There exist two main mechanisms of allelopathy: 1) direct transfer of metabolites between neighbouring plants during their life and 2) accumulation and transformation of bioactive substances in the environment with their subsequent influence on higher plants. The latter mechanism is similar to that causing soil sickness or soil toxicity. Allelochemicals affect all functions of the living system: photosynthesis, respiration, mineral nutrition, transpiration, immunity, and growth. The initial biochemical effect of allelochemicals seems to be on the synthesis of protein mediated by RNA/DNA. Knowledge of the mechanisms of concrete signals during realization of hereditary codes would serve as a powerful tool for the improvement of crop plant development and productivity.

Activity of endogenous cytokinins and gibberellins was determined in the shoots and roots of oak seedlings following the submersion of roots in water acidified with sulphuric add to pH 3.0, 2.5, 2.0, 1.7 and 5.6 (control) for 6 and 72 h. After 6 h the cytokinins increased in the shoots and in roots at pH 2.5 and 2.0, respectively. None of the tested pH values decreased the cytokinin activity. The acidification of solutions exhibited no remarkable influence on gibberellin activity. Only slight stimulation of the gibberellin activity was exhibited at pH 2.0. After 72 h a slight decrease in the activity of cytokinins was observed in the shoots at pH 2.5, remarkable decrease was revealed at pH 2.0 and 1.7. Nevertheless, in the roots stimulation of cytokinin activity appeared up to pH 2.0 (the highest at pH 2.5), and significant decrease was shown at pH 1.7. The activity of gibberellins in shoots showed similar dependence of changes as the activity of cytokinins in roots. Acidification stress exerted no influence on the gibberellin. activity in roots.

7 days or 7 weeks old alfalfa plants (Medicago sativa L.), susceptible (S) and resistant (R) to bacterial wilt, were inoculated withCorynebacterium michiganense pv.insidiosum and on day 8 and 15 after inoculation the levels of acid-soluble phosphate esters (P-esters) were determinated by means of³²P labelling in the shoots or roots. The most significant changes were recorded in the roots of the older R plants grown in full Knop nutrient solutions on day 8 after inoculation. The marked reduction of inorganic phosphate (P₁) uptake by whole R plants is accompanied by a decrease in the levels of fructose-l, 6-bisphosphate (Fru-P₂), glucose-6-phosphate (Glc-6-P), fructose-6-phosphate (Fru-6-P), adenosine mono-, and diphosphate (AMP and ADP), phosphorylcholine (P-choline) and a proportional increase in the level of P₁. In the S plants, infection affected neither P₁ uptake nor P₁ proportions. In the plants grown after inoculation in diluted Knop's solutions (0.147 mM KH₂PO₄), infection induced a reduction of the radial transport of P₁ to the segments of R roots whereas a reduction of the levels was only recorded in some P-esters [AMP, ADP, dihydroxyacetone phosphate (DHAP), and P-choline, but no decrease of Fru-P₂, Glc-6-P and Fru-6-P]. In the S plants, P₁ transport and the levels of P-esters were increased by the infection. P₁ transport exhibited considerable metabolic dependence (DNP, DCCD). Bacterial infection probably had no influence on the activity of the plasma membrane ATPases.

Application of kinetin to four interspecific crosses in Brassica, either singly or in combination with NAA, resulted in larger length and retention time of ovaries, increased frequency of ovules with embryos and aided embryo development.

The cross B. campestris ssp. japonica x B. oleracea var. botrytis fails due to incompatibility barrier at the stigma. To realize this cross, irradiated compatible pollen (mentor pollen) was used before the incompatible pollination. The presence of mentor pollen stimulated the incompatible pollen to germinate and effect fertilization and seed set. One hybrid was thus obtained. Most of the seeds were inviable. Of the 5 plants raised one was a hybrid and 4 resembled the female parent.

The quantitative changes of chlorogenic acid in shoots and leaves of two groups of grape vines differing in their flower- and fruit-formation have been studied. It was established that in most cases the acid level was lower in both shoots and leaves of grape vines with suppressed growth and intensified generative processes. The results obtained suggest that the lower content of chlorogenic acid is related to the reduced growth of shoots and acceleration of generative processes.

The content and changes in the activity of phosphogluconate dehydrogenase were followed in leaf tissues of tobacco plants infected with the potato virus Y (PVY) in the acute phase of infection. The activity of the enzyme was higher in virus-infected tissues during the entire experimental period compared with the values found in healthy control plants in both crude homogenate and partially purified enzyme preparation. The courses of the activity curves of both the crude extract and partially purified enzyme preparations were consistent and correlated with the reproduction curve of the virus. These results suggest that increased activity of the enzyme was a result of its coarse regulation and indicate its immediate involvement in"de novo" biosynthesis of the virus via the oxidative pentose phosphate pathway.

The content of endogenous cytokinin-like substances and the release of ethylene were determined in leaves of different insertion of sunflower plants during their ontogeny. The content of cytokinin-like substances was highest in the leaves on the middle part of the stem (that is in leaves just before full expansion), with a decrease occurring both towards the base and the apex of the stem, when followed at four growth phases (vegetative plants, plants with inflorescence diameter up to 0.5 cm, plants with inflorescence diameter up to 3 cm, and plants in flower). Changes in the content of cytokinin-like substances during the ontogeny of the leaf also corresponded to this pattern. Data obtained with the leaf at the third node from the basis of the stem showed that the level of cytokinin-like substances first sharply increased, and then after reaching maximal value (at the time when leaf blade area reached approximately 70 % of the final value) slowly and continuously decreased.
The highest amount of ethylene released from the leaves was recorded in basal leaves and then also in apical leaves, whereas the leaves with the largest blade area situated at the central part of the stem released the lowest amount of ethylene. This pattern was repeatedly found at all four selected growth phases of sunflower plants.

Benzylaminopurine and thidiazuron stimulated shoot proliferation ofTilia, Sorbus andRobinia. Low concentration of BAP (0.2-1.0 mg I^-1) promoted axillary bud formation and shoot elongation. Thidiazuron displayed high cytokinin activity at very low concentrations (0.002-0.05 mg I^-1). Shoot number induced on media containing thidiazuron was large. Numerous shoots were produced on the media containing BAP together with thidiazuron. Shoots produced on media containing thidiazuron or BAP together with thidiazuron rooted after transfer to medium supplemented with low concentration of auxin (IBA or NAA).

In negatively photoblastic seeds of Cucumis anguria L. the scarification of micropyle overcame the inhibitory effect of "white light ", although far-red radiation inhibited the germination in both intact and scarified seeds. The moistened testa powder transmitted more far-red than red radiation. Thus the testa could act as a filter of radiation and maintain a high far-red/red ratio at the photosensitive site of the embryo, which must be localized near the micropyle.

The effect of potassium administration in early germination of wheat (Triticum durum L., cv. Appulo) was studied by incubating isolated embryos at different viability levels with 1 mM KC1. The stimulation of germination and growth in aged embryos by K⁺ was accompanied by a significant increase in (³H) leucine incorporation into protein and in phospholipid synthesis as evidenced by TLC analysis of embryo phospholipids extracted after 24 h of imbibition. The metabolic improvement mediated by the cation would in turn repair aging damage occurring at subcellular sites including membranes.

Different species of yam tubers were examined for the presence of pyruvate kinase and phosphatase activities, Pyruvate kinase was purified 25 fold with a yield of 50 %. using ammonium sulphate precipitation and ion exchange chromatography on DEAE-Sephadex. Partially purified enzyme showed normal Michaelis-Menten kinetics. However, pyruvate kinase from crude extract of dormant yam tuber showed slight sigmoid response towards phosphoenol-pyruvate and magnesium and to a certain extent ADP. The enzyme is activated by AMP and inhibited by ATP and citrate in both crude and partially purified preparations. Further studies on the effect of energy charge on the enzyme strongly suggest that pyruvate kinase from D. alata is a regulatory enzyme. No evidence was found for the presence of more than one pyruvate kinase in germinating D. alata tuber. With the exception of D. dumentorum, all the other three species of yams studied contain very little or no detectable phosphatase activity during dormancy. However, phosphatase activity increased during germination in all the species, thus excluding the use of sprouting yam tubers for kinetic study of pyruvate kinase.

Uptake of³⁵S-sulphate by bundle sheath strands (BSC) from leaves of maize plants (Zea mays L. ev. Dekalb L 72 A) was higher than that by isolated mesophyll protoplasts (MC) of maize. Ion uptake followed the Michaelis-Menten kinetic satuiation curves. SO² ₄-uptake increased after addition of malate, NADPH, malate + NADP+ to BSC suspensions, but not to MC susp: nsions.

The season of collection of shoot tips from field-grown plants ofChrysanthemum morifolium cv. Birbal Salmi was found to be crucial for their proliferation and establishment of plants in vitro. Shoot tips collected only during the period of March to April proliferated and survived. Shoot apices and segments of leaf, stem (nodal und internodal) and root, excised from aseptically established plants, were cultured on Murashige and Skoog's medium supplemented with different concentrations and combinations of Kn, BAP, IAA and NAA. A maximum number of 9 off-shoots differentiated, without intervening callus formation, from a shoot tip in a treatment containing 1.5 mg l^-1 BAP and 0.1 mg^-1 NAA in 60 days. A single-node stem segment produced less shoots in the same treatment. Other kinds of expiants produced only callus. About 2-cm-long shoots, excised from cultures of proliferating shoots, were rooted 100 %, acclimatized and grown in soil. They grew normally and flowered true-to-type.

The change in protein composition of whole cotyledons and cotyledon aleurone grains ofLupinus luteus L. during seed germination was studied. SDS-polyacrylamide gel electro-phoresis showed a clear change in composition of cotyledon proteins as well as in composition of the aleurone grains during 5 days of seed germination. At this time, both in whole cotyledons as well as in aleurone grains, two subunits of β-conglutin with mol. m. 53 000 and 39 000 were rapidly hydrolyzed. After 5 days of germination traces of α-conglutin subunits could be detected in the cotyledons, whereas in aleurone grains this globulin fraction disappeared. In whole cotyledons and in cotyledon aleurone grains the γ-conglutin subunits with mol. m. 28 000 and 17 000 were not mobilized during the study period. These results indicate that the protein components with lower mol. m. were degraded later than those withhigher mol. m. during seed germination.

The effect of choline, acetylcholine, carbamylcholine and of the acetylcholinesterase inhibitors, eserine and neostigmine, on the germination of seeds exhibiting different photosenitivities was studied in two light variants. Both in the darkness and in continuous white light the substances in question had no effect or only slightly modified germination of photoblastically neutral seeds. On the other hand, the above substances, particularly aeetylcholine and carbamylcholine, stimulated germination of photoblastically positive seeds, and inhibited germination of photoblastically negative seeds in continuous white light.

High rates of oak shoot multiplication were achieved on low salt nutrient media (BTM, WPM) supplemented with adenine type cytokinin or BAP. Low concentration of (0.2-0.6 mg 1^-1) stimulated formation of a large number of axillary shoots and their elongation. Thidiazuron in very low concentration (0.001 - 0.002 mg 1^-1) promoted shoot proliferation, in high concentration stimulated formation of large callus. More than twelve thousand micropropagated shoots were rooted in low salt agar media supplemented with low concentration of auxin (IBA or NAA 0.2 - 0.5 mg 1^-1). High rooting percentages (81 %) were obtained. Survival of the mieropropagated plantlets transplanted into soil was high (78 %). Micropropagated trees planted in the field withstood severe winter frosts without significant losses. At the end of the fifth growing season trees attained considerable size.

The rooting ability of 2 cm long shoots ofPisum sativum L., derived from differentin vitro shoot-tip cultures in two pea cultivars Bohatýr and Kleine Rheinländerin was evaluated. In three mutually independent experiments the full and half-strength Murashige-Skoog medium (containing full or half concentration of macro and microelements), with sucrose concentrations 10-30 g l-¹, and with various NAA and IAA combinations, was tested. The variant with half concentration of macro- and microelements, supplemented with 30 g l¹ sucrose, and with growth regulators in the quantity of 1 μM proved optimum.

Response of stomata in epidermal strips from green leaves ofTradescantia sillamontana and anthocyanin-rich purple leaves ofT. virginiana and from green and pigmented regions ofPedilanthus tithymaloides leaves, to ABA have been compared. Stomata from anthocyanin-rich leaves or leaf parts appeared to be relatively insensitive to ABA as compared to those from green leaves or leaf parts. Observations indicate the possibility of the involvement of endogenous anthocyanins in antagonising ABA in preventing the stomatal opening.

In hexaploid triticale, cultivar UPT 72142 having highly shrivelled grains, there were only five pairs of rye chromosomes, whereas cultivars UPT 78268 and UPT 79245 having medium grains had six and five pairs, respectively. Plump grained cultivars UPT 79347 and UPT 79339 had 2 pairs of rye chromosomes. A decrease in the number of rye chromosomes in triticales is con-elated with grain improvement.

Second and third leaves of 25-day old wheat (Triticum aestivum L.) planta, cultivar WL-711, were inoculated with three races of the brown rust pathogen,Puccinia recondita f. sp.tritici. The host reflected a differential interaction response towards these races, viz., susceptible reaction to race 77 (reaction type 4) intermediate reaction to race 104 (reaction type 1 - 3) and resistant reaction to race 63 (reaction type 0). Post-inoculation mutual interaction brought about observable changes in peroxidase activity along with leakage of electrolytes and phenolic constituents indicating altered permeability of host tissue. Differential increase in leakage of electrolytes and phenols occurred with increase of peroxidase activity during 12 h and 24 h post-inoculation, however, during 36 h, 48 h and 72 h post-inoculation, electrolyte leakage increased continuously with corresponding increases in peroxidase activity in all three interactions, while leakage of phenols showed a differential pattern. The differential changes in isoenzymic pattern of peroxidase activity were also noted during the progressive infection process which may also be an indication of altered permeability of the host issue at the very onset of brown rust infection.

Uncoupling of oxidative phosphorylation by 2,4 dinitrophenol stimulated leaf respiration rate measured as CO₂ production or O₂ uptake after low temperature treatment.