied in extracts from each organelle. It has been found that the enzymes from both organelles show some differences from other plant organellar polymerases in terms of response to specific inhibitors, DNA templates and cations. A comparison of different plant organellar DNA polymerases is presented.

Some of the seed-evolved volatiles, which were mainly composed of methanol, acetaldehyde, ethanol and acetone, caused the loss of seed germinability during storage. In general, the deleterious effects of the volatiles increased with increasing RH and temperature during storage. Acetaldehyde had the strongest deleterious effect regardless of RH and temperature, while ethanol caused seed deterioration only at high RHs. Acetone was slightly deleterious to some species, while methanol was almost inert in most seeds. Various aldehydes applied during storage showed some toxicity to seed germinability, which decreased, except for 3-methylbutanal, with increasing molecular size, suggesting that the endogenous volatile compounds with an aldehyde group cause seed deterioration. On the other hand, the contents of volatile compounds in seeds were higher when they were stored at 44% RH (water sorption zone 2), than when stored at 12% RH (water sorption zone 1). Acetaldehyde, the most deleterious volatile, was more abundantly accumulated within the seeds stored at −3.5°C than at 23°C. Based on these facts, it is suggested that endogenous volatiles, especially acetaldehyde, may be an important factor that accelerates seed deterioration which often occurs under lower RHs and/or temperatures throughout long-term storage.