The percentage of sucrose in sugar beet storage root fresh and dry matter is closely related to root structure. It has been suggested that the sucrose content might be increased by using plant growth regulators to modify storage root structure through control of cambial development, cell division and cell expansion. During storage root development correlations were found between the changing phytohormone profiles and the formation of secondary cambia and their subsequent cell division and expansion. Sugar beet root derived cell suspension cultures were used for detailed studies of the roles of endogenous phytohormones. The gibberellin synthesis inhibitor paclobutrazol was tested in cell cultures and whole plants. The observations provide a basis for development of plant growth regulator regimes to optimise sucrose yield from sugar beet.

The senescence of detached leaves of the Australian seagrassZostera muelleri Irmisch ex Aschers was associated with decreased levels of chlorophyll, soluble protein, and alkaline pyrophosphatase and catalase activities. Senescence was faster in artificial seawater than in distilled water. Irradiance has a retarding effect on senescence while cycloheximide did not retard senescence appreciably. The action of cycloheximide was studied usingCyperus rotundus L.,Hydrangea macrophylla (Thunb.) Ser.,Solandra maxima (Sesśe & Moc.) P. S. Green, andDatura suaveolens Humb. & Bonpl., and the cycloheximide sensitivity correlated with low chlorophyllase activity of the leaf.

A comparative study of dark respiration and effects of high and low temperature on dark respiration in five greenhouse cultivated tropical and sub-tropical species was made. The respiration rates determined manometrically were low in all species. Respiration rates increased with increasing temperature. Low temperature treatment accelerated the rate of respiration at 25 °G except in two species. An attempt has been made to determine the critical and optimum temperature for these species. The respiratory behaviour of these species has been discussed with respect to their original habitats and prevailing environmental conditions.

With the aim to reduce the period of flowering and of fruit maturation, we investigated the effect of auxins on flower formation. For these experiments we used young decapitated plants with two plagiotropic branches. Both the auxins, indol-3-ylacetic acid (IAA) and 2,4-dichlorophenoxyacetic acid (2,4-D), retarded flower formation in coffee, the latter one being more effective. The effects of 2,4-D if applied on only one of the two plagiotropic branches can be observed only in this treated one. Furthermore, the auxins seem to act in coffee plant directly by affecting flower formation and not indirectly by inducing endogenous ethylene production.

Abscisic acid was analysed in seeds of oak, at. five stages of development in threeweek intervals, starting on the 6th week after anthesis until maturation in the 18th week after anthesis. The physico-chemical methods and bioassays were used for a closer characterization and quantification of the examined compounds. Differentiation of the ABA level depending upon the stages of seed development has been observed in various parts of acorn.

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.

6-benzylaminopurine (BAP), β-indolylbutyric acid (IBA), phenylacetic acid (PAA) and a triazine herbicide (TH) affected the development of vascular bundles both in coleoptiles and the last intemodium below the ear of wheat plants. These substances were applied before the conductive tissues in the organs investigated were fully differentiated, the degree of their effect being dependent not only on the concentration, but also on the developmental stage of the plants and on cultivation conditions. In coleoptiles the area of vascular bundles was increased especially in the dark. In the stalk, BAP, PAA, IBA and IBA + TH increased the area of phloem. BAP also stimulated the activity of parenchymatous cells of xylem. TH had the same effect only on xylem, which became evident by an increase in vascular bundle area, but not in that of phloem. These anatomical aberrations were confirmed by cytological analysis which revealed that the stimulatory effect of the substances investigated consisted in activation both of companion cells of phloem and parenchymatous cells of xylem, whose function is analogous. As a result, both types of cells contained numerous organelles characteristic of meristematic cells, which suggests protein synthesis and an active transport of substances.

The radiotracer technique is described for transport and metabolic studies in latex producing bark. This technique provided further evidence that carbohydrates are supplied to latex vessels as sucrose and that sucrose is readily used for rubber synthesis. Bark treatment with ethylene resulting in latex flow stimulation is shown to activate within a few hours sucrose and water transport into the latex vessels. The available knowledge suggests an implication of an energy-dependent process of proton excretion which could explain the earlier observed ethylene induced alkalization of latex cytosol enhancing invertase and sucrose utilization.
An increase of tapping intensity decreases the level of latex sucrose and changes its distribution pattern in drained bark. There is a clonal difference in the early response to exploitation. Bark excision by conventional tapping inhibits downward sucrose translocation, the inhibition being proportional to the surface of consumed bark above the tapping cut. Seasonal variation in the level of latex sucrose is related to variations in sunshine duration and sucrose concentration rises during the process of refoliation. The results stress the importance of sucrose supply to latex vessels for latex producing potentialities and indicate an association of low sucrose availability in latex vessels with premature latex vessel degeneration and bark dryness.