Grand Rapids Lettuce Research Articles

Optical properties of Lactuca and Taraxacum seed and fruit coats: Their role as light filters

The optical properties of seed and fruit coats were examined from several varieties of light‐sensitive achenes. Taraxacum vulgare L. and Lactuca sativa L. cv. Grand Rapids achenes with dark fruit coats and L. sativa cvs Huvudsallat and Issallat with white fruit coats were examined. Transmission spectra varied among the different achenes: white fruit coats of Lactuca acted as neutral density filters between 450 and 780 nm, whereas Taraxacum transmitted 2–36% in this region. The ribbed fruit coat structure greatly affected transmission so that at different locations in the same coat, transmission varied between 20 to 80% at 660 and 730 nm. Fruit coats of Grand Rapids lettuce and Taraxacum transmitted more far‐red than red light with T660/T730 ratios of 0.8 and 0.4, respectively. The relationship between the optical properties of fruit coats and light‐stimulated germination is discussed.

The use of light-emitting diodes (LEDs) as green and red/far-red light sources in plant physiology

The use of green, red, and far-red light-emitting diodes (LEDs) as light sources for plant physiological studies is described. LED lamps have the advantage over conventional light sources in that they are lightweight, low cost, portable, easily constructed and, more important, do not require colour filters. Using LED lamps, red/far-red reversal of germination of Grand Rapids lettuce seeds was demonstrated and the effectiveness of a green LED safelight was compared with that of a conventional filtered safelight, using extension growth of photosensitive Avena fatua L. seedlings as an indicator. The green LEDs gave comparable performance to the conventional filter-type safelight. An infra-red LED light source was also evaluated.

Changes in germination and respiratory potential of embryos of dormant Grand Rapids lettuce seeds during long-term imbibed storage, and related changes in the endosperm

Grand Rapids lettuce (Lactuca sativa L.) seeds were stored in an imbibed state for up to two years. Embryos dissected from stored seeds showed a progressive loss with time in their ability to germinate on polyethylene glycol (PEG) solutions. Little germination of dissected embryos from one-month imbibed seeds occurred on-6 bar PEG but only after four months of storage did the dissected embryos fail to germinate on-4 bar PEG. After two years storage 30% of dissected embryos still were able to germinate on-2 bar PEG. This loss of germination potential, which may be a symptom of the development of an embryo dormancy, could be reversed by N(6)-benzyladenine (BA) and red light (R) applied together or separately to dissected embryos. Two weeks of chilling of 12-month imbibed seeds restored sensitivity to R and a 48-h BA pretreatment prior to R resulted in germination rates similar to those of seeds emerging from primary dormancy. There was loss of embryo control of endo-β-mannanase activity after two weeks of storage even though the endosperms themselves retained their capacity for enzyme synthesis for six more weeks. Eventually, then, endo-β-mannanase synthesis is not possible because of inherent changes in both the embryo and endosperm, although each tissue undergoes changes at its own rate. Oxygen uptake by embryos dissected from two-month imbibed seeds did not increase to the same extent as embryos dissected from freshly imbibed seeds. In intact seeds germinating from a skotodormant state, oxygen uptake increased at a time coincident with radicle protrusion, but did not achieve the levels of uptake of those seeds germinating from a primary dormant state. The decline in uptake of oxygen by secondary dormant seeds is the result of a lowered respiratory capability of the embryo itself, rather than of changes in permeability of the surrounding structures.

Long-term storage of dormant Grand Rapids lettuce seeds in the imbibed state: physiological and metabolic changes

Grand Rapids lettuce (Lactuca sativa L.) seeds retained their viability for up to ten months when maintained in the fully imbibed state on moist filter paper at 25°C in darkness. The ability of red light (R) to promote their germination was essentially lost within one week from the start of imbibition; sensitivity to gibberellic acid was retained for two weeks longer. Seeds which did not respond to either treatment had entered the state of secondary dormancy (skotodormancy). This could be relieved at all times by a combination of benzyladenine and R applied to the intact seed, or by isolation of the embryo and incubation on water. Protein synthesis increased initially following imbibition but declined after 72 h to a constant low level. Respiration declined over the first week of storage in the imbibed state to a much-reduced steady level. Cotyledonary lipid declined between four and ten months of storage but the axial lipid remained unchanged. Sucrose in the embryo increased after five months, but no changes in glucose, galactose, fructose or mannose were found. The total N content of the cotyledons declined over the first three months of storage in the imbibed state, with a concomitant rise in axial N; the latter declined slowly thereafter. Basal α-galactosidase (EC 3.2.1.22) activity decreased over seven months, but the phytochrome-induced component could not be raised by a 15-min R treatment even after one month. Germination induced by R and benzyladenine was achieved at later times without a rise in α-galactosidase levels.

Gibberellic Acid-induced Structural Alterations in the Endosperm of Germinating Lactuca sativa L. Achenes

Summary Exogenously applied gibberellic acid induces structural alterations in the micropylar region of the endosperm in Grand Rapids lettuce ( Lactuca sativa L.) achenes prior to radicle protrusion. Besides the pattern of storage material degradation and vacuole formation, both timing and localization of endosperm alterations are similar to the previously reported phytochrome-mediated ones. These data favor the concept that gibberellic acid and phytochrome promote lettuce achene germination, at least in the later stages, through a similar channel.

Lettuce Endosperm Structural Changes During Germination under Different Light, Temperature, and Hydration Conditions

The endosperm structure of dark-germinating Grand Rapids lettuce (Lactuca sativa L.) achenes was examined by light microscopy under different germination conditions. Achenes imbibed in darkness at 25 C show significant structural modifications in a restricted area of the endosperm, opposite the radicle tip, before radicle emergence. These preemergence changes, similar to those observed in light-requiring Grand Rapids lettuce achenes after phytochrome activation, are mainly characterized by the mobilization of storage materials and the vacuolation of the cytoplasm Achenes imbibed under conditions not permitting the completion of germination (continuous far-red light, high temperature, and appropriate osmotic solution) do not show any structural change in their endosperm cells. We conclude that cytoplasmic modification of the endosperm cells in the micropylar region is necessary for the completion of germination in lettuce achenes, in addition to the necessity for decrease in embryo water potential.

A role for ?-galactosidase in the degradation of the endosperm cell walls of lettuce seeds, cv. Grand Rapids

Isolated endosperms of Grand Rapids lettuce (Lactuca sativa L.) seeds undergo extensive cell-wall degradation and sugars are released into the surrounding incubation medium. One sugar so released is galactose. α-Galactosidase (EC 3.2.122) is present at the same level in both dry and imbibed isolated endosperms and is responsible for the release of galactose. However, this enzyme does not act upon the native endosperm cell wall, but requires first its partial hydrolysis and the production of oligomers by the action of endo-β-mannanase (EC 3.2.1.787). Galactose is then cleaved from these oligomers, allowing their further subsequent hydrolysis by endo-β-mannanase. Thus α-galactosidase and endo-β-mannanase act cooperatively to effect the hydrolysis of the lettuce endosperm cell walls.

Regulation of Phenylalanine Ammonia-lyase in Germinating Lettuce Seeds (Lactuca sativa L. cv. Grand Rapids): Effects of Abscisic Acid and Water Stress

Summary Abscisic acid (ABA) and polyethylene glycol (PEG, 35%) completely inhibited germination of Grand Rapids lettuce seeds in light. A parallel inhibition of the activity of L-phenylalanine ammonia-lyase (PAL) was also observed under these conditions. Effects of ABA were not reversed by GA 3 and only slightly reversed by benzyladenine. Seeds germinated in water showed a steady increase in PAL activity during a 20 h period. If seeds germinated in water were transferred to 0.02 mM ABA, the activity of PAL in these seeds significantly declined during the next 24 h. Transfer of germinated seeds to PEG resulted in a much faster loss of PAL activity during this period. The disappearance of PAL activity on transfer to ABA and PEG occurred with calculated half-lives of 11 and 3 h, respectively. The loss of PAL caused by PEG was readily reversible on removal of the osmoticum.

Phytochrome control of skotodormancy release in Grand Rapids lettuce achenes

Light‐requiring Grand Rapids lettuce (Lactuca sativa L.) achenes develop skotodormancy when imbibed in darkness for 7 days at 25°C. Redried skotodormant achenes maintain this type of dormancy upon subsequent rehydration. At 25°C full germination of skotodormant achenes can be induced by continuous and intermittent red light illumination as well as by several brief red irradiations given daily. One brief (10 min) red light irradiation can partly break skotodormancy at 20°C, while at lower temperatures the same treatment results in full induction of germination. Phytochrome control of the release from skotodormancy is proven by a) the dependence of the germination response on the relative sequence of red and far‐red light in cyclic irradiations, and b) the reversion of red action by subsequent far‐red irradiation. The time course of germination of skotodormant achenes treated with intermittent red light depends upon the length of dark interval between the light pulses. Germination is considerably delayed compared to that of non‐skotodormant ones, induced by a single brief red light treatment. This fact in combination with the requirement, over a long period of time, of Pfr action for full manifestation of germination, indicates that skotodormancy is a deeper form of dormancy. It is concluded that the germination of lettuce achenes may always be subjected to phytochrome control.

Lipid participation in intracellular freezing avoidance mechanisms of lettuce seed

Freeze-fracture electron microscopy was used to study water content related freezing resistance in Grand Rapids lettuce seeds. Consistent and recognizable conformational changes occurred in lipid-water phases of lettuce seeds at different moisture contents. In air-dry lettuce seed cotyledons, the lipids lying in spherical lipid bodies near the cell wall appeared amorphous, while the structure was crystalline above 20% water content. The lipid bodies interassociated into membrane bilayers in seeds containing 20 to 25% water. Such lyotropic phase transitions in membrane lipids during lettuce seed hydration are believed to contribute to the biphasic freezing behavior observed in lettuce seeds at different moisture contents and to provide a natural freezing tolerance mechanism for highly desiccated plant tissues such as seeds.

Immediate phytochrome action in inducting α-galactosidase in lettuce seeds

The light-induced breaking of dormancy of Grand Rapids lettuce seeds imbibed in darkness at 25°C is a classical phytochrome-controlled phenomenon1. Although it has been shown that irradiation of seeds 1–2 h after the start of imbibition produces the active form of phytochrome (Pfr) which acts rapidly after its formation2,3, the completion of germination (observed as radicle protrusion) is only evident several hours later. Clearly, then, radicle emergence is temporally removed from the rapid (and presumably primary) action of phytochrome. Very little is known about the processes set in motion by Pfr that lead to the completion of germination. Here we report that red light-induced stimulation of α-galactosidase (EC 3.2.1.22) activity occurs substantially before germination is completed, and show that the action of phytochrome in promoting an increase in the activity of this enzyme is very rapid.

Lettuce Production in a Controlled Environment Plant Growth Unit

ABSTRACT THE growth and yield responses of Grand Rapids let-tuce to physical and environment factors of a Con-trolled Environment Plant Growth (CEPG) unit are presented. Changes in the environment were made and resulting yields are given. Data presented also shows losses in the root, unmarketable leaves and mortality.

The growth physics and water relations of red-light-induced germination in lettuce seeds

Irradiation of Grand Rapids lettuce (Lactuca sativa L.) seeds with red light (R) results in an increase in the growth response in the embryonic axes which enables them to overcome the mechanical restriction of the endosperm or the osmotic restriction of an external solution. When the growth rates of axes from R-and far-red(FR)-treated seeds are equalized with polyethylene-glycol-4000 solutions, a water potential difference of 3.4 bars between the axes is obtained. Measurements of the osmotic potential indicate a 1.0-2.0-bar difference between the treatments, depending upon incubation time. Although the difference in water potential remained constant during the growth of the axes, the osmotic constituents of the axes of R-treated seeds were diluted to a greater extent than those of the axes of FR-treated seeds. The length-to-volume ratio of the growing axes of R-treated seeds increased at a greater rate than that of the axes of FR-treated seeds. These data indicate that changes in the cell-wall properties may account for the remainder of the observed water potential difference. We propose that the phytochrome-mediated growth increase in these embryonic axes is an integrated function of the cells: increased wall loosening may be inextricably linked to simultaneous increases in the osmotic constitutents. The driving force for growth may not involve solely a change in the osmotic potential or the pressure potential, with passive readjustment of the other parameter, but the activation of a cellular component that links changes in the wall properties with the accumulation of osmotic constituents.

Kinetic and Thermodynamic Parameters That Describe Isothermal Seed Germination

The purpose of this study was to see if the kinetic and thermodynamic seed germination model that has been proposed by Hageseth is indeed applicable to other seed systems. The data fits both the kinetic model and thermodynamic model and enables one to determine reaction rates, chemical concentrations, activation energies, and changes in enthalpy, entropy, and the Gibbs free energy. The experiments used Grand Rapids lettuce seeds and were carried out at constant temperatures of 10·2, 11·5, 14·5, 17·3, 19·2, 22·5, 26·0, and 28·5 °C. Over this temperature range there are three different processes occurring.

Respiration of Grand Rapids lettuce (<italic>Lactuca sativa</italic> L.) seeds in relation to chemical and photocontrol of germination

Respiration of Grand Rapids lettuce (<italic>Lactuca sativa</italic> L.) seeds in relation to chemical and photocontrol of germination

Abscisic Acid and the Germination of Thermodormant Lettuce Fruits (Lactuca sativa cv. Grand Rapids). The Fate of Isotopically Labelled Abscisic Acid

AbstractABA inhibits the germination of Grand Rapids lettuce seeds at a temperature at which photoblastism is not observed. This ABA inhibition can be overcome by GA, or cytokinin, or red light acting alone or in concert. At high levels of ABA the ameliorative effects of GA, cytokinin or red light are not manifest and it is suggested that lettuce seeds' response to a given level of ABA is temperature dependent, and any interaction which may be observed between ABA and other regulators, or red light, likewise is temperature dependent.Seeds only actively metabolize ABA after germination is complete though ABA does influence the germination process proper. Some radioactivity, originally applied as 14C ABA, cannot be extracted from 14C ABA treated seeds by methanol, and this “fixed” ABA can constitute more than 50% of the activity taken up by the seeds. High levels of “fixed” ABA are found in seeds which do not germinate. As well as this “fixed” ABA two labelled metabolites arise in seeds treated with 14C ABA. The levels of these rise in seed extracts as the level of extractable ABA decreases.Whilst ABA can affect germination our results do not allow us to assign a specific regulatory role to ABA in seed germination.

Effects of Light, Abscisic Acid, and 6N-Benzyladenine on the Metabolism of [3H]Gibberellin A4 in Seeds and Seedlings of Lettuce, cv. Grand Rapids

Gibberellin A(4) (GA(4)) can substitute for light in the germination of Grand Rapids lettuce seeds. Seeds imbibed in [(3)H]GA(4) do not convert this to other GAs prior to, or immediately following, visible germination: thus GA(4) alone can promote radicle expansion. Abscisic acid inhibited [(3)H]GA(4)-induced germination, but did not significantly affect [(3)H]GA(4) uptake or metabolism during germination. (6)N-benzyladenine overcame the inhibitory effect of abscisic acid and increased [(3)H]GA(4) uptake, although radicle emergence was delayed somewhat.During hypocotyl extension there was a large conversion of [(3)H]GA(4) to [(3)H]GA(1) in light or darkness, the major conversion site being the growing root. Hypocotyls of dark-grown seedlings contained more [(3)H]GA(1) than those of light-grown seedlings. The apparent inability of exogenous GA(1) to promote greater hypocotyl extension than GA(4) is related to its poorer uptake. Abacisic acid markedly inhibited hypocotyl expansion, root growth, and the conversion of [(3)H]GA(4) to [(3)H]GA(1).

Effects of 35 C Heat Treatments on Photosensitive Grand Rapids Lettuce Seed Germination

Grand Rapids lettuce (Lactuca sativa L.) seeds were given 35 C heat treatments to increase photodormancy in a subsequent 20 C dark period. Short heat treatments (1-5 hours) induced a significant germination percentage increase of from 16% to over 50% depending on seed lot. With longer heat treatments dark germination percentage was gradually reduced to zero. If given at the end of 35 C, far red or red followed by far red further increased the amount of dark germination.Thermodormancy also delayed red-stimulated germination by 10 hours or more when red was given following a long 35 C treatment. The presence of Pfr was required during this time since far red light remained effective in reversing at least 50% of the red stimulation for up to 16 hours compared to only 4 hours in nonheat-treated seeds.

Promotive Effects of Organic Solvents and Kinetin on Dark Germination of Lettuce Seeds

Significant promotion in dark germination was observed when Grand Rapids lettuce (Lactuca sativa L.) seeds were soaked in acetone or dichloromethane, vacuum-dried, and imbibed at 25 C. Permeation of kinetin via these organic solvents further enhanced the dark germination. Those seeds that were affected by acetone and acetone-kinetin treatments and germinated in the dark escaped red-far red photocontrol of germination. Although abscisic acid was not detected in the organic solvent leachates, they did contain other inhibitory substances affecting lettuce seedling growth. In the light, acetone and acetone-kinetin treatments also enhanced the rate of germination and the increased germination by acetone-kinetin treatment was correlated with increased polyribosome formation. The possible mechanisms involved in promotion of lettuce seed germination by organic solvents and kinetin are discussed.

Interactions between Abscisic Acid, Gibberellins and Cytokinins in Grand Rapids Lettuce Seeds Germination

AbstractExperiments with Grand Rapids lettuce seeds (Lactuca sativa L.) maintained in darkness or irradiated with red light have shown that the inhibition of germination induced by low concentrations of ABA (2, 4, 6 μM) could be overcome by gibberellins (GA3 or GA4). The same results were obtained, although to a lesser extent, under the influence of two out of the four cytokinins tested (K and BAP) for seeds maintained in darkness.To suppress the block induced by higher concentrations of ABA (for example 8 μM), it was necessary to apply a cytokinin (K, BAP, Z or 2iP) and a gibberellin (GA4 or GA3) simultaneously, or a cytokinin following a red light treatment.Experiments conducted in darkness in which ABA (8 μM) was applied together with a cytokinin (BAP) and a gibberellin (GA4) showed that the gibberellin and the cytokinin played similar roles towards each other and towards ABA.