α-Amylase in the Barley Grain Influenced by the Auxin-Cytokinin Interaction in the Coleoptile

Two phases are distinguished in the α-amylase production in barley (Hordeum vulgare) grains. There is an increase in activity extended to the third or fourth day of germination, then a slight decrease follows. This decrease is accelerated by kinetin while it is prevented by IAA applied at the top of the embryo coleoptile. IAA reverses partially the kinetin action. IAA applied in the germination medium has practically no effect. Removal of the coleoptile stops further increase in α-amylase activity and induces complete insensitivity to hormone treatment. The results indicate that auxin metabolism in the coleoptile participates in the control of α-amylase evolution in the barley grain and that kinetin could act through auxin metabolism in this coleoptile.     

Anoxia tolerance in rice seedlings: exogenous glucose improves growth of an anoxia-'intolerant', but not of a 'tolerant' genotype

This study demonstrated that, in rice seedlings, genotypic differencein tolerance to anoxia only occurred when anoxia was imposedat imbibition, but not at 3 d after imbibition. When seeds wereimbibed and grown in anoxia, IR22 (anoxia-‘intolerant’)grew much slower and had lower soluble sugar concentrationsin coleoptiles and seeds than Amaroo (anoxia-‘tolerant’),while Calrose was intermediate. After 3 d in anoxia, the sugarconcentrations in embryos and endosperms of anoxic seedlingswere nearly 4-fold lower in IR22 than in Amaroo. Sugar deficitin the embryo of IR22 is presumably due to the limitation ofsugar mobilization rather than the capacity of transport asshown by similar sugar accumulation ratios of 1.8 between embryoand endosperm in IR22 and Amaroo at 3 d in anoxia. With 20 molm^–3 exogenous glucose, coleoptile extension and freshweight increments in anoxic seedlings of IR22 were much closerto those in the two other genotypes, nevertheless protein concentrationremained lowest on a fresh weight basis in the coleoptiles ofIR22; indicating that protein synthesis has a lower priorityfor energy apportionment during anoxia than processes crucialto coleoptile extension. In contrast to these responses to anoxiaimposed at imbibition, IR22 had nearly the same high toleranceto anoxia as Calrose and Amaroo, when anoxia was imposed onseedlings subsequent to 48 h aeration followed by 16 h hypoxicpretreatment. In fact, coleoptiles of anoxic IR22 had highersugar concentrations and grew faster than Calrose, and exogenousglucose had no effect on the coleoptile extension of IR22. Excisedcoleoptile tips of IR22 and Amaroo with exogenous glucose hadsimilar rates of ethanol production and were equally tolerantto anoxia. In conclusion, much of the anoxia ‘intolerance’of IR22 when germinated in anoxia could be attributed to limitedsubstrate availability to the embryo and coleoptile, presumablydue to slow starch hydrolysis in the endosperm. Key words: Anoxia, coleoptile, embryo, endosperm, ethanol production, germination, growth, Oryza sativa L., solute net uptake or loss, sugar availability.     

Abscisic acid in developing wheat grains and its relationship to grain growth and maturation

Summary During the later stages of growth of grains of wheat (Triticum aestivum L. cvs. WW15 and Gabo) there is a dramatic increase (up to 40fold) in the content of abscisic acid (ABA) to 4–6 ng per grain. This level remains high from 25 to 40 days after anthesis. Then, in association with natural or forced drying of the grain, there is a rapid drop (5–10 fold) in the ABA content and a brief increase in the content of bound ABA. The bulk of ABA in an ear was in the grain (95%) and although the embryo contributed 19% of this ABA it was less than 5% of the grain by weight. There was no clear relationship between ABA content and the growth of grains in various spikelet or floret positions. Application of (±)-ABA to the ear had no effect on grain growth rate but led to an earlier cessation of grain growth and hastened the drying of the grain. Isolated embryos and whole grains were capable of germinating during the mid grain growth period (15–25 days), but germination capacity declined subsequently as ABA accumulated. Later, still, with grain drying and loss of ABA, embryo and grain became germinable again. At this time there was also a dramatic increase in the ability of the grain to synthesize -amylase. It is suggested that the accumulation of ABA at the later stages of grain growth prevents precocious germination and premature hydrolysis of starch reserves of the morphologically mature but still unripe grain. An inevitable consequence of such action may be in triggering grain maturation.     

Manipulation of Grain Dormancy in Wheat

To help in understanding the mechanisms of dormancy in wheat(Triticum aestivum L.), brief drying of intact ears has beenexamined as a technique for rapid imposition of dormancy. Beforenatural grain desiccation at 40 d post anthesis (dpa), wholeplants were moved for 15 to 20 d to ‘wet’, humidconditions (90–100% RH) or to ‘dry’ conditions(35–40% RH). The duration of subsequent dormancy was atleast doubled by the ‘dry’ treatment, however, onlyif the grain was allowed to dry before 50 dpa. In the ‘wet’ears there was a low level (up to 22%) in-ear germination by55 dpa and the remaining, non-sprouted grain no longer becamedormant on drying. Brief (4 d) drying episodes could also preventsprouting and induce some dormancy, but only over a 4 to 10d window of sensitivity before 50 dpa. Grain drying when properlytimed not only arrests development but, perhaps through damagingeffects of drying, causes dormancy. Dormancy imposition wasnot related to embryo abscisic acid (ABA) or sucrose content. Key words: Abscisic acid, desiccation, dormancy, in-ear sprouting, wheat     

The Role of Maturation Drying in the Transition from Seed Development to Germination: V. RESPONSES OF THE IMMATURE CASTOR BEAN EMBRYO TO ISOLATION FROM THE WHOLE SEED: A COMPARISON WITH PREMATURE DESICCATION

Kennode, A. R, and Bewley, J. D. 1988. The role of maturationdrying in the transition from seed development to germination.V. Responses of the immature castor bean embryo to isolationfrom the whole seed; a comparison with premature desiccation.—J.exp. Bot. 39: 487–497. Desiccation is an absolute requirement for germination and post-germinativegrowth of whole seeds of the castor bean, whether desiccationis imposed prematurely during development, at 35 d after pollination(DAP) or occurs naturally during late maturation (50–60DAP). Desiccation also plays a direct role in the inductionof post-germinative enzyme synthesis in the cotyledons of embryosin the intact seed; this event is not simply due to the presenceof a growing axis. Isolation of embryos from the developingcastor bean seed at 35 DAP results in both germination and growth,despite the absence of a desiccation event. We have comparedthe metabolic consequences of premature drying of whole seeds(35 DAP) and isolation of the developing 35 DAP embryos. Inboth cases, hydrolytic events involved in the mobilization ofstored protein reserves proceed in a similar manner and mirrorthose events occurring within germinated mature seeds. Thereare differences, however, for post-germinative enzyme (LeuNAaseand isocitrate lyase) production occurs to a lesser extent innon-dried isolated embryos than in those from prematurely dried(35 DAP) whole seeds, or from mature dry (whole) seeds. Desiccationof the 35 DAP whole seed does not alter the subsequent responseof the embryo upon isolation. Thus, while drying does not affectthe metabolism of isolated embryos, it has a profound effecton that of embryos within the intact seed. Tissues surroundingthe embryo in the developing intact seed (viz. the endosperm)maintain its metabolism in a developmental mode and inhibitgermination. This effect of the surrounding tissues can onlybe overcome by drying or by their removal. Key words: Metabolism, isolation, desiccation, embryo, endosperm, castor bean, development, germination     

The Morphology of Germination in Setaria lutescens (Gramineae): The Effects of Covering Structures and Chemical Inhibitors on Dormant and Non-dormant Florets

The sequence of organ emergence in embryos of the yellow foxtailgrass (Setaria lutescens) is similar to that reported for othergrasses: coleorhiza, radicle, coleoptile and first leaf. Thecoleorhiza emerges by forcing open a hinged flap on the lemma.Coleorhiza trichomes form soon after emergence. The radicleprotrudes through the abaxial surface of the coleorhiza. Thecoleoptile elongates in a downward direction initially as itpasses between the lemma and palea, but immediately turns upward.The first leaf emerges by protruding through a slit-like creaseon the adaxial surface of the coleoptile. Embryos excised fromdormant florets are shown to germinate as well as those fromnon-dormant florets. Experiments are described which show theinhibitory effects of the embryo covering structures; the lemma,palea and caryopsis coat. Other experiments are discussed concerningthe effects on germination of the inhibitors abscisic acid andcycloheximide. An intermediate concentration of cycloheximide(10^–4 M) does not decrease germination percentage, butrather inhibits radicle growth but no coleoptile elongation.The significance of these observations is discussed.     

THE ULTRASTRUCTURAL MORPHOLOGY AND ONTOGENY OF OAT COLEOPTILE PLASTIDS

Structurally similar proplastids occur in the shoot, scutellum, and root of the oat embryo at the start of germination. These proplastids follow several pathways of differentiation, depending on their location within an organ and on previous exposure to light. During the first 24 hr of germination morphologically similar amyloplasts are formed from the preexisting proplastids in most of the cells of the seedling. After about 24 hr in the light, unique chloroplasts begin to develop in a subepidermal ring of small cortical parenchyma cells in the coleoptile and give the organ a pale green color. At 48 and 72 hr the coleoptile chloroplasts and etioplasts are conspicuously different from the corresponding leaf plastids in morphology and ontogeny but contain typical photosynthetic grana and prolamellar bodies. Study of the ontogeny of plastids in the epidermal and nongreening parenchymal regions of dark grown coleoptiles shows that these plastids undergo significant losses in starch content, and some increase of membranes within the plastid, related to the age of the cell. Light has little effect on the structure of these plastids. It is suggested that the ontogeny of all the plastid types of the oat seedling begins with a common precursor—a relatively simple proplastid that is present at the time of germination. Starch grains showing two distinct types of erosion, apparently enzymatic, were observed in oat coleoptile plastids. In one type (grooved appearance) the starch grains are consistently associated with plastid membranes, while in the other type (irregular, spiny appearance) the starch grains are associated with the plastid stroma only. We suggest that there are two enzyme systems for metabolizing starch in oat plastids—one membrane-bound and the other free in the stroma.     

GERMINATION AND SEEDLING GROWTH IN ECHINOCHLOA CRUS-GALLI VAR. ORYZICOLA UNDER ANOXIC CONDITIONS: STRUCTURAL ASPECTS

The morphological and cellular basis of anoxic germination in Echinochloa crus-galli var. oryzicola is reported. The embryo of E. crus-galli var. oryzicola is typically panicoid in its overall morphology, and is relatively large with a prominent coleoptile and mesocotyl. The response to anoxia is essentially the same in light and dark. Shoot growth occurs in both mesocotyl and coleoptile by cell elongation with no cell division. There is no emergence of the radicle without oxygen. Under anoxia the growth response is not the same as etiolation; there is no plumule elongation within the coleoptile, no protochlorophyll(ide) is found, and limited mesocotyl elongation occurs without oxygen. Air-dark treatment after anoxic germination results in a typical etiolated morphological response, including a resumption of mesocotyl growth, elongation of the plumule within the coleoptile, and initiation of pigment synthesis. These results indicate the effects of anoxia are not permanent but rather limiting and reversible.     

Growth and Cellular Differentiation in the Wheat Coleoptile (Triticum vulgare L.): I. ESTIMATION OF CELL NUMBER, CELL VOLUME, AND CERTAIN NITROGENOUS CONSTITUENTS

The numbers and volumes of cells were determined for consecutivestages in the growth and development of the wheat coleoptile(var. ‘King II’) when grown at 25° C. in darknessfrom soon after germination to senescence. Cell expansion occurredthroughout growth and development up to 96 hours, and was accompaniedby cell division between 18–60 hours. Evidence is presentedthat suggests there are two phases of cell expansion concernedin coleoptile growth. Determinations were made at daily intervals from 24 to 120 hoursafter sowing of protein nitrogen, trichloroacetic acid solublenitrogen (TCA-sol. N), ribonucleic acid (RNA), and deoxyribonucleicacid (DNA) in the coleoptile, and the results expressed on aper-coleoptile and per-cell basis. The maximum rate of net proteinsynthesis took place during or after the cell-multiplicationphase of growth, depending on whether the results were expressedper coleoptile or per cell respectively. The ratio of proteinnitrogen to TCA-sol. N changed considerably during growth, from4·7 for young cells to 0·68 for mature cells. The fluctuations in the values for RNA and protein are consistentwith the template theory of protein synthesis and the DNA dataare discussed in relation to polyploidy in differentiated cells.No significant difference was found in the nucleotide compositionof RNA during the growth and development of the coleoptile.     

Weedy adaptation in Setaria spp. IV. Changes in the germinative capacity of S. faberii (poaceae) embryos with development from anthesis to after abscission

Studies were conducted evaluating germinability states in giant foxtail (Setaria faberii) embryos, as well as surrounding tissues (hull, caryopsis), with germination assays. Further, seed age, fascicle arrangement, flowering patterns, and elongation in the inflorescence were evaluated. Both qualitatitive and quantitative morphological observations of the hull and the caryopsis were revealed by precisely determined fertilized spikelet age from anthesis until after seed abscission. Red coloration of the placental pad at ≈ 11 d after anthesis is probably a morphological indicator of physiological maturity. Germinability of giant foxtail embryos changed with development. Four qualitatively different types of embryo germination were observed during development of the seed: early disorganized callus growth at the basal, coleorhizal end of the embryo; germination of immature embryos with shortened and thickened axes; germination of the scutellum; and germination and growth of the coleoptile and coleorhiza in embryos aged 7 d after anthesis and older. Axis-specific embryo germinability was also observed. Inhibition of the embryo could be localized to the coleoptile, the coleorhiza, or both. These studies provide evidence for a complex model of germinability regulation based on the independent, asynchronous actions of the embryo, caryopsis, and hull compartments, as well as on their dependent, synchronous action. These studies provide evidence for a dynamic, developmental model of giant foxtail germinability regulation resulting in phenotypes with a wide range of germinability shed from an individual panicle. These diverse germinability phenotypes are found at all stages of development, but particularly when the seed is shed and the soil seed bank is replenished.     

Suppression of glucan,water dikinase in the endosperm alters wheat grain properties,germination and coleoptile growth

Starch phosphate ester content is known to alter the physicochemical properties of starch, including its susceptibility to degradation. Previous work producing wheat (Triticum aestivum) with down‐regulated glucan, water dikinase, the primary gene responsible for addition of phosphate groups to starch, in a grain‐specific manner found unexpected phenotypic alteration in grain and growth. Here, we report on further characterization of these lines focussing on mature grain and early growth. We find that coleoptile length has been increased in these transgenic lines independently of grain size increases. No changes in starch degradation rates during germination could be identified, or any major alteration in soluble sugar levels that may explain the coleoptile growth modification. We identify some alteration in hormones in the tissues in question. Mature grain size is examined, as is Hardness Index and starch conformation. We find no evidence that the increased growth of coleoptiles in these lines is connected to starch conformation or degradation or soluble sugar content and suggest these findings provide a novel means of increasing coleoptile growth and early seedling establishment in cereal crop species.     

The Role of Maturation Drying in the Transition from Seed Development to Germination: VII. EFFECTS OF PARTIAL AND COMPLETE DESICCATION ON ABSCISIC ACID LEVELS AND SENSITIVITY IN RICINUS COMMUNIS L. SEEDS

During mid-development (25–40 d after pollination: DAP)of the castor bean seed the amount of abscisic acid (ABA) increasesin both the endosperm and the embryo, declining substantiallythereafter until there is little present in the mature dry (60DAP) seed. Premature desiccation of the seed at 35 DAP alsoleads to a major decline in ABA within the embryo and endosperm.Partial water loss from the seed at 35 DAP which, like naturaland premature desiccation, leads to subsequent germination uponreturn of the seed to full hydration, causes a much smallerdecline in ABA levels. In contrast, ABA declines substantiallyin the non-dried (hydrated) control at 35 DAP, but the seedsdo not germinate. Hence, a clear negative correlation betweenABA content and germinability is not observed. Both drying,whether natural or imposed prematurely, and partial drying decreasethe sensitivity of the isolated embryo to exogenous ABA by about10-fold. The protein synthetic response of the castor bean embryo exposedto 0.1 mol m^–3 ABA following premature desiccation exhibitssome similarity to the response of the non-dried developingembryo—in both cases the synthesis of some developmentalproteins is enhanced by ABA, and germination is suppressed.Germination of mature seeds is also suppressed by 0.1 mol m^–3ABA, but the same developmental proteins are not synthesized.In the cotyledons of prematurely-desiccated seed, some proteinsare hydrolysed upon imbibition in 0.1 mol m^–3 ABA, a phenomenonthat occurs also in the cotyledons of similarly treated matureembryos, but not in developing non-dried embryos. Hence theembryo exhibits an ‘intermediate’ response uponrehydration in 0.1 mol m^–3 ABA following premature desiccation;viz. some of the responses are developmental and some germinative.Following natural or imposed drying, the isolated embryo becomesrelatively insensitive to 0.01 mol m^–3 ABA: germinationis elicited and post-germinative reserve breakdown occurs inthe radicle and cotyledons. The reduced sensitivity of the embryoto ABA as a consequence of desiccation may be an important factorin eliciting the switch to germination and growth within thewhole seed. Key words: Abscisic acid, desiccation, astor bean endosperm, seed development, germination, protein synthesis, isolated embryos, hormone sensitivity     

Effect of desiccation to low moisture content on germination,synchronization of root emergence,and plantlet regeneration of black spruce somatic embryos

A desiccation protocol was developed to evaluate the effect of different levels of desiccation on germination and plantlet regeneration of black spruce somatic embryos. Large desiccation chambers (80 l) with four liters of saturated salt solutions provided constant relative humidities (RH) of 63, 79, 88, and 97% (± 2%). Under these conditions, an embryo mass of 10 mg always dried fast even at 97% RH. In contrast, an embryo mass of 80 mg generated different kinetics of water loss, from fast drying at 63% RH to slow drying at 97% RH. Drying rates similar to those obtained with 80 mg embryos were also generated by combining 40 mg embryos with 40 mg water. The effects of drying rate and embryo MC on germination rate, root elongation, and plantlet regeneration were examined. A fast drying rate to 4–5% embryo MC, obtained under 63% RH, was detrimental to germination and plantlet development. However slower drying rates, obtained under 79–97% RH and generating 7–19% MC in the embryos, gave developmental responses similar to the control. Synchronization of root emergence was improved only for embryos desiccated to approx. 16% MC under 97% RH. The optimal desiccation protocol using large desiccation chamber at 97% RH and a constant embryo mass of 40 mg embryos plus 40 mg water was applied to five genotypes of black spruce. For all genotypes, desiccated embryos gave plantlet regeneration rates similar to the control undesiccated embryos. This revised version was published online in June 2006 with corrections to the Cover Date.     

Studies on the Physiology of Rice: XV. CHANGES IN THE NITROGEN METABOLISM OF SEEDS DURING GERMINATION AND THEIR RELATION TO THE APPLICATION OF GROWTH REGULATORS

Seeds of rice variety ‘Chinsurah Boro’ were treatedwith two concentrations of indoleacetic acid (IAA) and maleichydrazide (MH), 0.1 mg. per litre and 0.01 mg. per litre, duringthe first 7 days of germination, and the resulting changes innitrogen metabolism of both endosperm and embryo were determined.The seedling growth at different times during the treatmentwas recorded. Uptake of water by the embryo increased up to 72 hours, butthereafter the percentage water content fell, owing to large-scaletranslocation of dry matter to it. The endosperm showed increasingwater content for 5 days. There are two marked stages of nitrogen metabolism of the seedduring germination. During the first 72 hours hydrolysis ofprotein in the endosperm and translocation of soluble nitrogento the embryo are the predominant features. Thereafter the embryoactively synthesizes protein from the products of translocation. IAA and MH affected the seedling growth similarly. Initiallythere was a small retardation of leaf growth, but at later periodsrecovery took place. Root growth was more adversely affectedthan shoot growth, the effect also persisting longer. Thesechanges in growth were reflected in the nitrogen metabolismof the seedlings. For the first few days IAA and MH retardedthe supply of soluble nitrogen from the endosperm to the embryo,consequently there was less soluble nitrogen in the embryo thoughprotein synthesis there was affected only slightly. The differencein soluble nitrogen between the treated and untreated embryospersisted throughout the seven days. An attempt is made to explainthis on the basis that primarily IAA and MH retard the enzymaticactivity responsible for the hydrolysis of protein in the endosperm.     

The Role of Maturation Drying in the Transition from Seed Development to Germination: III. INSOLUBLE PROTEIN SYNTHETIC PATTERN CHANGES WITHIN THE ENDOSPERM OF Ricinus communis L. SEEDS

Kermode, A. R., Gifford, D. J. and Bewley, J. D. 1985. The roleof maturation drying in the transition from seed developmentto germination. III. Insoluble protein synthetic pattern changeswithin the endosperm of Ricinus communis L. seeds.—J.exp. Bot. 36: 1928–1936. Immature seeds of Ricinus communisL. cv. Hale (castor bean) removed from the capsule at 30 or40 days after pollination (DAP) can be induced to germinateby being subjected to drying. This desiccation–inducedswitch from development to germination is mirrored by a change,upon subsequent rehydration, in the pattern of insoluble proteinsynthesis within the endosperm storage tissue. During normaldevelopment from 25–40 DAP there is rapid synthesis ofthe insoluble (11S) crystalloid storage protein. At later stagesof development (45 and 50 DAP), crystalloid protein synthesisdeclines markedly and synthesis of new insoluble proteins commences.Following premature drying at 30 or 40 DAP, the pattern of insolubleprotein synthesis upon rehydration is virtually identical tothat following imbibition of the mature seed. Proteins synthesizedduring normal late development (at 45 and 50 DAP) are producedup to 48 h after imbibition; a subsequent change in the patternof insoluble protein synthesis occurs between 48 and 72 h. Thus,in contrast to the rapid switch in the pattern of soluble proteinsynthesis induced by drying, insoluble protein syntheses withinthe endosperm are redirected towards those uniquely associatedwith a germination/growth programme only after a considerabledelay following mature seed imbibition, or following rehydrationof the prematurely dried seed. Nevertheless, these results supportour contention that drying plays a role in the suppression ofthe developmental metabolic programme and in the permanent inductionof a germination/growth programme. Key words: Desiccation, crystalloid storage proteins, castor bean, seed development, seed germination     

A Critical Study of the Auxin Theory of Growth Regulation in the Mesocotyl of Avena sativa

A method of growing Avena seedlings is described, which allowsthem to be handled individually in darkness. Mesocotyls of seedlinge from which the tip of the coleoptileis repeatedly removed are as long as those of control plantsnot so decapitated. Mesocotyls of seedlings which are deseeded on the 3rd day ofgrowth, followed by decapitation of the cleoptile tip on the4th day, are, at 7 days old, as long as those of controls notso decapitated. When deseeded plants are decapitated, regeneration of auxinproduction occurs at the tip of the coleoptile stump. Where a reduction in the length of the mesocotyl results fromdecapitation, a wound reaction is probably concerned in additionto any auxin changes. Removal of the coleoptilar node causes a sharp decrease in thefinal length of the mesocotyl. Heating intact seedlings at 40° C. for 3 hours causes areduction in the length of the mesocotyl but not of the coleoptile.The effect of heating is not reversed by subsequent treatmentat low temperature, which instead appears to augment these effects. When seedlings are exposed to the action of KCN, iodoacetate,or anaerobic conditions, and illuminated while so exposed, perceptionof light takes place, resulting in a reduction in the lengthof the mesocotyl. Perception of light takes place in seedlings germinated at normaltemperatures, but maintained at low temprature during illuminationand also in seedlings grown for 6 weeks at 2° C. withoutany previous growth at normal temperature. Light perception takes place in embryos excised from dry grainand grown on a culture medium. No difference in free amino-acid content is apparent betweendark grown and illuminated seedlings. The effects of illumination survive a period of drying downand become apparent upon subsequent germination of the grainin darkness. The drying process itself causes an additionalreduction in mesocotyl length. It is concluded that auxin itself is not the primary reactantin the perception process, and that the growth of the mesocotylis probably controlled by the coleoptilar node and plumulargrowing point, rather than by auxin diffusing downward fromthe tip of the coleoptile.     

Cytokinin-Inhibitor Antagonism in the Hormonal Control of α-Amylase Synthesis and Growth in Barley Seed

The effect of cytokinins and gibberellic acid on the inhibition of growth and α-amylase synthesis by germination inhibitors was investigated in intact and embryoless seed halves. The cytokinins, kinetin and benzyladenine, effectively reversed the inhibition of coleoptile growth and α-amylase synthesis by abscisic acid and courmarin in barley seed. An antagonism between cytokinins, kinetin and benzyladenine, effectively reversed the inhibition of coleoptile growth and α-amylase synthesis by abscisic acid and coumarins in barley seed. An antagonism between cytokinins and germination inhibitors was also shown in root growth. Abscisic acid inhibited coleoptile growth to a greater extent than the root growth while the opposite held true in the case of coumarin. The apparent increase in coleoptile growth and α-amylase synthesis by gibberellic acid plus abscisic acid (or coumarins) over abscisic acid (or coumarin) appears to be a result of the overall stimulation of growth and metabolism by exogenous gibberellic acid and probably does not involve an interaction of gibberellic acid with the inhibitors. Gibberellic acid reversed root inhibition to some extent. Abscisic acid inhibition of gibberellic acid induced α-amylase synthesis in the embryoless endosperm was not reversed by excess gibberellic acid or kinetin Cytokinin reversal of inhibition of growth and enzyme synthesis probably depends on some factor(s) in the embryo. Cytokinin reversal of inhibitor action leading to enzymen synthesis and growth may be at the level of genome or at the site protein assembly.     

Endosperm sterol phenotype and germination in wheat

Free and conjugated sterols of endosperm, coats, scutellum, coleoptile and roots have been analysed at different germination stages in two wheat cultivars with different endosperm sterol phenotypes. It seems that sterol metabolism of the developing tissues, namely coleoptile and roots, is not affected by the sterol conjugation profile of the endosperm. Enough sterol is present in the mature embryo to supply the germinating axis during the observation period (144 hr at 16°). The data suggest that sterol is transferred from scutellum to coleoptile and roots during germination.     

Nitric oxide accelerates seed germination in warm-season grasses

The nitric oxide (NO) donor sodium nitroprusside (SNP) significantly promoted germination of switchgrass (Panicum virgatum L. cv Kanlow) in the light and in the dark at 25°C, across a broad range of concentrations. SNP also promoted seed germination in two other warm-season grasses. A chemical scavenger of NO inhibited germination and blocked SNP stimulation of seed germination. The phenolic (+)-catechin acted synergistically with SNP and nitrite in promoting seed germination. Acidified nitrite, an alternate NO donor also significantly stimulated seed germination. Interestingly, sodium cyanide, potassium ferricyanide and potassium ferrocyanide at 200 μM strongly enhanced seed germination as well, whereas potassium chloride was without effect. Ferrocyanide and cyanide stimulation of seed germination was blocked by an NO scavenger. Incubation of seeds with a fluorescent NO-specific probe provided evidence for NO production in germinating switchgrass seeds. Abscisic acid (ABA) at 10 μM depressed germination, inhibited root elongation and essentially abolished coleoptile emergence. SNP partially overcame ABA effects on radicle emergence but did not overcome the effects of ABA on coleoptile elongation. Light microscopy indicated extension of the radicle and coleoptiles in seeds maintained on water or on SNP after 2 days. In contrast, there was minimal growth of the radicle and coleoptile in ABA-treated seeds even after 3–4 days. These data indicate that seed germination of warm-season grasses is significantly influenced by NO signaling pathways and document that NO could be an endogenous trigger for release from dormancy in these species.     

Salt tolerance of two differently drought-tolerant wheat genotypes during germination and early seedling growth

Summary Osmotic and specific ion effect are the most frequently mentioned mechanisms by which saline substrates reduce plant growth. However, the relative importance of osmotic and specific ion effect on plant growth seems to vary depending on the drought and/or salt tolerance of the plant under study. We studied the effects of several single salts of Na⁺ and Ca²⁺−NaCl, NaNO₃, Na₂SO₄, NaHCO₃, Na₂CO₃, and Ca(NO₃)₂—on the germination and root and coleoptile growth of two wheat (Triticum aestivum L.) cultivars, TAM W-101 and Sturdy, the former being more drought tolerant than the latter. The concentrations used were: 0, 0.02, 0.04, 0.08, 0.16, and 0.32 mol L⁻¹. Significant two- and three-way interactions were observed between cultivar, kind of salt, and salt concentration for germination, growth of coleoptile and root, and root/coleoptile ratio. Salts differed significantly (P<0.001) in their effect on seed germination, coleoptile and root growth of both cultivars. Germination of TAM W-101 seeds was consistently more tolerant than that of Sturdy to NaCl, CaCl₂, Ca(NO₃)₂, and NaHCO₃ salts at concentrations of 0.02, 0.04, 0.08, 0.16 mol L⁻¹. The osmotic potential, at which the germination of wheat seeds was reduced to 50% of that of the control, was different depending on the kind of salt used in the germination medium. NaCl at low concentrations (0.02 and 0.04 mol L⁻¹) stimulated the germination of both wheat cultivars. At concentrations of 0.02 to 0.16 mol L⁻¹, Ca²⁺ salts (CaCl₂ and Ca(NO₃)₂) were consistently more inhibitory than the respective Na⁺ salts (NaCl and NaNO₃) for germination of Sturdy. This did not consistently hold true for TAM W-101. Among the Na⁺ salts, NaCl was the least toxic and NaHCO₃ and Na₂CO₃ were the most toxic for seed germination. Root and coleoptile (in both wheat cultivars) differed in their response to salts. This differential response of coleoptile and root to each salt resulted in seedlings with a wide range of root/coleoptile ratios. For example, the root/coleoptile ratio of cultivar TAM W-101 changed from 2.09 (in the control) to 3.77, 3.19, 2.8, 2.44, 1.31, 0.32, and 0.0 when subjected to 0.08 mol L⁻¹ of Na₂SO₄, NaCl, CaCl₂, NaNO₃, Ca(NO₃)₂, NaHCO₃, and Na₂CO₃, respectively. Na₂CO₃ at 0.08 mol L⁻¹ inhibited root growth to such an extent that germinated wheat seeds contained coleoptile but no roots. The data indicate that, apart from the clear and more toxic effects of NaHCO₃ and Na₂CO₃ and lesser toxic effect of NaCl on germination and seedling growth, any toxicity-ranking of other salts done at a given concentration and for a given tissue growth may not hold true for other salt concentrations, other tissues and/or other cultivars. The more drought-tolerant TAM W-101, when compared to the less drought tolerant Sturdy, showed higher tolerance (at most concentrations) to NaCl, CaCl₂, Ca(NO₃)₂ and NaHCO₃ during its seed germination and to Na₂SO₄ and CaCl₂ for its root growth. This supports other reports that some drought-tolerant wheat cultivars are more tolerant to NaCl. In contrast, the coleoptile growth of drought-sensitive Sturdy was noticeably more tolerant to NaNO₃, Ca(NO₃)₂ and NaHCO₃ than that of drought-tolerant TAM W-101. Based on the above and the different root/coleoptile ratios observed in the presence of various salts, it is concluded that in these wheat cultivars: a) coleoptile and root tissues are differently sensitive to various salts, and b) at the germination stage, tolerance to certain salts is higher in the more drought-tolerant cultivar.