T. H. Thomas

Some reflections on the relationship between endogenous hormones and light-mediated seed dormancy

The relationships between phytochrome and endogenous hormones in the light-mediated control of seed dormancy are discussed. It is concluded that gibberellins are primarily involved in post-dormancy metabolic processes leading to embryo growth and radicle emergence, such as food reserve mobilisation and endosperm softening. Evidence is considered that germination inhibitors, particularly abscisic acid, are involved in the establishment and maintenance of primary dormancy. The role of cytokinins not fully elucidated but there is considerable evidence to suggest that phytochrome control may involve cytokinin effects on transmembrane ion fluxes. In terms of hormonal control, phytochrome mediated dormancy is a complex phenomenon. There is a need for molecular studies of processes controlled by phytochrome, GAs, CKs and ABA during dormancy and germination to unravel the complexities of the dormancy mechanisms. Such studies would be facilitated by the availability of CK-deficient mutants of classical light-sensitive species.

Changes in colour, polygalacturonase monosaccharides and organic acids during storage of tomatoes

Abstract The appearance of polygalacturonase and red pigmentation in mature-green tomato fruit was prevented by storing the fruit in 5% O 2 , 5% CO 2 and 90% N 2 . However, the breakdown of starch to give monosaccharides and the change in concentration of organic acids which is normally associated with ripening still took place. On removal of the fruit to ambient conditions, polygalacturonase was synthesized and the fruit changed colour but monosaccharide and organic acid concentrations did not change.

THE ROLE OF CYTOKININS IN THE PHYTOCHROME-MEDIATED GERMINATION OF DORMANT IMBIBED CELERY (APIUM GRAVEOLENS) SEEDS

The dormancy of celery seeds was broken by red light treatment given during imbibition and this effect was reversed by far‐red light. The exact quantitative relationship between the timing and duration of red light treatment and dormancy‐break has not been elucidated. However, the accumulated effect of daily 5 min exposures was greater than a single 5 min exposure on the second day of imbibition. The effects of red light treatment were simulated by treatment with a mixture of the gibberellins A4 and A7 and N6‐‐benzyladenine. A correlation between the requirement for red light and the requirement for exogenous cytokinins in the presence of GA4/7 was demonstrated by using six cultivars with different dormancy characteristics. In order to investigate the role of natural cytokinins in dormancy‐break, quantitative and qualitative changes in cytokinins were measured in celery seeds immediately after red‐light treatments. Rapid increases in n‐butanol‐soluble cytokinins following irradiation were associated with concomitant decreases in water‐soluble cytokinins, suggesting a red light induced cytokinin conversion. Three of the cytokinins present in the n‐butanol fraction of celery seed extracts were chromatographically similar to zeatin, zeatin riboside and N6‐‐A2‐‐isopentenyladenosine (i6Ado). The elution profiles from a PVP column of two others were similar to BA and its riboside. The possibility that these two cytokinins act as specific dormancy‐breaking cytokinins in celery seeds is discussed. There was little evidence of reversal of the cytokinin conversion mechanism by far‐red light exposure.

Gibberellin involvement in dormancy-break and germination of seeds of celery (Apium graveolens L.)

The temperature-dependent, primary dormancy of cv. Florida 683 celery seeds in darkness was partially broken by a 30 min light exposure on the third day of incubation at 20–22°C, resulting in c 50 percent germination after 20 days. This light stimulation was negated by including different inhibitors of gibberellin biosynthesis in the incubation medium. Subsequent addition of a solution of the gibberellins A4 and A7 or of the gibberellin-active compound (1-3-chlorophthalimido)-cyclohexane carboxamide (AC94,377) overcame the inhibitory effects on germination of these GA-biosynthesis inhibitors. It is suggested that light stimulates the biosynthesis of gibberellins which are essential for dormancy-break in celery seeds and that this biosynthesis is either directly or indirectly controlled through phytochrome.

Possible control of gibberellin-induced release of temperature-dependent primary dormancy in seeds of celery (Apium graveolens L.) by transmembrane ion fluxes

The temperature-dependent primary dormancy of cv Florida 683 celery seeds in darkness was broken by GA4/7 (2 × 10-4 M) alone but other growth regulators such as BA, ethephon or daminozide were necessary to break dormancy of cv Lathom Blanching seeds in the presence of GA4/7 at this concentration. Although AgNO3 partially inhibited both the ethephon- and BA- induced germination of cv Lathom Blanching seeds in the presence of GA4/7 in the dark it did not affect the promotive action of daminozide. Ethephon did not overcome the inhibitory action of high concentrations of AgNO3 in the light. The ethylene synthesis inhibitor aminoethoxyvinylglycine (AVG) did not inhibit the germination of cv Lathom Blanching seeds induced by growth regulators in the dark or in the absence of growth regulators in the light. Fusicoccin (FC) did not break celery seed dormancy unless applied in the presence of GA4/7. Germination of cv Lathom Blanching celery seeds treated with GA4/7 at 16°C in the dark was inhibited by the K+ ionophore benzo-18-crown-C-6 (18-C-6) and in the presence of Ca2+ by the Ca2+ ionophore A23187; the 18-C-6 inhibition was reversed by BA.It is concluded that the involvement of gibberellin in celery seed dormancy is not dependent on endogenous ethylene and is directly or indirectly controlled through the action of other hormones on transmembrane ion fluxes.

Responses of dormant heather (Calluna vulgaris) seeds to light, temperature, chemical and advancement treatments

Two seed lots of Calluna vulgaris were obtainedfrom English Nature (seed of Cornish provenance) (EN) and John ChambersWildflower Seeds (JCWS). In laboratory tests, under continuous light untreatedseeds of both seed lots were partially dormant at temperatures between14–35 °C, but JCWS seeds were more deeply dormant thanENseeds. The optimum temperature for germination for both lots was ca 18°C. Germination of EN seeds was much lower in the dark than inthe light at all temperatures; JCWS seeds did not germinate in the dark. In thelight at 22 °C, dormancy of both seed lots was broken whenseeds were incubated in GA4/7 solution(2 × 10−4 M). Dormancy ofJCWSseeds at 22 °C in the light was broken when seeds wereincubated in four different smoke solutions but more so when used incombinationwith GA4/7. Soaking seeds for 4h insmoke/GA4/7solutions before sowing improved both the speed andpercentage germination in pot experiments on a mist bench in the glasshouse byat least 10-fold. Soaking with GA4/7 alone produced a 5-fold increasein germination but seedlings were more etiolated than with thesmoke/GA4/7 mixtures. A seed advancement treatment modified from thatused commercially on sugar beet seeds also promoted germination in bothlaboratory and glasshouse tests. This entailed soaking seeds in 0.2% thiramsuspension for 4h followed by incubation in excess solution at 22°C for 4 days. This treatment was not as effective as thesmoke/GA4/7 seed soaks.

Comparative effects of gibberellins and an N-substituted phthalimide on seed germination and extension growth of celery (Apium graveolens L.)

The N-substituted phthalimide AC 94377 (1-(3-chlorophthalimido)-cyclohexanecarboxamide) was equally effective as a mixture of the gibberellins A4 and A7 (GA4/7) in breaking dormancy and stimulating germination of celery seeds when either was used in combination with ethephon or daminozide as a seed soak. Whereas seedlings emerging from GA4/7-treated seeds became etiolated in comparison with those from untreated seeds, those from AC 94377-treated seeds showed normal development. Preharvest sprays of gibberellic acid (GA3) increased the height of mature plants in comparison with untreated controls by about 16 per cent whereas AC 94377 was ineffective. The yield from GA3-treated plots was about 10 per cent greater than that from AC 94377-treated plots.

Evidence for the accumulation of a germination inhibitor during progressive thermoinhibition of seeds of celery (Apium graveolens L.)

The germination of seeds of celery (Apium graveolens L.) becomes progressively thermoinhibited on incubation in the dark at high temperatures, the inhibitory temperature being dependent on the cultivar used. In two high-dormancy cultivars of celery, the production of germination inhibitors in seeds incubated in the dark at 26°C gradually increased over a 7-day period. Inhibitor production was measured by incubating seeds of the low-dormancy cultivar Florida 683 in homogenates of the thermoinhibited seeds of the high-dormancy cultivars and recording germination either in the light or with the gibberellins A4 and A7 (GA4/7) in the dark. Most Florida 683 seeds which failed to germinate in the homogenates after 15 days were induced to germinate by addition of N6-benzyladenine (BA). The presence of BA in addition to GA4/7 throughout incubation in the dark completely overcame the inhibitory effects of homogenates. This indicates that thermoinhibition of celery seeds is associated with the accumulation of a germination inhibitor which interacts with cytokinins. This does not appear to be abscisic acid (ABA) since ABA levels in thermoinhibited seeds were lower than in untreated seeds and did not increase with duration of high temperature treatment.

Changes in endogenous cytokinins of celery (Apium graveolens L.) seeds following an osmotic priming or growth regulator soak treatment

Celery seeds were less thermoinhibited when dried back after a seed soak treatment with the gibberellins A4 and A7 (GA4/7) plus ethephon (G+E) or an osmotic priming treatment in the light with polyethylene glycol (PEG). At temperatures between 18 and 25° in the dark, 50 percent of the PEG-treated seeds germinated after 3 days whereas G+E-treated seeds required 7 days and untreated seeds did not germinate at all.Irrespective of treatment, dry control and dried-back, treated seeds contained very little detectable cytokinin activity. However, when such seeds were imbibed for 18 h in the dark and then analysed immediately with the soybean callus bioassay, less cytokinin activity was detected in both G+E and PEG seeds than in the untreated seeds. In particular, cytokinins with the HPLC properties of zeatin and its riboside were decreased in G+E seeds and virtually absent from PEG seeds. Conversely, extracts from PEG and to a lesser extent G+E seeds contained activity which chromatographically resembled cytokinin glucosides whereas this was absent from untreated seeds.

Responses of Florence fennel (Foeniculum vulgare azoricum) seeds to light, temperature and gibberellin A4/7

The percentage and the rate of germination of seeds of three varieties of Florence fennel was higher in the dark than in the light, the high temperature cut-off points being between 27.2 and 29.4°C. The optimum temperature for germination was between 20 and 25°C. Seeds of all three varieties responded to incubation in solutions containing gibberellin A4/7 mixture (GA4/7; ‘Regulex’), giving higher germination in the light at temperatures from 20 to 30°C. Seeds steeped for 4 h at 25°C or for 24 h at 5°C in GA4/7 solutions gave a higher percentage and increased rate of emergence as compared with untreated dry seeds, when sown in compost at 25°C; steeping in water alone was also beneficial. In general, drying the treated seeds before sowing reduced the rate but sometimes increased the percentage of germination as compared with seeds sown when still moist. Seeds harvested from secondary umbels of var. Zefa fino germinated better both in the light and dark than those taken from primary umbels.