Seed Maturity and the Effects of Different Drying Conditions on Desiccation Tolerance and Seed Longevity in Foxglove (Digitalis purpurea L.)

The effects of different drying methods on desiccation toleranceand longevity in seeds of foxglove (Digitalis purpurea L.) wereassessed from just prior to mass maturity (when seeds have attainedmaximum dry weight), and at intervals during the post-abscissionphase of development. Tolerance of drying under seed conservationconditions (15% relative humidity, RH, and 15 °C), was acquiredclose to mass maturity at 36 d after flowering (DAF). Increasesin desiccation tolerance were induced when drying was delayedfor 4 d by placing seeds in a near-saturated atmosphere (approx.100% RH), or if seeds were pre-dried for 7 d at either approx.32% or approx. 73% RH. Irrespective of the drying treatment, seed longevity increasedthroughout the sampling period, i.e. beyond the point of massmaturity and throughout the post-abscission phase, up to thepoint of incipient natural dispersal. At each developmentalstage, delayed drying or pre-drying led to an increase in seedlongevity under controlled ageing conditions compared with seedsdried directly under seed conservation conditions. Increasesin longevity were apparent as increases in the estimates forthe intercept of transformed seed survival curves (K_i) and forthe standard deviation of the normal distribution of seed lifespans,and also in the mean time to death of individuals in storage,consistent with a continuation of ripening events. The results are discussed in relation to the assessment of seedlongevity and to current post-harvest drying practices for seedsintended for long-term ex-situ conservation.Copyright 1995,1999 Academic Press Digitalis purpurea L., foxglove, seed development, seed drying, seed longevity     

Post-abscission, pre-dispersal seeds of Digitalis purpurea remain in a developmental state that is not terminated by desiccation ex planta

Background and Aims

Seed quality may be compromised if seeds are harvested before natural dispersal (shedding). It has been shown previously that slow or delayed drying can increase potential quality compared with immediate rapid drying. This study set out to investigate whether or not there is a critical moisture content, below which drying terminates maturation events for seeds harvested after mass maturity but before dispersal.

Methods

Seeds of foxglove (Digitalis purpurea) in the post-abscission pre-dispersal phase were held at between 15 and 95 % RH for 4 or 8 d, with or without re-hydration to 95 % RH for a further 4 d, before drying to equilibrium at 15 % RH. In addition, dry seeds were primed for 48 h at −1 MPa. Subsequent seed longevity was assessed at 60 % RH and 45 °C.

Key Results

Rate of germination and longevity were improved by holding seeds at a wide range of humidities after harvest. Longevity was further improved by re-hydration at 95 % RH. Priming improved the longevity of the seeds dried immediately after harvest, but not of those first held at 95 % RH for 8 d prior to drying.

Conclusions

Maturation continued ex planta in these post-abscission, pre-dispersal seeds of D. purpurea dried at 15–80 % RH at a rate correlated positively with RH (cf. ageing of mature seeds). Subsequent re-hydration at 95 % RH enabled a further improvement in quality. Priming seeds initially stored air-dry for 3 months also allowed maturation events to resume. However, once individual seeds within the population had reached maximum longevity, priming had a negative impact on their subsequent survival.Key words: Digitalis purpurea, seed development, post-harvest treatment, priming, longevity, ex situ conservation     

Production of a Fungistat and the Role of Fungi during Germination of Digitalis purpurea L.cv. Gloxiniaflora Seeds

The seed coats of Digitalis purpurea L. cv. Gloxiniaflora werecultured for indigenous fungi. Alternaria alternata (Fries)was identified as the sole fungus on seeds freshly harvestedfrom unopened capsules, whereas A. alternata, Rhizopus arrhizus(Fischer), Penicillium sp. and other fungi appeared on storedseeds. The appearance of fungi in seed cultures was seasonal,being more frequent in winter and early spring than in summerand autumn. Alternaria grew well on autoclaved seeds, on dehiscentseed coats, or on seed coats separated from the embryos of ungerminatedseeds. Rhizopus did not grow on these but grew weakly on theculture medium from viable seeds. A. alternata appears to functionas a degradation agent for the seed coat subsequent to germination.Neither fungus was found to be essential to germination of Digitalisseeds. Bioassay of the culture medium from germinating seedsshowed that a fungistat effective against both Alternaria andRhizopus is produced coincident with germination. Based on chromatographicanalysis, the fungistat appears to be a cardenolide. Alternaria alternata (Fries), cardenolides, Digitalis purpurea L. cv, Gloxiniaflora, fungistats, germination, Rhizopus arrhizus (Fischer)     

Plastid Ontogeny in the Corolla Cells of Digitalis purpurea L. cv. Foxy

Initially the corolla plastids of Digitalis purpurea containsmall grana with negatively stained thylakoids. Degenerationof the grana, loss of chlorophyll and the transient accumulationof starch accompany corolla expansion Starch disappears by thetime the anthers dehisce and granular and amorphous phytoferrtindeposits become prominent in the stroma Concomitant with theseparation of the stigmatic lobes the thylakoid system is reducedto a central membranous network enveloping the phytofemtm aggregatesJust prior to corolla abscission the stroma becomes packed withplastoglobuh Although this developmental sequence closely resemblesthat for chromoplasts in yellow and red flowers, fruits andautumn leaves there is no synthesis of carotenoid pigments inthe corollas of Digitalis purpurea At maturity the plastidsare therefore best described as elaioplasts. Digitalis purpurea L., foxglove, corolla, plastids, elaioplasts, phytofemtin, ultrastructure, X-ray microanalysis     

A Comparison of Leek (Allium porrum) and Onion (Allium cepa) Seed Development

Leek and onion seed dry weight increased exponentially for thefirst 31 days after flowering (DAF) but thereafter the increasein dry weight was slower. Before maximum seed dry weight wasreached at 45 DAF in onion and 59 to 66 DAF in leek, seed moisturecontent, seed oxygen uptake and conductivity of the seed steepwater fell from initially high levels. Although some seeds germinated31 DAF in both species, full germination in both was not reacheduntil 66–80 DAF. Tolerance of the seed to artificial dryingimmediately after harvesting occurred 45 DAF in onion and 74DAF in leek. Free nuclear division continued in the endospermuntil 17–22 DAF in onion and until 31–35 DAF inleek but it was not until 45 DAF in onion and 66 DAF in leekthat the embryo and endosperm filled the cavity formed by thepericarp. After formation of cell walls in the endosperm thepattern of change in cell number in both species was similar.The shrunken appearance of the seed coat in leek, which occurredearly in seed development, was associated with the period offree nuclear division in the endosperm and, in addition, thepericarp was thinner than in onion. There was no evidence thatthe shrunken seed coat early in development was associated withself as opposed to open-pollination. Allium porrum, Allium cepa, seed development, endosperm, embryo, cell number, germination, respiration, seed leachates     

Synthesis of Canavalin and Concanavalin A in Maturing Canavalia gladiata Seeds

In successive stages of seed development of Canavalia gladiata,seeds were harvested, and time-course changes of the accumulationof canavalin and concanavalin A (Con A) were investigated bySDS-gel electrophoresis and protein blot analysis. Results showedthat the synthesis of both seed proteins started as early as30 days after flowering (DAF) and that they had different accumulationpatterns during seed development. The synthesis and accumulationof canavalin were most active at SOSO DAF, whereas the contentof Con A continued to increase gradually until the seed maturationwas nearly completed (80 DAF). Next, an RNA fraction was preparedfrom seeds of 40 DAF, translated in a wheat germ system andits translation products analyzed by the immunoprecipitationand SDS-gel electrophoresis. The mol wt (about 48,000) of thein vitro product immunoprecipitated with the antiserum to canavalinwas very closed to that of canavalin. The mol wt (34,000) ofthe in vitro product immunoprecipitated with the antiserum toCon A was about 4,000 higher than that of Con A. Canavalin-mRNAwas present as early as at 30 DAF and its translational activitywas the highest at 35–40 DAF. Con A-mRNA was also detectedat 30 DAF and, in contrast to canavalin-mRNA, high activitieswere maintained until 75 DAF. (Received November 25, 1986; Accepted January 29, 1987)     

The Floral Nectary ofDigitalis purpureaL., Structure and Nectar Secretion

The floral nectary of the foxglove (Digitalis purpureaL.), locatedat the base of the ovary, was examined by: scanning electronmicroscopy; quantitative bright-field microscopy via computer-aided3-D reconstruction from serial sections; morphometric procedures;transmission electron microscopy and measurement of nectar effluxunder different experimental conditions. Time-lapse video recordingvia a microscope with incident light clearly showed that thenectar escaped from the apertures of modified stomata. The volumeflux via individual stomatal apertures was 0.31±0.1 nlmin^-1; therefore only a fraction of the total number of stomataper nectary (115±8) would be sufficient to dischargethe amount of nectar reported in previous publications. Thestomatal apertures are continuous with intercellular spacestraversing the small-celled nectariferous tissue. The latteris vascularized only by phloem, whose termini consists of rowsof slender cells. These sieve-like cells are surrounded by moreor less isodiametrical sheath cells with dimensions similarto the secretory cells. Details of nectary functioning are basedon enhanced structural information, complementary data on nectardischarge after experimental manipulations and the nature ofthe effluence.Copyright 1998 Annals of Botany Company Digitalis purpureaL.; foxglove; floral nectary; (ultra-)structure; 3-D reconstruction; morphometry; nectar flow; time-lapse video recording.     

Priming and re-drying improve the survival of mature seeds of Digitalis purpurea during storage

Background and Aims

Most priming studies have been conducted on commercial seed lots of unspecified uniformity and maturity, and subsequent seed longevity has been reported to both increase and decrease. Here a seed lot of Digitalis purpurea L. with relatively uniform maturity and known history was used to analyse the effects of priming on seed longevity in air-dry storage.

Methods

Seeds collected close to natural dispersal and dried at 15 % relative humidity (RH), 15 °C, were placed into experimental storage (60 % RH, 45 °C) for 14 or 28 d, primed for 48 h at 0, −1, −2, −5, −10 or −15 MPa, re-equilibrated (47 % RH, 20 °C) and then returned to storage. Further seed samples were primed for 2 or 48 h at −1 MPa and either dried at 15 % RH, 15 °C or immediately re-equilibrated for experimental storage. Finally, some seeds were given up to three cycles of experimental storage and priming (48 h at −1 MPa).

Key Results

Priming at −1 MPa had a variable effect on subsequent survival during experimental storage. The shortest lived seeds in the control population showed slightly increased life spans; the longer lived seeds showed reduced life spans. In contrast, seeds first stored for 14 or 28 d before priming had substantially increased life spans. The increase tended to be greatest in the shortest lived fraction of the seed population. Both the period of rehydration and the subsequent drying conditions had significant effects on longevity. Interrupting air-dry storage with additional cycles of priming also increased longevity.

Conclusions

The extent of prior deterioration and the post-priming desiccation environment affect the benefits of priming to the subsequent survival of mature seeds. Rehydration–dehydration treatments may have potential as an adjunct or alternative to the regeneration of seed accessions maintained in gene banks for plant biodiversity conservation or plant breeding.Key words: Digitalis purpurea, priming, re-drying, seed longevity, seed storage, ageing     

Subcuticular Secretion by Cactus Seeds Improves Germination by Means of Rapid Uptake and Distribution of Water

We report a formerly unknown mechanism in seeds which improvesgermination under relatively dry conditions. During seed developmentof several South American cacti, epidermal cells produce proteinaceousmaterial that appears to pass through ectodesmata in the outercell wall and which accumulates under the cuticle. Once moistened,this secretory layer readily absorbs water and distributes itover the seed surface. It thus improves water uptake and ensuresgermination with the minimum amount of available water, whichmay be advantageous in (semi-)arid regions. In experiments withseeds ofEchinopsis thionantha(Speg.) Werd. andGymnocalyciumgibbosum(Haw.) Pfeiff. under water stress, intact seeds tookup significantly more water and germinated better than seedsfrom which the hydrophilous layer had been artificially removed. Cactaceae; seeds; ectodesmata; imbibition; germination     

Relationships between indices of seed maturity and carrot seed quality

Carrot seeds cvs Chantenay and Amsterdam were harvested on several occasions from crops grown in 1985 and 1986, from the time they had reached or were close to their maximum dry weight and were starting to turn brown. Maximum seed dry weight occurred approximately 40–45 days after flowering (DAF) in both cultivars. Maximum germination (International Seed Testing Association 14-day count) occurred 40 and 55 DAF in cvs Chantenay and Amsterdam, respectively, but the maximum 7-day count and the minimum coefficient of variation of embryo length did not occur until 60 DAF in cv. Chantenay and 55 to 65 DAF in cv. Amsterdam. Percentage germination was negatively and linearly related to seed moisture content, chlorophyll a + b content in the seed coat and seed distortion, measured on a modified tensile-testing machine, the relationships accounting for 77, 71 and 64% of the variance in the 7-day germination count, respectively. The corresponding values for the 14-day count were 63, 61 and 50%. A simple field test for monitoring seed maturity is proposed.     

The Effect of Temperature on Seed Set and Seed Development in Detached Spikelets of Perennial Ryegrass (Lolium perenne L.)

In perennial ryegrass seed yield is low. Pollination, fertilizationand early stages of seed development are critical periods forthe realization of the yield potential. As intact plants aredifficult to handle, we used detached spikelets to study effectsof temperature and genotype on seed set and seed development. Plants of four cultivars were grown in pots. Just prior to flowering,spikelets were detached from the ears and put on water. Afterpollination, various spikelets of each plant were placed atdifferent temperatures (14/14, 17/12 or 20/15°C). Sevendays after pollination the percentage seed set and the lengthof the developing seeds (caryopses) were assessed. Only floretsin the four basal positions were used. For each cultivar an analysis of variance for unbalanced datawas carried out. There were highly significant difference amonggenotypes for seed set and caryopsis length. There was no evidencefor genotypic variation for tolerance to low temperatures. Neithertemperature nor floret position had an effect on seed set. Thecaryopsis length, however, increased with increasing temperatureand basal florets produced longer caryopses than distal florets.The detached spikelets proved to be a useful experimental tool.Copyright1993, 1999 Academic Press Lolium perenne L., perennial ryegrass, seed yield, seed set, detached spikelets culture, temperature, floret position     

Changes in the Molecular-size Distribution of Carbohydrates during Ripening in Attached and Detached Seeds of Lolium temulentum L.

Seed samples of Lolium temulentum L. were harvested at variousstages of maturity and subjected to successive cold- and hot-waterextractions. These extracts were then fractionated by gel-filtrationand the changes in the carbohydrate elution pattern with increasingseed maturity were characterized. Extracts of seeds from earlyharvests contained only free sugars, but the mean chain-lengthincreased with maturity. The hot-water fraction contained nosalient features until 17 days after anthesis but, after thispoint, a completely excluded, high-molecular-weight polysaccharideaccumulated. Infiltration studies using ^I4C-sucrose and detached seeds showedthat sugar was readily incorporated into polysaccharide, butthat the degree of uptake was directly proportional to the developmentalstage of the seed at the time of detachment. Hydrolysis dataon the various components are given and the results are discussedin terms of their practical significance.     

Moisture Loss as a Prerequisite for Seedling Growth in Soybean Seeds (Glycine max L. Merr.)

Rosenberg, L. A. and Rinne, R. W. 1986. Moisture loss as a prerequisitefor seedling growth in soybeanseeds (Glycine max L. Merr.).—J.exp. Bot. 37: 1663–1674. As soybean seeds [Glycine max (L.) Merr.] develop, they undergoa change in seed moisture. When excised prematurely from thepod and planted, seeds do not exhibit seedling growth until63 d after flowering (DAF) when the seed moisture has fallenbelow 60%. In contrast, seed germination (radicle protrusion)can occur when seeds as young as 35 DAF (68–79% moisture)are excised, but this germination docs not lead to comparableseedling growth frequencies unless seeds are first given a moistureloss treatment to artificially reduce their moisture below 60%.A moisture loss treatment applied at 35 DAF thus enables seedto undergo the transition from germination (cell expansion)to seedling growth (cell division and expansion) to the extentthat treated immature seed have a vigour index comparable toseeds matured on the plant (100%). The pattern of protein synthesisin vivo was examined in 35 DAF seed using [³⁵S]-methionine incorporation.When moisture loss treatment was applied for 24 h to 35 DAFseeds, seeds synthesized several new polypeptides when comparedwith untreated seeds at the same developmental stage. The sameseed samples showed 0% seedling growth in the absence of moistureloss treatment and 80% seedling growth when the treatment hadbeen applied. Moisture loss from soybean seeds appears to bea prerequisite for the synthesis of new proteins which may bepart of the metabolic process or processes that allow the soybeanseed to undergo the transition from seed germination to seedlinggrowth. Key words: Moisture loss, germination/growth, soybean     

Developmental Changes in Relation to Desiccation Tolerance of Archontophoenix alexandrae (Palmae) Seeds

Germination of Archontophoenix alexandrae seeds and embryos were studied under gradient water content treatments throughout the seed development phases of maturation in 2005 to investigate seed desiccation tolerance and storage characteristics. During the maturation process, seed water content decreased gradually from55 DAF (days after flowering) to 70 DAF, and seeds reached the maximum dry-weight at 90 DAF. Seed germinability appeared after 60 DAF. Seeds germinated with a temperature range from15℃- 40℃ under alternating photoperiod (14 h light, 10 h dark, 12μmol m^{- 2}s ^{- 1} ), while the best germination percentage was obtained between 30℃- 35℃. A maximum germination capacity reached at 70 DAF. However, seed germination was greatly inhibited by light. Desiccation tolerance of seeds and embryos increasedgradually from 55 DAF to 90 DAF and reached the maximum at 90 DAF with a semilethal water content of 0.18 g/g ( seed) and 0.3 g/g ( embryo) respectively. Rapid dehydration maintained higher seed germination percentage than thatof slow dehydration when drying to the same water content. Seeds with without water content treatments failed to germinate after 1 month storage under - 18℃, whereas appropriate desiccation treatment prolonged seed longevity under 4℃, 10℃ and 15℃ storage temperatures. It revealed obviously the recalcitrant characteristics of Archontophoenix alexandrae seeds torage behaviour which are tolerant toward neither deep desiccation nor low temperatures.     

Longevity of pearl millet (Pennisetum glaucum) seeds harvested at different stages of maturity

The storage potential of seeds harvested at weekly intervals after controlled pollination was studied in three diverse cytoplasmic male sterile pearl millet (Pennisetum glaucum) lines. In the first experiment in 1989, a comparison of p₅₀ (time for viability to decline to 50% during storage) among seeds of the line DSA 105A harvested 14, 21, 28, 35 and 42 days after pollination (DAP), and then stored at 35°C with 15% moisture content or 40°C with 13% moisture content, showed that those harvested 35 DAP had the greatest longevity. In the second experiment in 1990, a comparison of p₅₀ within the lines 5141A and L 67A harvested 28, 35 and 42 DAP, and then stored at 40°C with 13% moisture content, showed that seeds of both lines harvested 42 DAP had the greatest longevity. In both the seasons, and in all three lines, maximum seed longevity (p₅₀) was attained one week after physiological maturity (defined as the end of the grain filling period), which is therefore the optimum time of harvest to obtain good quality seeds for conservation.     

The Dry Storage of Citrus Seeds

The survival of seeds of lemon (Citrus limon L.), lime [C. arantifolia(Christm.) Swing.] and sour orange (C. aurantium L.) was examinedunder a wide range of constant moisture contents and temperatures.Seed longevity was increased by decreasing the moisture contentand temperature of the storage environment. Maximum viabilitywas maintained in the combination of storage conditions includingthe lowest moisture content (5 per cent) and lowest temperature(–20 °C) investigated. The practicality of dry storageof citrus seed for genetic conservation is discussed. Citrus limon L., lemon, Citrus aurantifolia (Christm.), Swing, lime, Citrus aurantium L., sour orange, dry storage, moisture content, temperature, seed viability, seed longevity     

Expression of a GALACTINOL SYNTHASE gene is positively associated with desiccation tolerance of Brassica napus seeds during development

Desiccation tolerance of seeds is positively correlated with raffinose family oligosaccharides (RFOs). However, RFOs’ role in desiccation tolerance is still a matter of controversy. The aim of this work was to monitor the accumulation of RFO during acquisition of desiccation tolerance in rapeseed (Brassica napus L.). Rapeseeds become desiccation tolerant at 21-24 d after flowering (DAF), and the time was coincident with an accumulation of raffinose and stachyose. A gene encoding galactinol synthase (GolS; EC2.4.1.123), involved in RFO biosynthesis, was cloned and functionally characterized. Enzymatic properties of recombinant galactinol synthase were also determined. Accumulation of BnGOLS-1 mRNA in developing rapeseeds was concomitant with dry weight deposition and the acquisition of desiccation tolerance, and was concurrent with the formation of raffinose and stachyose. The physiological implications of BnGOLS-1 expression patterns in developing seeds are discussed in light of the hypothesized role of RFOs in seed desiccation tolerance.     

Desiccation Tolerance and Potential Longevity of Developing Seeds of Rice (Oryza sativa L.)

The longevity and desiccation tolerance of samples of seedsof a japonica rice (Oryza sativa L.) harvested serially duringdevelopment from plants grown in two temperature regimes, viz28/20 °C and 32/24 °C (12/12 h) were determined. Massmaturity (defined as the end of the seed-filling phase) occurred19·7 and 18·3 d after 50% anthesis, respectively.Longevity (determined at 40 °C with 15% moisture contentand quantified by the value of the constant K_i of the seed viabilityequation) improved during seed development and maturation until17 and 14 d after mass maturity in the cooler and warmer regimes,respectively, but declined thereafter. Changes in K_i with timewere similar in the two environments until mass maturity, butthe increase in K_i values after mass maturity was much greaterin the cooler regime. Tolerance of desiccation to low (4%) moisturecontents improved until 22 and 14 d after mass maturity in thecooler and warmer regimes, respectively, when maturation dryinghad reduced seed moisture contents naturally to 24 and 32% moisturecontent, respectively. Further delays to seed harvest reduceddesiccation tolerance, particularly in the warmer environment.Comparison among 15 samples of seeds harvested at differenttimes in the two environments showed a strong correlation (r= 0·947, P < 0·01) between longevity (K_i) anddesiccation tolerance (to 4% moisture content). Hence, it issuggested that the regulation of desiccation tolerance to lowmoisture contents and potential air-dry longevity during seeddevelopment and maturation determined here may have a commoncause.Copyright 1994, 1999 Academic Press Oryza sativa L., rice, desiccation tolerance, genebanks, seed development, seed longevity, temperature     

The Effect of Chilling and Moisture Status on the Germination, Desiccation Tolerance and Longevity ofAesculus hippocastanumL. Seed

Effects of 2 °C chilling on the threshold moisture contentsand water potentials for various physiological processes wereestimated forAesculus hippocastanumL. seed. Seed harvested atthe time of maximum seed fall exhibited a dual response to drying:partial drying from near 50% to 32–40% moisture contentprogressively increased germination percentage (at 16 °C)up to various peak values; further desiccation was detrimental,confirming that the seeds are ‘recalcitrant’. Themoisture content for optimum germination was increased by atleast 10% as the chilling period was raised from 0 to 9 weeks.A negative linear relationship was found between log₁₀mean timeto germinate and probit final germination, regardless of pre-treatment,indicating that partial desiccation and chilling are interchangeablein promoting germination of hydrated seed. For nearly fullyhydrated seeds, increasing the chilling period from 6 to 26weeks increased the viability-loss onset point for desiccationinjury from near 40% to about 48% moisture content without alteringthe drying rates of seed tissues. Extending moist chilling invarious seed lots from 0 to 26 weeks decreased subsequent longevityat 16 °C. For 26-week-chilled seeds longevity (the periodto lose one probit of germination) differed above and belowa threshold moisture content of 48%. It remained constant inthe moisture-content range 48–38%, but increased progressivelyas moisture content was raised above 48%. This threshold moisturecontent coincided with the value above which chilled seed pre-germinatedin storage. The results indicate that post-harvest desiccationand chilling alter the water relations of various physiologicalprocesses and a schematic summary is presented which relatesthe results to an axis water sorption isotherm.Copyright 1998Annals of Botany Company Aesculus hippocastanumL., horse chestnut, chilling, moisture content, water potential, desiccation tolerance, longevity, recalcitrant seed, embryo axis, maturation, germination.