Allelopathy in Heracleum laciniatum: Inhibition of Lettuce Seed Germination and Root Growth

Both dark and red light germination of lettuce seeds (cv. “Maikönig”) as well as their root and hypocotol elongation were inhibited when the seeds were sown in petri dishes together with a few seeds of Heracleum laciniatum Horn. This inhibition was not significantly counteracted by the presence of gibberellic acid (GA₃) or/and 6-benzylaminopurine (BA). However, a large proportion of the lettuce seeds germinated abnormally (only cotyledons emerged) when treated with BA in the presence of Heracleum seeds. GA₃ had alone no significant effect on abnormal germination, but it counteracted the effect of BA to some extent. The inhibitory effect of Heracleum seeds gradually disappeared during a moist incubation period of one to seven days in darkness at 25°C. When lettuce seeds were pre-incubated together with Heracleum seeds for one to five days the remaining, non-germinated lettuce seeds had lost their ability for subsequent germination in darkness in distilled water. This induced dark dormancy was to a great extent broken by red light, but not by GA₃ or/and BA. H. laciniatum seeds inhibited the germination of Salix pentandra seeds and to some extent also the germination of radish but had no effect on the germination of spruce.     

Effect of exogenous gibberellins on flowering in Pinus sylvestris grafts

Ethanolic sprays of gibberellins were applied to developing shoots of about 12-year-old Scots pine (Pinus sylvestris L.) grafts during the shoot elongation period in two consecutive years. Both male and female flowering was increased by these treatments in both years. The effect was particularly distinct in male flowering. However, different clones showed varying responses to the treatment. This variation was associated with both the genotype and the environmentally determined year-to-year fluctuation in flowering. Differences among clones were analysed further by introducing a model earlier developed for comparisons of the growth rhythm in various woody and herbaceous species.     

The Freezing Response of an Arctic Cushion Plant, Saxifraga caespitosa L.: Acclimation, Freezing Tolerance and Ice Nucleation

Cold hardiness in actively growing plants of Saxifraga caespitosaL., an arctic and subarctic cushion plant, was examined. Plantscollected from subarctic and arctic sites were cultivated ina phytotron at temperatures of 3, 9, 12 and 21 °C undera 24-h photoperiod, and examined for freezing tolerance usingcontrolled freezing at a cooling rate of 3–4 °C eitherin air or in moist sand. Post-freezing injury was assessed byvisual inspection and with chlorophyll fluorescence, which appearedto be well suited for the evaluation of injury in Saxifragaleaves. Freezing of excised leaves in moist sand distinguishedwell among the various treatments, but the differences werepartly masked by significant supercooling when the tissue wasfrozen in air. Excised leaves, meristems, stem tissue and flowerssupercooled to –9 to –15 °C, but in rosettesand in intact plants ice nucleation was initiated at –4to –7 °C. The arctic plants tended to be more coldhardy than the subarctic plants, but in plants from both locationscold hardiness increased significantly with decreasing growthtemperature. Plants grown at 12 °C or less developed resistanceto freezing, and excised leaves of arctic Saxifraga grown at3 °C survived temperatures down to about –20 °C.Exposure to –3 °C temperature for up to 5 d did notsignificantly enhance the hardiness obtained at 3 °C. Whenwhole plants of arctic Saxifraga were frozen, with roots protectedfrom freezing, they survived –15 °C and –25°C when cultivated at 12 and 3 °C, respectively, althougha high percentage of the leaves were killed. The basal levelof freezing tolerance maintained in these plants throughoutperiods of active growth may have adaptive significance in subarcticand arctic environments. Saxifraga caespitosa L., arctic, chlorophyll fluorescence, cold acclimation, cushion plant, freezing stress, freezing tolerance, ice nucleation, supercooling     

The Cessation of Apical Growth in Latitudinal Ecotypes and Ecotype Crosses of Salix pentandra L

Apical growth cessation in seedling progenies of three latitudinalecotypes and ecotype crosses of Salix pentandra L. was investigatedunder controlled growth conditions and out-of-doors. The southernecotype (59° 40' N) was characterized by late growth cessationunder short day conditions, short critical photoperiod for growthcessation, and high light sensitivity; the northern ecotype(69° 39' N) by rapid growth cessation, long critical photoperiod,and low light sensitivity. The geographically intermediate ecotype(64° 28' N) had an intermediate photoperiodic response.The hybrid progenies were, however, closer to the southern thanto the northern parent ecotype indicating a tendency to favourinheritance of southern characters. Fluctuating temperatures(21 °C/9 °C and 18 °C/9 °C) induced growth cessationin the northern ecotype but not in the other ecotypes or ecotypecrosses with the exception of a few plants of the intermediateecotype and of two hybrid progenies. The time course of apicalgrowth cessation in various progenies grown out-of-doors at69° 39' N was similar to that observed under SD conditionsexcept that the differences between the northern ecotype andthe other ecotypes and crosses were greater.     

Field Studies of Photosynthesis as Affected by Water Stress, Temperature, and Light in Birch

This study continues the investigations previously conducted as laboratory experiments. The results from the present study confirm our earlier observations made on alder seedlings concerning the effect of water stress, temperature and light on the net uptake of CO₂. A variable that we could call the physiological water stress is proposed as a measure of the intrinsic factor of photosynthesis during and after drought. A physiological water stress builds up and discharges slowly and interacts strongly with temperature. Our model for the effects of physiological water stress, temperature, and light intensity explains satisfactorily the net uptake of CO₂ in birch in the field. Thus, our earlier results concerning the effects of physiological water stress on photosynthesis are not artifacts generated by the unnatural laboratory environment.     

Flower bud differentiation in Salix pentandra as affected by photoperiod, temperature and growth regulators

Flower bud initiation in seedlings and vegetatively propagated plants of Salix pentandra from different locations has been studied under controlled conditions. In mature plants flower bud formation was induced by 2-chloroethyltrimethylammoniumchloride (CCC) and by short day treatment. The effect of CCC was antagonized by GA₃. The critical photoperiod for flower bud formation was about 18 h for a southern clone (from 49°48'N), but cuttings of a northern ecotype (from 69°39'N) formed flower buds even at 24 h photoperiod. Generally, flower bud formation occurred simultaneously with apical growth cessation. However, apical growth cessation was not a prerequisite for floral initiation and flower buds were also found in elongating plants. Seedlings up to 60 days old did not form flower buds in growth chamber studies. The length of the juvenile phase has not been studied in detail, but cuttings taken from seedlings approximately 20 cm high and 60 days old were able to develop flower buds when treated with CCC. A gradual transition from the juvenile to the mature phase was obtained by repeated pruning of seedlings grown at 18°C and 24 h photoperiod.     

Activity of Various Growth Substances in the Avena First Internode Test

The elongation growth of the Avena first internode segments was studied in the presence of one or several of the following growth substances: indoleacetic acid (IAA), 6-fur-furylamino purine (FAP, kinetin), 6-benzylamino purine (BAP), gibberellin A₃ (GA₃) and A₄₊₇ (GA₄₊₇), and abscisic acid (ABA). The cytokinins at concentrations of 10^?7 to 10^?6M stimulated growth with 4 to 6 per cent but this effect was not statistically significant. Concentrations higher than 5 × 10^?6M inhibited growth. FAP and BAP (from 10^?8M to 10^?6M) had no significant interaction with any other growth substance used. The two-factor interactions of IAA × ABA, IAA × GA₃, and GA₃× ABA, as well as the three-factor interaction IAA × ABA × GA₃ were significant. However, the IAA × ABA interaction was significant only when high concentration (10^?6M) of ABA was used. The growth inhibition produced by 10^?7 and 10^?6M ABA was overcome by about equimolar concentrations of IAA. The stimulation of growth by GA₃ and GA₄₊₇ (10^?9 to 10^?7M) was prevented by simultaneous application of ABA, and it was reduced significantly by application of IAA (10^?7 to 10^?8M). GA3 at 10^?8M combined with different concentrations of IAA gave slightly higher elongation than IAA alone but the observed values were significantly lower than expected assuming independent additive action.     

Effect of Gibberellic Acid on Dark and Light Germination at Different Temperatures of Calluna, Ledum andRhododendron Seeds

The aim of the experiments was to study the effects of gibberellic acid (GAs) on the germination of Calluna vulgaris L., Ledum palustre L. and Rhododendron lapponicum (L.) Wahlenb. seeds under different environmental conditions. Under continuous light from white fluorescense tubes (3000 lux), untreated seeds of Calluna were partly dormant at all temperatures studied (9, 15, 21, 27, 27/9, 8/16 hours). Percentage of dormant seeds increased, however, with decreasing temperature, and it varied also from seed lot to seed lot. Untreated seeds of Ledum were dormant in light at 9° and 15°C but not at higher temperatures. Untreated seeds of Rhododendron were completely dormant in light at temperatures from 13° to 24°C. Seeds of all species were completely dormant in darkness both at 15° and 27°C. GA₃ stimulated greatly the germination of all species under all studied environmental conditions. The used concentrations (0.2–3.2 mM) gave nearly 100% germination in most cases. At 9°C the dormancy in some seed lots of Calluna and Ledum was only partly broken by the used concentrations of GA₃.     

Arrested Leaf Abscission in the Non-Abscising Variety of Pubescent Birch: Developmental,Morphological and Hormonal Aspects

A novel type of abscission development, arrested abscission,is described for a rare variety of pubescent birch (Betula pubescensEhrh. f. hibernifolia Ulvinen), the leaves of which dehydrateand remain attached to the plant. Anatomical examination ofthe abscission zone revealed that its development is temporallyand spatially similar to that of normal leaves of Betula pubescens.Floodingand exogenous ethylene were effective in inducing shedding ofthe leaves at the beginning of the growing season, butthis effectwas lost in senescing leaves. Leaf hydration and abscissionwere retained in the presence of external abscisic acid (ABA)inthe non-abscising variety, which also had a lower level of endogenousABA. The observed responses together with the alteredhormonallevel in this variety suggest that apart from ethylene, ABAis involved in autumnal abscission indirectly by regulatingtheleaf water status. Key words: ABA deficiency, abscisic acid, abscission zone, ethylene, senescenc     

Allelopathic Inhibitors in Seeds of Hevacleum laciniatum

Biological inhibitors present in the dry seeds of Heracleum laciniatum Horn have been studied by means of thin layer chromatography and various bioassays. The ethylacetate fraction of methanol extracts contained three main groups of inhibitors having the following R values when chromatographed on silica gel with chloroform: (1) 0.60–0.80; (2) 0.20–0.40; (3) 0.0–0.10. The group (1) inhibited lettuce seed germination and root growth, elongation of Avena first internode segments, growth of the fungus Cladosporium cucumerinum, and it caused leakage of betacyanin from red beet tuber slices. The groups (2) and (3) had no activity in red beet betacyanin leakage test, but they were inhibitory in the other bioassays used. The group (3) were generally less active than the group (2). The group (2) contained the furanocoumarins pimpinellin, bergapten, isobergapten and angelicin. Inhibitors present in the groups (1) and (3) have not been identified. Effects of five synthetic furanocoumarins (pimpinellin, bergapten, isobergapten, isopimpinellin, sphondin) on lettuce seed germination and Cladosporium growth were investigated. Pimpinellin was inhibitory to Cladosporium growth, and lettuce seed germination was inhibited by pimpinellin and sphondin.