Cell cycle activity during seed priming is not essential for germination advancement in tomato

Seed priming is a technique of controlled hydration and drying that results in more rapid gemination when the seeds are reimbibed. Advancement of radicle meristem cells into the S and G2 phases of the cell cycle, increasing the percentage of nuclei having a 4C DNA content, has been reported to occur during priming. It has been suggested that the efficiency of priming is related to the accumulation of 4C nuclei in the radicle meristem, but the extent of cell cycle activity varied among different treatments and seed lots. A wide range of priming treatments across temperatures, water potentials and durations can be compared on a common basis using the hydrothermal priming time model. Flow cytometry was used to monitor cell cycle activity in a number of tomato (Lycopersicon esculentum Mill.) seed lots during priming in relation to the accumulation of hydrothermal priming time and the subsequent germination rate response. In some seed lots, the percentage of 4C nuclei in the radicle meristems prior to emergence increased in proportion to accumulated hydrothermal priming time, while in other lots, no increase in nuclear DNA content was detected. All lots, however, demonstrated rapid radicle emergence following priming. Thus, replicative DNA synthesis in radicle meristem nuclei often occurred during seed priming, but an increase in the percentage of 4C nuclei was not essential for germination advancement.     

Effect of tannin content of faba bean (Vicia faba) seed on seed vigour, germination and field emergence

Differences in seed vigour of zero- and high-tannin faba beans were investigated using 25 seed lots of 12 cultivars following earlier reports of poor emergence in the zero-tannin types. Field emergence ranged from 54–96% indicating differences in seed vigour between cultivars all having high laboratory germination (>91%). Seed from zero-tannin accessions with poor emergence had a higher incidence of testa and cotyledon cracking, a smaller percentage of hard seeds, more rapid water uptake, a lower percentage of vital staining of cotyledons and a greater leaching of solutes than high-tannin types. Nevertheless, variation in these characteristics existed between cultivars and lines of both types. Seeds with more cracks in the seed coat and fewer hard seeds imbibed water more rapidly and consequently showed lower levels of vital staining and more cracks in the abaxial surface of the cotyledons. Slower imbibition in polyethylene glycol lessened the incidence of these deleterious characteristics and may provide a practical resolution to the problem of poor field emergence in zero-tannin lines of faba bean with low seed vigour.     

Evidence for Repair Processes in the Invigoration of Seeds by Hydration

Seeds with low vigour as a result of ageing showed a large improvementin seed quality following a 2 h soak in water, reflected inan increased rate of germination, higher emergence in soil,and the retention of high germination after 24 h rapid ageingin a controlled deterioration vigour test. Similar, but smallerimprovements were seen after a priming treatment using polyethyleneglycol. Seeds dried to their initial moisture content aftertreatment retained these beneficial effects. The treatmentshad little effect on high vigour seed. These findings suggestthat seed invigoration by hydration treatments results fromthe repair of previously sustained deterioration. Brussels sprouts, Brassica oleracea var. gemmifera, priming, repair, seed moisture content, ageing     

Canola seed germination and seedling emergence from pre-hydrated and re-dried seeds subjected to salt and water stresses at low temperatures

Low soil temperatures and low water potentials reduce and delay the seed germination of canola (Brassica rapa L., B. napus L.) in western Canada. Germination is also very sensitive to the salinity effects of nitrogen fertiliser placed with the seed, especially when the seed bed is relatively dry. The effects of pre-hydration and re-drying treatment on canola (Brassica rapa L. cv. Tobin) seed germination and seedling emergence at 10°C subjected to either a water or salt stress were determined. Low water potentials, induced by polyethylene glycol (PEG 8000), low soil moisture, or high concentrations of salts, reduced both germination and seedling emergence, and increased the time to 50% germination and emergence of seeds at 10°C. At equal osmotic potentials, Na₂SO₄ was less inhibitory on low temperature germination than either NaCl or PEG, suggesting that the sulphate ion partially alleviated the inhibitory effects of low water potential. Solutions of NaCI produced more abnormal seedlings compared to Na₂SO₄, suggesting that NaCl was more toxic than Na₂SO₄ during seedling development. Pre-hydration and re-drying partially overcame the inhibitory effects of both low water potential and salts on seed germination and seedling emergence at 10°C. The seed treatment increased the germination rate in Petri dishes and seedling emergence from a sandy loam soil. Water potentials or soil water contents required to inhibit 50% germination or emergence at 10°C were lower for treated seeds compared to control seeds. Salt concentrations inhibiting 50% emergence were higher for treated seeds than control seeds. Neither treated nor control seeds produced seedlings which emerged if the soil water content was lower than 9% or when the soil was continuously irrigated with salt solutions of 100 mmol kg^-1 of NaCl or 50 mmol kg^-1 of Na₂SO₄. These results suggest that the pre-hydration and re-drying treatment did not lower the base water potentials at which seedling emergence could occur. Abnormal seedlings were observed in both treated and control seeds, particularly if the soil was watered with NaCl solutions; however, the seed treatment reduced the number of abnormal seedlings.     

Germination characteristics of Rhinanthus minor influence field emergence,competitiveness and potential use in restoration projects

• The facultative root hemi‐parasite Rhinanthus minor is often used in grassland habitat restoration projects to regulate ecosystem structure and function. Its impact on community productivity and diversity as a function of resource supply, sward composition and management has been widely investigated. However, there is a lack of information about the possible influence of seed quality on the efficacy of the hemi‐parasite.
• Ten seed lots from commercial sources were sown in the field and their germination characteristics investigated in the laboratory. Seeds from four lots were also germinated and sown in pots alongside plants of two host species, Lotus corniculatus and Holcus lanatus. Plant establishment, height and flowering density were evaluated for the hemi‐parasite, while plant biomass was measured for both R. minor and its host.
• Two aspects of seed quality influenced the field emergence of seed lots of R. minor, the radicle emergence (%) and the length of the lag period from the beginning of imbibition to germination (mean germination time), which indicates seed vigour. A longer lag period (lower vigour) was associated with higher levels of seedling mortality and lower plant vigour, in terms of plant height and biomass accumulation and was also reflected in the parasitic impact of the seed lots.
• Seed quality, specifically germination and vigour, can influence the establishment, survival, subsequent plant productivity and parasitic impact of R. minor in vegetation restoration projects. Seed quality is discussed as a key factor to consider when predicting the impact of the hemi‐parasite on community productivity and diversity.

     

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.     

Nucleotide Levels and Loss of Vigour and Viability in Germinating Wheat Embryos

At an optimal germination temperature loss of viability in wheatseed is reflected in reduced levels of nucleoside triphosphatesand nucleotide sugars in the embryo compared to the levels foundin high viability embryos during the early hours of imbibition.These differences are magnified on germination at 10° Cand are accompanied by a greatly reduced rate of protein synthesisin embryos of low viability compared with the correspondingrate in embryos of high viability. Loss of vigour precedes lossof viability in seed but differences in biochemical parametersbetween high and medium vigour seed do not become apparent unlessgermination under stress conditions, e. g. low temperature,occurs. Under these stress conditions, wheat seed of reducedvigour can be distinguished from high vigour seed lots of similarviability by the decreased levels of nucleotides and nucleotidesugars found in embryos from reduced vigour seed lots duringthe early hours of imbibition and by reduced rates of proteinsynthesis in these embryos during this same period. Nucleotideanalysis is a potentially useful method for rapid assessmentof the viability or vigour of a seed lot. Key words: Nucleotides, Vigour, Viability, Wheat     

The Influence of Testa Condition on the Imbibition and Vigour of Pea Seeds

Seed lots of peas with high rates of water uptake which containeda large proportion of seeds with at least one crack in the testaand were low in vigour, as indicated by electrolyte leakageand poor field emergence. Comparisons of the tetrazolium chloridestaining of the cotyledons after imbibition in water and polyethyleneglycol of two seed lots both with and without scarificationshowed that cracks in the testa could cause cell death by increasingthe rate of water uptake. Although the major cause of poor seedvigour appeared to be the incidence of cracks in the testa whichresulted in rapid water uptake, cell death, and solute leakage,the condition of the embryo also influenced the response towater uptake. The practical significance of these observationsto the problems of field emergence and seed production in peasis discussed.     

Measurement of the Water Potential of Intact Plant Tissues2PISUM SATIVUM L.)

Through the use of a microthermocouple psychrometer it has becomepossible to measure the water potential over part of the surfaceof a pea seed, starting about 19 hours after sowing in distilledwater, or later in the case of seeds germinating at lower osmoticpotentials, in both instances until well after radicles haveemerged. It has been shown that the potential of air-dry seedsis well below –6,000 joules/kg but increases rapidly duringimbibition, depending upon the water potential of the germinationmedium. Pea seeds subjected to lower external water potentialsgerminate at lower internal water potentials than they exhibitin distilled water. The water potentials of the seeds decreasejust after radicle emergence till the radicle establishes contactwith its germination medium, possibly as a result of demandfor moisture during that period due to incipient cell elongation.No detectable amounts of osmotically active substances are exudedfrom the seed during germination; the pea seed coat restrictsthe entry of polyethylene glycol molecules into the seed untilemergence of the radicle.     

Systematic overestimation of Salicaceae seed survival using radicle emergence in response to drying and storage: implications for ex situ seed banking

Biodiversity conservation programmes are underpinned by seed banking following drying to low water contents (WC), and supported by both the assessment and prediction of seed viability over time. The means of judging viability is thus crucial to the comprehension of seed vigour. We selected seeds of three species and one hybrid in the Salicaceae likely to have variation in tolerance to drying, processing and storage, including in relation to cryobanking, and compared survival growth as radicle emergence (germination) and normal seedling production. With three seed lots of Salix gracilistyla, air-drying to 8–10 % WC enhanced seed survival after 40 days’ storage at 5 °C as compared with non-treated seeds at 14–20 % WC. Four seed lots of Populus alba × P. glandulosa showed equally high germination (88–100 %) and proportions of normal seedlings (81–99 %) when stored at 5 °C for 7–10 weeks. Among seven seed lots of S. gracilistyla, two groups with different storage behaviour could be statistically distinguished with normal seedling production ranging from 0 to 45 % after storage at 5 °C for 13 weeks. Seed tolerance to WC manipulation and cryopreservation was very variable among species and seed lots. Seed lots of S. hallaisanensis and S. gracilistyla with ~80 % germination survived cryopreservation at 10 % WC, but were sensitive to lower WCs. In contrast, Populus seeds had greater desiccation tolerance combined with cryopreservation capability. With seed lots of all species and hybrids, cryopreservation had little effect on viability unless the high moisture freezing limit had been exceeded (~10–20 % WC, depending on seed lot). However, under all conditions of handling (drying, rehydration, storage at 5 °C or cryopreservation) using germination as the only indicator of viability over-estimated survival compared with normal seedling production.     

The performance of carrot seeds in relation to their viability

The germination and emergence characteristics, and early seedling growth, of carrot seeds cv. Chantenay red-cored from different sources with a range of germination from 54–94%, was compared. Seeds from protected crops (mean temperature of growth 21°C, r.h. 45–70%) gave higher percentage germination than those from crops grown outdoors (mean temperature 15°C, 70–100% r.h.). Germination was also higher from mature (seed moisture content at harvest 20% or lower) than immature seed (seed moisture content at harvest between 20 and 60%). High percentage germination (>90%) was associated with low mean germination times and low spreads of germination times whilst the reverse was true for low percentage germination. Similar relationships were found for seedling emergence characters in the field although a lower proportion of the viable seeds produced seedlings from slowly than rapidly germinating seed lots. In general, seed lots having a low percentage germination gave greater variability in plant weight than those of higher percentage germination. There was no effect of seed source on radicle or shoot relative growth rates or on post-emergence seedling growth rates.     

The use of an ELISA to quantitate the extent of 11S globulin mobilization in untreated and primed sugar beet seed lots

Seed priming (controlled imbibition) is a widely used technique for improving crop establishment, because it allows a reduction of the time to radicle emergence following seed imbibition and synchronization of individual seeds within seed lots with respect to germination timing. The major problem encountered in seed priming is the control of seed imbibition to a level permitting pre-germinative processes to proceed but that blocks radicle emergence. If not, the consequence of drying back the seeds to initial moisture content for storage purposes could be a total loss of the treated batch. This is because, as long as radicle growth has not begun, seeds may be re-dried without any permanent deleterious effects upon subsequent germination or growth. Recently, we reported the discovery of a molecular marker of sugar beet seed priming, corresponding to the basic B-subunit of the seed storage protein 11S globulin. An ELISA based upon this molecular marker has been used to analyse how different sugar beet seed lots respond to a priming treatment. The results demonstrate that this ELISA allows us to readily distinguish between the primed seeds and the corresponding untreated seeds.     

A water relations analysis of seed germination rates

Seed germination culminates in the initiation of embryo growth and the resumption of water uptake after imbibition. Previous applications of cell growth models to describe seed germination have focused on the inhibition of radicle growth rates at reduced water potential (Ψ). An alternative approach is presented, based upon the timing of radicle emergence, to characterize the relationship of seed germination rates to Ψ. Using only three parameters, a `hydrotime constant' and the mean and standard deviation in minimum or base Ψ among seeds in the population, germination time courses can be predicted at any Ψ, or normalized to a common time scale equal to that of seeds germinating in water. The rate of germination of lettuce (Lactuca sativa L. cv Empire) seeds, either intact or with the endosperm envelope cut, increased linearly with embryo turgor. The endosperm presented little physical resistance to radicle growth at the time of radicle emergence, but its presence markedly delayed germination. The length of the lag period after imbibition before radicle emergence is related to the time required for weakening of the endosperm, and not to the generation of additional turgor in the embryo. The rate of endosperm weakening is sensitive to Ψ or turgor.     

Nucleic acid and protein synthesis and loss of vigour in germinating wheat embryos

A study has been made of the RNA and protein synthesising systems of wheat embryos isolated from seed lots having high viability but differing in vigour. The rate of RNA and protein synthesis in wheat embryos during the early hours of germination is related to the vigour of the seed lot. The imposition of a stress factor, in the nature of a sub-optimal germination temperature, during germination of isolated wheat embryos magnifies the differences in rates of protein and RNA synthesis between high and low vigour seed. Using cell-free protein synthesising systems it has been demonstrated that an important difference between high and low vigour embryos lies in the relative levels of messenger RNA in the embryo. High vigour embryos contain relatively higher levels of poly A⁺-RNA (i.e. potential mRNA species) than lower vigour embryos and furthermore the level of poly A⁺-RNA in high vigour embryos increases during early germination whilst in lower vigour embryos the level decreases. The difference in poly A⁺-RNA levels accounts, at least partially, for the differences in rates of protein synthesis observed between embryos from high and low vigour wheat seed during early germination at both optimal and sub-optimal germination temperatures.Abbreviations HEPES N-2-hydroxyethylpiperazine-N-2-ethane sulfonic acid - poly A⁺-RNA polyadenylated RNA - GM germination medium - PMS post-mitochondrial supernatant fraction     

Seed Germination and Seedling Growth in Sopubia delphinifolia - a Hemi-root Parasite: Germination Requirements

Requirements for seed germination (emergence of radicle) andseedling formation (emergence of both radicle and cotyledons)of a hemi-root parasite Sopubia delphinifolia were studied inthe absence of any host stimulus, in Petri dish and asepticcultures. Both water washing in trickling tap water as wellas cold treatment were effective in inducing/stimulating germinationand seedling formation in the light. Although ethrel, an ethylenereleasing compound, stimulated radicle emergence it inhibitedthe emergence of the cotyledons. Light was found to be essentialfor germination; none of the growth substances could replacethe light requirement. Light responses seem to be mediated throughthe phytochrome system. The results indicate that the emergenceof the radicle, its further growth into the root and the emergenceof cotyledons are controlled by different factors. Sopubia delphinifolia, hemi-root parasite, seed germination requirements, pretreatment, seedling formation     

The Effect of Accelerated Aging of Sorghum Seeds on Seedling Vigour

Six different levels of vigour of sorghum seeds from an identicallot were obtained through accelerated aging for various periodsof time. Vigour was estimated according to yielding abilityof the seed in the field. After 6 d of aging, seed invigoration,to the extent of a 20% increase in yield, was observed. However,this was followed by a gradual decrease in yield following agingof up to 48 d. All seed lots showed 88–92% germinationor emergence capacity when tested in the laboratory, which correlatespoorly with vigour. However, vigour correlated well with therates of germination and root emergence when tested in the laboratory,as well as with emergence rate in the field.     

Hydrothermal time analysis of tomato seed germination responses to priming treatments

Controlled hydration of seeds followed by drying (seed priming) is used to break dormancy, speed germination, and improve uniformity of radicle emergence. To date, empirical trials are used to predict optimal priming conditions for a given seed lot. Since priming is based upon seed water relations, it was hypothesized that the sensitivity of germination to reduced water potential before priming might be mechanistically related to, and therefore predictive of, priming responsiveness. Analyses of germination of 13 tomato (Lycopersicon esculentum Mill.) seed lots at two temperatures (15C and 20C) and three water potentials (0, -0.28 and -0.43 MPa) showed that seed lot germination responses could be quantitatively characterized by parameters derived from thermal time, hydrotime, and hydrothermal time models (R²0.73-0.99). Six of the seed lots were primed at two temperatures (15°C and 20°C) and three water potentials (-1.0, -1.5 and -2.0 MPa) for various durations, dried, and their subsequent germination rates analysed according to hydropriming time and hydrothermal priming time models. The responses of germination rates to priming were characterized by hydropriming time (HP) and hydrothermal priming time (HTP) constants and the minimum water potential (min) and temperature (Tmin) for achieving a priming effect. The values of min and Tmin varied relatively little among tomato seed lots, and the generalized values of min=2.39 MPa and Tmin=9.10°C accounted for 74% (15°C), 57% (20°C), and 62% (across both temperatures) of the increase in germination rates following priming. Nonetheless, while the hydrothermal time models described germination patterns both before and after priming, there was relatively little predictive relationship between them.     

Mineral Ion Contents, Solute Release and Aspects of Ion Transport in Embryos from Germinating Low and High Vigour Wheat (Triticum aestivum)

Both low and high vigour seed lots were more than 90% viablewhen germinated at 25 °C. The tissue weights and the contentsof mineral ions and glycinebetaine in the ungerminated embryoswere the same for each lot. The release of K⁺ and phosphateions from isolated embryos into aqueous media were also thesame. These similarities remained for at least 36 h of germination.At 48 h the coleoptiles of the low vigour embryos were about20% shorter and their fresh weight was about 30% less than thoseof the high vigour embryos. These differences were reduced againto about 10% at 72 h. In contrast to these differences, H⁺ efflux and ³⁶Rb⁺ uptakeby isolated embryos were up to 50% slower in low vigour embryoscompared with the high vigour ones. The difference was maximalat 24 h of germination and appeared to depend upon the anionpresented and the pH of the incubation medium. The H+/³⁶Rb⁺exchange ratio was not, however, changed in low vigour embryoscompared with high vigour ones.     

Expression and inhibition of aquaporins in germinating Arabidopsis seeds

Extensive and kinetically well-defined water exchanges occur during germination of seeds. A putative role for aquaporins in this process was investigated in Arabidopsis. Macro-arrays carrying aquaporin gene-specific tags and antibodies raised against aquaporin subclasses revealed two distinct aquaporin expression programs between dry seeds and young seedlings. High expression levels of a restricted number of tonoplast intrinsic protein (TIP) isoforms (TIP3;1 and/or TIP3;2, and TIP5;1) together with a low expression of all 13 plasma membrane aquaporin (PIP) isoforms was observed in dry and germinating materials. In contrast, prevalent expression of aquaporins of the TIP1, TIP2 and PIP subgroups was induced during seedling establishment. Mercury (5 microM HgCl(2)), a general blocker of aquaporins in various organisms, reduced the speed of seed germination and induced a true delay in maternal seed coat (testa) rupture and radicle emergence, by 8-9 and 25-30 h, respectively. Most importantly, mercury did not alter seed lot homogeneity nor the seed germination developmental sequence, and its effects were largely reversed by addition of 2 mM dithiothreitol, suggesting that these effects were primarily due to oxidation of cell components, possibly aquaporins, without irreversible alteration of cell integrity. Measurements of water uptake in control and mercury-treated seeds suggested that aquaporin functions are not involved in early seed imbibition (phase I) but would rather be associated with a delayed initiation of phase III, i.e. water uptake accompanying expansion and growth of the embryo. A possible role for aquaporins in germinating seeds and more generally in plant tissue growth is discussed.