Photosensitivity of Rumex obtusifolius seeds for stimulation of germination: Influence of light and temperature

A population of Rumex obtusifolius L. seeds imbibed for 24 h at 25°C exhibits a sigmoid logarithmic fluence-response relationship for stimulation of germination by red light (R), 11.0 μmol m⁻² being necessary for 50% of the response. After 24 h imbibition at 35°C the fluence-response relationship for stimulation of germination by R is biphasic. For 50% response the very sensitive phase (very low fluence-response) requires 4.7 − 10⁻²μmol m⁻² whereas the less sensitive phase (low fluence-response) requires 4.0 μmol m². A few seconds of far-red light (FR) satisfies the germination requirement of the sensitive seeds after 24 h at 35°C. However, a longer period of FR (2 h) results in low germination. The fluence-response relationship for induction of these seeds by R is sigmoid, 4.8 μmol m⁻² being necessary for 50% response, demonstrating that 2 h FR desensitizes the sensitive proportion of the seed population induced by 24 h at 35°C. A proportion of the seed population can be further sensitized by 60 min at 35°C following this desensitization.     

Induction of extreme light sensitivity in buried weed seeds and its role in the perception of soil cultivations

Abstract. Light, probably acting through the photo-receptor phytochrome, promotes germination of weed seeds when the soil is disturbed by tillage operations. A short period of burial is shown to induce an enormous ∼10000-fold increase in light sensitivity in the seeds of the arable weed Datura ferox which is interpreted as a natural transition to the 'very-low-fluence' mode of phytochrome action. Field experiments indicated that germination of buried seeds may be triggered by millisecond-exposures to sunlight and suggested a key role for the process of sensitization in the mechanisms whereby light requiring seeds detect the occurrence of soil cultivation events in arable lands.     

Fluence-response curves and action spectra for promotion and inhibition of seed germination in wildtype and long-hypocotyl mutants of Arabidopsis thaliana L.

The fluence-response curves of wildtype and long-hypocotyl mutants of Arabidopsis thaliana L. for induction and inhibition of seed germination, expressed as percentage germination on probit scale against logarithm of fluence, are very different in shape. The mutants show reduced photoinhibition of hypocotyl growth in white light compared with wildtype, suggesting they are either mutated in phytochrome, the blue/UV-absorbing photosystem or some other red-absorbing photosystem. Calculations of the amount of the far-red-absorbing form of phytochrome (Pfr), by a given fluence have been made taking into account pre-existing Pfr in the seeds. This pre-existing Pfr can change dramatically the slope of a fluence-response curve. Other factors such as an overriding factor, stimulating germination by a non-phytochrome-related process, the total phytochrome content, the range of normal distribution of logarithm of Pfr requirement of individuals in the population and differential screening can influence the form and-or position of a fluence-response curve. Action spectra calculated for germination induction and for the inhibition of induction for the different genotypes are qualitatively the same, having peaks of effectiveness at 660 nm and 730 nm respectively. In the blue region of the spectrum very little activity is seen in comparison with that of red light. Differences in bandwidth of effectiveness for induction of germination are attributed to different amounts of screening pigments in the seed batches. The long-hypocotyl mutants therefore have a normal phytochrome system operative in the control of seed germination, by short-term irradiation and no other photosystem appears to be involved.Abbreviations and symbols FR far-red light - P phytochrome - Pfr far-red-absorbing form of P - Pr red-absorbing form of P - R red light - SD standard deviation of logarithm Pfr around - logarithm Pfr required for 50% germination - aparent molar conversion cross section - maximum Pfr/Ptot established by a given wave-length - ₀ initial Pfr     

Sensitivity of Polygonum aviculare seeds to light as affected by soil moisture conditions

BACKGROUND AND AIMS: It has been hypothesized that soil moisture conditions could affect the dormancy status of buried weed seeds, and, consequently, their sensitivity to light stimuli. In this study, an investigation is made of the effect of different soil moisture conditions during cold-induced dormancy loss on changes in the sensitivity of Polygonum aviculare seeds to light. METHODS: Seeds buried in pots were stored under different constant and fluctuating soil moisture environments at dormancy-releasing temperatures. Seeds were exhumed at regular intervals during storage and were exposed to different light treatments. Changes in the germination response of seeds to light treatments during storage under the different moisture environments were compared in order to determine the effect of soil moisture on the sensitivity to light of P. aviculare seeds. KEY RESULTS: Seed acquisition of low-fluence responses during dormancy release was not affected by either soil moisture fluctuations or different constant soil moisture contents. On the contrary, different soil moisture environments affected seed acquisition of very low fluence responses and the capacity of seeds to germinate in the dark. CONCLUSIONS: The results indicate that under field conditions, the sensitivity to light of buried weed seeds could be affected by the soil moisture environment experienced during the dormancy release season, and this could affect their emergence pattern.     

Arabidopsis thaliana seed germination and early seedling growth are inhibited by monoethanolamine salts of para-halogenated benzoic acids

Abstract

In spite of the simplicity of its molecules, the complex effects of benzoic acids on the regulation of plant growth are an increasingly attractive field of research to chemists and biologists. Halide substituted benzoic acids offer an excellent opportunity to explore the effect of electron withdrawing substituents (fluoro-, chloro-, bromo- and iodo-) on the response of plant growth stage. Under normal physiological conditions, benzoic acids are ionized molecules that exhibit low solubility in water. Monoethanolamine, a natural alkanolamine, was used to generate salts of monoethanolamine of halogenated para-substituted benzoic acids, new compounds with biological activity. This study reports on the biological effects of these substances at different concentrations on Arabidopsis thaliana seed germination and early seedling growth. Seed germination at 22°C, in a vertical position, under a photoperiod of 16 h light and 8 h darkness, was variable depending on the concentration of the compounds applied. Final germination percentages were similar for all treatments and control at 0.05 mM and 0.1 mM (exception p-Br BA and p-I MEASPBA). No germination occurred when seeds were treated with more than 0.5 mM. The results also revealed that the primary root length and the number of secondary roots are reduced in a concentration-dependent manner and also in relation to increasing atomic size of the substituents (F < Cl < Br < I). It is concluded that uptake rates of benzoic acid anions by roots decrease with a decrease in hydrophilic character of the anion and with an increase in molecular size.     

Mathematical approach to effects of repeated treatments in the study of very low fluence, low fluence and high fluence germination responses

Theoretical calculation of the germination response induced by repeated treatments, separated by a dark period long enough to enable fixation of the effect of the preceding treatment, is possible when defining the percentage germination induced by the first treatment as the responding proportion (p) of the total treated seed population. Consequently the germination response induced by a second treatment should be relative to the proportion (q) of the seed population not responding to the first treatment (q = 1 - p).
The fitting of these calculations with experimental data for the Very Low Fluence Response (VLFR) for germination of seeds of Kalanchoë blossfeldiana Poelln. cv. Vesuv, induced by repeated light pulses, suggests the independency of the effect of each treatment, i.e. the effect of the second treatment is neither positively nor negatively influenced by the first treatment.
This hypothesis is not valid for calculation of the Low Fluence Response (LFR) for germination of Kalanchoë seeds induced by repeated light pulses, since the first light pulse does not result in a germination response. At least two irradiations are needed for an LFR while the third and following pulses increase the response much more than calculated with the proposed equation. It is suggested that the LFR in Kalanchoë , in contrast to the VLFR, includes the involvement of some pre-existing far-red absorbing form of phytochrome (P_fr) and the involvement of dark reactions are to be considered.
The effect of long irradiation times (up to 2xl0⁵ s) resulting in a (defined in this paper) high fluence response (HFR) for germination of Kalanchoë seeds is also discussed in terms of independently responding seed population fractions.     

Activities of sulfhydryl-related and phenylpropanoid-synthesizing enzymes during the germination ofArabidopsis thaliana seeds

Seeds ofArabidopsis thaliana were sowed in a Murashige and Skoog’s medium and incubated in a tissue culture chamber at 26.2°C. They appeared to germinate at 60–72 hours after sowing. We measured the time-course activities of sulfhydryl-related enzymes such as thioredoxin, thioltransferase (glutaredoxin), thioredoxin reductase, and glutathione reductase. The activities of two enzymes, phenylalanine ammonia-lyase and tyrosine ammonia-lyase, which are involved in phenylpropanoid synthesis, were also measured and compared. Phenylalanine ammonia-lyase activity increased from 48 hours after sowing, whereas tyrosine ammonia-lyase activity increased transiently at the early stage and decreased slightly afterwards. Thioredoxin activity gradually decreased during the germination process. However, thioltransferase activity increased drastically from 60 hours after sowing. Thioredoxin reductase and glutathione reducatase increased rapidly from 36 hours after sowing. Thioredoxin activity was found to be relatively high in the dormant seeds.     

A single positive phototropic response induced with pulsed light in hypocotyls of Arabidopsis thaliana seedlings

The fluence-response curves were measured for phototropic curvature in response to unilateral 450-nm light in hypocotyls of Arabidopsis thaliana (L.) Heynh. These show the classical first positive (peak curvature of 9–10°), indifferent and second positive phototropic response. Reciprocity is valid only for the first positive response; the fluence requirements for its induction are similar to those for induction of the first positive phototropic response of coleoptiles. Large angles of curvature also may be induced by multiple pulses if the individual pulses are separated by an optimum dark period of about 15 min. The curvature induced by a given fluence, whether applied in continuous irradiation or a sequence of pulses, is a linear function of the duration of continuous irradiation or the duration between first and last pulse, respectively. For a given fluence applied in a sequence of pulses, reciprocity remains valid provided the duration between first and last exposure is kept constant. When the duration between first and last pulse is sufficiently long, the fluence required for high phototropic curvature falls in the first positive fluence range. These results are interpreted to indicate the existence of a kinetic limitation in the transduction sequence, and a relatively short lifetime of an initial physiologically active photoproduct. The apparent existence of more than one positive response may have resulted from these characteristics of the transduction sequence.     

Effects of preconditioning and temperature during germination of 73 natural accessions of Arabidopsis thaliana

BACKGROUND AND AIMS: Germination and establishment of seeds are complex traits affected by a wide range of internal and external influences. The effects of parental temperature preconditioning and temperature during germination on germination and establishment of Arabidopsis thaliana were examined. METHODS: Seeds from parental plants grown at 14 and at 22 degrees C were screened for germination (protrusion of radicle) and establishment (greening of cotyledons) at three different temperatures (10, 18 and 26 degrees C). Seventy-three accessions from across the entire distribution range of A. thaliana were included. KEY RESULTS: Multifactorial analyses of variances revealed significant differences in the effects of genotypes, preconditioning, temperature treatment, and their interactions on duration of germination and establishment. Reaction norms showed an enormous range of plasticity among the preconditioning and different germination temperatures. Correlations of percentage total germination and establishment after 38 d with the geographical origin of accessions were only significant for 14 degrees C preconditioning but not for 22 degrees C preconditioning. Correlations with temperature and precipitation on the origin of the accessions were mainly found at the lower germination temperatures (10 and 18 degrees C) and were absent at higher germination temperatures (26 degrees C). CONCLUSIONS: Overall, the data show huge variation of germination and establishment among natural accessions of A. thaliana and might serve as a valuable source for further germination and plasticity studies.     

Enhancement of NaCl tolerance in Arabidopsis thaliana by exogenous L-asparagine and D-asparagine

The tolerances of Columbia Arabidopsis thaliana (L.) Heynh. to NaCl, L-asparagine (L-Asn) and D-asparagine (D-Asn) during seedling establishment on sterile agar medium were determined. Germination and the establishment of upright seedlings with expanded green cotyledons were increasingly inhibited by NaCl concentrations from 20 to 180 m M and radicle growth was prevented at 225 m M NaCl. Tolerance of established seedlings to NaCl was similar at these concentrations. Seedling establishment was prevented at 20 m M L-Asn and 60 m M D-Asn, but L-Asn was not toxic to established seedlings. At lower concentrations, exogenous L- and D-Asn enhanced NaCl tolerance during germination and seedling establishment. Inhibition of seedling establishment by NaCl concentrations below 225 m M was reduced by the addition of L- and D-Asn to the medium. Maximal reduction of NaCl inhibition occurred between 2 and 4 m M for both L- and D-Asn. Higher concentrations of NaCl prevented establishment whether exogenous Asn was present or not. Reduction of NaCl inhibition occurred to the same extent whether L-Asn was presented simultaneously with the NaCl or preloaded for up to 24 h. The total seedling content of Na⁺ increased about 4-fold to 55 μg (mg dry weight)⁻¹ as the medium concentration of NaCl was increased from 9 μ M to 150 m M NaCl. Total K⁺ content declined about 80% from about 34 μg (mg dry weight)⁻¹ over the same range of NaCl concentrations. The Na⁺ uptake and K⁺ efflux by whole seedlings were similar whether or not NaCl tolerance was increased by exogenous Asn.     

Effects of light and temperature on seed dormancy and gibberellin-stimulated germination in Arabidopsis thaliana: studies with gibberellin-deficient and -insensitive mutants.

Effects of light and temperature on gibberellin (GA)-induced seed germination were studied in Arabidopsis thaliana (L.) Heynh. with the use of GA-deficient ( gal ) mutants, mutants with a strongly reduced sensitivity to GA ( gai ) and with the recombinant gai/gal . Seeds of the gal mutant did not germinate in the absence of exogenous GAs, neither in darkness, nor in light, indicating that GAs are absolutely required for germination of this species. Wild-type and gai seeds did not always require applied GAs in light. The conclusion that light stimulates GA biosynthesis was strengthened by the antagonistic action of tetcyclacis, an inhibitor of GA biosynthesis. In wild-type, gal and gai/gal seeds light lowered the GA requirement, which can be interpreted as an increase in sensitivity to GAs. In gai and gai/gal seeds light became effective only after dormancy was broken by either a chilling treatment of one week or a dry after-ripening period at 2°C during some months. The present genetic and physiological evidence strongly suggests that temperature regulates the responsiveness to light in A. thaliana seeds. The responsiveness increases during dormancy breaking, whereas the opposite occurs during induction of dormancy (8 days at 15°C pre-incubation). Since light stimulates the synthesis of GAs as well as the responsiveness to GAs, temperature-induced changes in dormancy may indirectly change the capacities to synthesize GAs and to respond to GAs. GA sensitivity is also directly controlled by temperature. It is concluded that both GA biosynthesis and sensitivity to GAs are not the primary controlling factors in dormancy, but are essential for germination.     

Analysis of short-term changes in the Arabidopsis thaliana glycerolipidome in response to temperature and light

Although the influence of temperature, particularly cold, on lipid metabolism is well established, previous studies have focused on long-term responses and have largely ignored the influence of other interacting environmental factors. Here, we present a time-resolved analysis of the early responses of the glycerolipidome of Arabidopsis thaliana plants exposed to various temperatures (4, 21 and 32°C) and light intensities (darkness, 75, 150 and 400 μmol m(-2) s(-1)), including selected combinations. Using a UPLC/MS-based lipidomic platform, we reproducibly measured most glycerolipid species reported for Arabidopsis leaves, including the classes phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI) phosphatidylglycerol (PG), monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG). In addition to known lipids, we have identified previously unobserved compounds, such as 36-C PGs and eukaryotic phospholipids containing 16:3 acyl chains. Occurrence of these lipid species implies the action of new biochemical mechanisms. Exposition of Arabidopsis plants to various light and temperature regimes results in two major effects. The first is the dependence of the saturation level of PC and MGDG pools on light intensity, likely arising from light regulation of de novo fatty acid synthesis. The second concerns an immediate decrease in unsaturated species of PG at high-temperature conditions (32°C), which could mark the first stages of adaptation to heat-stress conditions. Observed changes are discussed in the context of current knowledge, and new hypotheses have been formulated concerning the early stages of the plant response to changing light and temperature conditions.     

Photoinhibition of radish (Raphanus sativus L) seed germination: control of growth potential by cell-wall yielding in the embryo

The biophysical mechanism underlying photoinhibition of radish (Raphanus sativus L.) seed germination was investigated using three cultivars differing in sensitivity to continuous irradiation with far-red light (high-irradiance reaction of phytochrome). Sensitivity of germination to the inhibitory action of light was assessed by probing germination under osmotic stress (incubation in media of low water potentials adjusted with polyethylene glycol 6000) and expressed in terms of ‘germination potential’ (positive value of the water potential at which germination is inhibited by 50%). Far-red light decreases the germination potential to various degrees in the different cultivars, reflecting the light-sensitivity of germination in water. Removal of the seed coat increases the germination potential by a constant amount in darkness and light. It is concluded that germination depends on the expansive force of the embryo which can be drastically diminished by far-red light. Seed-coat constraint and expansive force of the embryo interact additively on the level of the germination potential. Photoinhibition of germination was accompanied by an inhibition of water uptake into the seed. Analysis of seed water relations showed that osmotic pressure and turgor assumed higher levels in photoinhibited seeds, compared to seeds germinating in darkness, while the water potential was close to zero under both conditions. Far-red light produced a shift (to less negative values) in the curve relating water-uptake rate to external water potential, i.e. a reduction in the driving force for water uptake. It is concluded that photoinhibition of germination results from the maintenance of a high threshold of cell-wall extensibility in the embryo.     

Contribution of gibberellin deactivation by AtGA2ox2 to the suppression of germination of dark-imbibed Arabidopsis thaliana seeds

Gibberellin levels in imbibed Arabidopsis thaliana seeds are regulated by light via phytochrome, presumably through regulation of gibberellin biosynthesis genes, AtGA3ox1 and AtGA3ox2, and a deactivation gene, AtGA2ox2. Here, we show that a loss-of-function ga2ox2 mutation causes an increase in GA(4) levels and partly suppresses the germination inability during dark imbibition after inactivation of phytochrome. Experiments using 2,2-dimethylGA(4), a GA(4) analog resistant to gibberellin 2-oxidase, in combination with ga2ox2 mutant seeds suggest that the efficiency of deactivation of exogenous GA(4) by AtGA2ox2 is dependent on light conditions, which partly explains phytochrome-mediated changes in gibberellin effectiveness (sensitivity) found in previous studies.     

利用激光扫描共聚焦显微镜研究拟南芥气孔发生与发育

通过激光扫描共聚焦显微镜,利用不同种类(波长)的激光研究拟南芥叶片气孔发生与发育。结果表明,利用紫外激光(351nm)扫描可以清楚观察到拟南芥表皮各种细胞及其发生发育的形态变化,包括表皮毛细胞、副卫细胞、保卫细胞、铺垫表皮细胞等。气孔发生过程中,首先原表皮细胞不对称分裂产生拟分生组织和副卫细胞,接着分化出保卫细胞母细胞,进一步发育形成保卫细胞,最终形成气孔器。气孔分化完成后,保卫细胞在紫外激光下不产生荧光,但利用蓝光激发(488nm)辅助荧光素染色,可清晰地看到保卫细胞。结果表明,激光扫描共聚焦显微镜在拟南芥叶表皮细胞形态研究上有独特的功能。