1-Amino-2-phenylethylphosphonie acid (PheP) retards growth inSpirodela oligor-rhiza causing morphological malformations, inhibits chlorophyll synthesis in progeny fronds, and markedly stimulates L-phenylalanine ammonia-lyase (PAL) activityin vivo and inhibits itin vitro; in prolonged experiments ribonuclease activity is stimulated. PheP severely inhibits anthocyanin synthesis in seedlings of red cabbage, with moderate inhibition of PAL activityin vivo; chlorophyll synthesis and growth of the seedlings are little affected.

Petunia hybrida plants were inoculated with differentAgrobacterium rhizogenes andA. tumefaciens strains and developed tumors were further cultivatedin vitro. Transformed flowering plants differentiated from tumors induced byA. rhizogenes strains 8196 and TRIOL Transformed but non-rooted plants developed also from tumors incited byA. tumefaciens T37. Cultures of roots transformed byA. rhizogenes strain 15834 did not show increased incidence of chromosomal aberrations in anaphases in comparison with untransformecl control. Permanent growth of isolated untransformedPetunia roots was not induced by addition of IAA into the medium.

Rooting cofactors and proanthocyanidins were studied in cuttings ofCastanea sativa Mill. from a natural mutant Nana and juvenile plants, both of which are easy to root, and from adult plants that do not root. Rooting cofactors and a high amount of proanthocyanidins were found in the easy-to-root cuttings, whereas in the adult plants no rooting cofactors were detected and the amount of proanthocyanidins was very low.

Occurrence and operation of sucrose synthetic system in randomly selected callus cultures such as persimmon, soybean and poplar cultures were examined by¹⁴C-tracer analysis and determining enzyme activities involved in sucrose metabolism. All the enzymes examined were present and¹⁴C-glueose was transformed into¹⁴C-sucrose in every callus. Sucrose synthetic capacity appears to be widely distributed in cultured plant tissues.

IAA synthesis proceeding by indol-3-ylpyruvate (IPyA) pathway seems to be regulated in two steps. In the first the L-trp conversion into IPyA is reduced by a low affinity of L-trp to the unspecific aminotransferase, by competition of L-trp with some aminoacids (e.g. L-asp) and indoles (e.g. indol-3-ylacetylaspartate). Simultaneously, a specific L-trp-dehydrogenase in dependence on the NAD(P)/NAD(P)H ratio regulates by its reversible effect the level of IPyA, connecting photosynthesis with growth. A second more "delicate" regulation of IAA level is carried out by the indol-3-ylacetaldehyde system. In pea plants two indol-3-ylacetaldehyde oxidases with pH optima 4.5 and 7.0 were found. The oxidases are differentially inhibited by an excess of IAA, different indoles as indol-3-ylacetylaspartate and aminoacids as L-asp. GA₃ and kinetin stimulate the conversion of indol-3-ylacetaldehyde to IAA.

The possibility that some of the variation in callus cultures involves epigenetic changes is examined in cultures established from the hypoootyls and roots ofEuphorbia heterophylla. It is shown that the responses of the cultures are affected by the light regimes under which they are grown and that in the dark and under short photoperiods, there are differences between the two types of culture with respect to pigmentation, auxin requirement, capacity to regenerate buds and roots and in certain isozyme patterns, whereas the two cultures are similar from the first passage under continuous light. However, these differences are only maintained for 2-3 passages, after which the root callus becomes similar to the hypoc otyl callus. Evidence is presented that these differences between cultures are epigenetic. Callus cultures established from the apical meristems of shoots and roots ofE. hetero phylla show similar differences to those observed between hypocotyl and root cultures and these differences are also lost after 3 passages. These results indicate that the cells of apical meristems are not totally uncommitted, but are determined as 'shoot' and 'root' meristem colls, respectively. The practical importance of a better understanding of epigenetic effects in plant cells is strassed.

The decrease of peroxidase (PO) and IAA-oxidase (IAAO) activities in stem bark of intact photoperiodically neutral tobacco plants was shown both in the course of development and in an acropetal direction along the stem as related to flowering regulation. In stem expiantsin vitro, the realisation stage of floral morphogenesis is related to an increase in PO and IAAO. This correlates with low auxin concentrations in the medium as required for floral morphogenesis. The changes in both PO and IAAO are suggested to be in the opposite directions during floral induction and floral morphogenesis.

Curves relating net photosynthetic rate to irradiance [P(I) curve relation] were estimated and analysed inMercurialis perennis L. plants stemming from three forest (spruce, beech and ash) stands with different tree leaf canopy development and different light regime.
The saturating irradiance (I_s) reached the highest values in plants of all three stands in spring (spruce forest: 438 W m^-2, beech forest: 440 W m^-2 and ash forest: 367 W m^-2), it declined sharply in the middle of the growing season (283, 285 and 297 W m^-2, respectively) and this I_s level persisted until autumn. A pronounced dynamics in plants from spruce and beech forests made itself manifest also in the adaptation (I_a) and compensating (I_c) irradiances, respectively. After a sudden decline in summer, values in autumn were close to those of the vernal season.
The most pronounced parameter, which optimally expressed the adaptation ofMercurialis perennis to various light conditions, was the photosynthetic efficiency (α) calculated as the slope of the linear part of the curve relating net photosynthetic rate to irradiance.
At the time of the highest P_{N sat.} value in course of the growing season (August) (spruce forest: 100, beech forest: 98.7 and ash forest: 85.8 μg CO₂ m^-2 s^-1), R_D was in its minimum; in autumn P_{N sat.} reached the lowest values which corresponded to the most intensive R_D.
It was found thatMercurialis perennis plants stemming from forest stands with different light conditions did not make use equally of the altering light conditions in the course of the growing season. By the underlying analysis of P(I) curves this rhizomatous perennial herb (geophyte) may be characterized as a shade tolerant species.

A possible involvement of the alternative oxidase pathway in proton translocation was investigated. Net H⁺ efflux- and elongation-rates were simultaneously and continuously measured by means of a pH-stat and an angular position transducer. Disulfiram, an inhibitor of the alternative path, reduces the IAA- and Fusicoccin-induced as well as endogenous proton secretion and growth. Fusicoccin-induced H⁺ secretion is very sensitive to reduced oxygen concentration values far apart from the K_m of cytochrome oxidase. The sensitivity of non stimulated proton secretion to reduction of oxygen concentration depends on the age of plant material. It is proposed that more than one system is responsible for proton translocation across the plasmalemma. One of them has a high sensitivity to reduced oxygen concentration which is within the same range of the high K_m value of the alternative oxidase.

Evidence for the existence of a neutral inhibitor specific for auxin has been in the literature for 45 years. The inhibitor is demonstrable by its effect in causing positive curvature of theAvena coleoptile. The growth of the mesocotyl of oat and corn seedlings in darkness and its inhibition by light are determined by the neutral inhibitor as is the phototropic response of the sunflower stem. Production of the inhibitor is promoted by red and fluorescent light. Irradiance at 730 nm promotes auxin production while irradiance at 660 nm promotes production of the inhibitor. The positive curvature induced by 2,3,5-triiodobenzoic acid can be used to quantify the neutral inhibitor. Since the benzoic acid is more effective than iodoacetate in reacting with the sulfhydryl of cysteine, a sulfhydryl group is indicated to bo one reaction site for auxin and to be the basis of polar transport.

The current status of our knowledge of auxin effects on floral induction is summarized. The most general effect is inhibition, although the concentration of synthetic auxins added to. plants tends to be too high for us to be certain that the inhibitory effects are truly physiological. Studies of endogenous levels of auxin have focused almost entirely on IAA-like bioassay activity. Chemical identifications of endogenous IAA are needed and feasible. In addition, a search for-other auxins involved in vegetative to floral transitions, their chemical identification, and measurement of their changing levels in the plant are urgently needed.

Inhibition of root growth was observed inChenopodium rubrum under photoperiodic conditions inducing flowering. That this inhibition is mediated by the cotyledons was shown directly by the effect of their excision, which changes the responsiveness of the roots to photoperiodic treatment. On the other hand, decapitation did not lead to such an effect. Some evidence is put forward suggesting that changes in IAA may be involved in these correlations. The existence of two different mechanisms of photoperiodic action in flowering and in root growth is proposed to explain these differences.

Recurrent exposures (17 administrations in 37 days) to the direct-acting mutagens N-methyl-N-nitrosourea and N-ethyl-N-nitrosourea and a chronic exposure (38 days) to the metabolism-requiring mutagen dimethylnitrosamine markedly increased the frequency of somatic mutations in a double heterozygous chlorophyll mutant ofNicotiana tabacum, whereas exposure to N-methyl-N'-nitro-N-nitrosoguanidine was ineffective.

The chemical and stereochemical properties of phytotropins are described in terms of a receptor model, based on their ability to bind to a receptor isolated from maize coleoptiles. Their root gravitropic properties are correlated with their ability to bind to the receptor. It is suggested that their ability to inhibit auxin transport, without more, may not be enough to explain their physiological effects.

The roles of the different cytokinin structures are discussed in relation to our knowledge of their biological activities, endogenous occurrence and metabolism.

When tissues ofCatharanthus roseus A6 crown gall were incubated on medium supplemented with 50 (μM N6-isopentenyladenine (i⁶Ade), endogenous i⁶Ade, N6-isopentenyladenosine (i⁶A) and i⁶A nucleotide (i⁶AXP) increased to. 6, 5 and 12 nmol g^-1, respectively, during 100 h. Whereas i⁶Ade and i⁶AXP increased rapidly during the initial 4 h and then remained relatively constant, the level of i⁶A continued to increase to 25 nmol g^-1 by 16 h and then decreased; Ribosylzeatin (io⁶A) and its nucleotide (io⁶AXP) remained constant at 1.5 and 1.7 nmol g^-1, respectively. Upon transfer to cytokininless medium, i⁶Ade and i⁶AXP declined rapidly but i⁶A increased to 10 nmol g^-1 after 4 h and then declined. Again, io⁶A and io⁶AXP were unchanged. Prolonged incubation of crown gall tissue on i⁶Ade completely inhibited growth. By contrast, nonrtransformed, autonomous tissue lines fromCalycanthus fertilis andActinidia chinensis Xarguta continued to proliferate on this medium. TheActinidia Une was shown to metabolize i⁶Ade to zeatin and to accumulate this cytokinin to levels in excess of 70 nmol g^-1.

In short-day plantChenopodium rubrum the photoperiodic requirement for flowering increases from one short day at the age of four days to two or three short days at the age of five to seven days. The photoperiodic requirement decreases again to one short day between the 10th and 12th day of cultivation. This feature, together with endogenous circadian rhythmicity of flowering, enabled us to test the effects of PGRs under different morphogenetic patterns. Representative PGR effects on flowering are quoted.

Nitrate consumption, nitrite production and nitrite loss were followed in the"in vitro" nitrate reductase (NR) assay mixtures with pea root extracts, and nitrite production in the "in vitro" NR assay was compared with anaerobic nitrite production in the "in vivo" assay with pea root segments. NR activity could not be reliably determined on the basis of nitrate consumption using three methods of NO₃-determination (salicylate, cadmium, enzymatic). NR activity based on nitrite determination was slightly underestimated due to nitrite loss (less than 10% with pea root extracts) in the assay mixture. Nitrite production in the "in vivo" NR assay under optimum conditions closely resembled nitrite production in the "in vitro" NR assay and thus reflected NR level in the tissue.

Glucose, fructose, sucrose, maltose, and lactose stimulated photoheterotrophic growth ofNostoc linckia (Roth.)Born. as well as its heterocyst frequency, chlorophyll and protein contents, ammoniacal nitrogen uptake and nitrogenase activities. Glucose, fructose and sucrose also supported slow chemoheterotrophic growth. α-ketoglutarate, pyruvate, ribose, succinate, acetate, sorbose and formate were inhibitory.

The content of spinach-leaf cells was compartmented by differential centrifugation. Three fractions were obtained,i.e. chloroplasts, pellet of remaining organelles sedimenting at 97 000g and cytosol. Enzyme activities of L-tryptophan aminotransferase (TAT) as well as L-tryptophan dehydrogenase (TDH) were demonstrated in all cell fractions. The highest activities of both enzymes were found in the pellet of organelles followed by the enzyme activities in the chloroplasts. The cytosol had the lowest enzyme activities. Chloroplasts are characterized by a relatively higher TDH activity, organelles sedimenting at 97 000g were marked by a relatively higher TAT activity. In all fractions both pyridine nucleotide coenzymes catalyzed the TDH activity. Ca²⁺ in a concentration of 0.8 minol l^-1 increased markedly the TDH activity in both directions of its activity.

The mechanisms by which exogenously applied plant growth regulators act to express those genes that are selectively involved in cell and tissue differentiation are not at all well comprehended. However, the ontogenetic sequences of events that enable receptor or target cells to be activated and to undergo dedifferentiation and redifferentiation, can often be followed experimentally and can lead to a better understanding of the causal relations and control mechanisms in coordinated cell growth and development.
Cytokinins, applied either in an agar medium or as high-concentration, short-duration pulses to expiants or as high-concentration, intermittent sprays to intact plants, induce switches in the normal developmental pattern of cells of certain explanted tissues. Examples of cells that are particularly receptive are the subsidiary cells of stomatal complexes and the differentiating cells of very young presumptive leaf primordia, that is before the latter have become irreversibly determined as leaves.

Several examples of transmission of rapid signals within plants are described. Most of these signals may be inhibited by pretreating plants with LiCl, with inhibitors of ionic permeability, or with substances interfering with Ca²⁺. Accordingly, they are dependent on ionic changes. LiCl also prevents or delays the onset of flowering in long-day plants. In addition isolated organs or tissues most often do not react to an external stimulus as do similar organs or tissues within an intact plant. All these facts lead to the conclusion that plants are highly integrated organisms. Any change in the ionic equilibrium occurring in one part of these organisms has rapid consequences in other parts. This hypothesis may be applied to the floral process: after perception of the appropriate environmental conditions, the leaves modify the orientation of meristem differentiation by promoting the redistribution of ions (especially Ca²⁺, which has a "second messenger" function).

Flower bud formation was studied in thin-layer tissue expiants of epidermis plus subepidermal cortex from the inflorescence ramifications ofNicotiana tabacum cv. Samsun. With appropriate hormone concentrations of BA and NAA expiants from flowerv and fruitbearing stalks regenerate flower buds only, while those from the internodes of the inflorescence ramifications produce generative as well as vegetative buds. In both types of expiants the number of buds formed depend mainly on the hormone concentrations but, in addition, the age of stalks and internodes from which expiants are taken also affects bud formation. Both ABA and JA inhibit flower bud formation in expiants of flower stalks. JA was shown to particularly inhibit bud initiation.