Mesocotyl growth of etiolated seedlings ofAvena sativa andZea mays in relation to light and diffusible auxin

Diffusible auxin levels were measured in coleoptiles and mesocotyls of dark-grown seedlings ofavena sativa (cv. Spear) andZea mays (cv. Golden Cross Bantam) using theAvena curvature bioassay. The coleoptile tip was confirmed as the major auxin source in etiolated seedlings. Auxin levels were found to decrease basipetally in sequent sections of theAvena coleoptile but not to decrease in apical sections of increasing length. An inhibitor capable of inducing positive curvatures ofAvena test coleoptiles was discovered in diffusates from the mesocotyls of oat and corn seedlings. The amount of this inhibitor was correlated with the cessation of mesocotyl growth of oat seedlings grown in darkness, and with the inhibition of mesocotyl growth of corn seedlings exposed to red light.     

A Technique for taking Successive Shadowgraphs of the Same Seedlings in Growth and Curvature Experiments with Oat Coleoptiles

A method is described for taking successive shadowgraphs ofthe same Avena seedlings, using an orange-sensitive paper andvery low-intensity orange light such that whereas shadowgraphscan be made with one minute's exposure no photo-tropic or growthresponse can be detected as a result of one hour's unilateralexposure of intact seedlings. Phototropic curvatures producedby somewhat higher intensities of red light emphasize the importanceof specifying not only the wave-length but also the intensityand duration of illumination used in experiments with Avenaseedlings. When not subjected to the unilateral illuminationthe seedlings were grown under deep red light (wave-length >0.63 µ) at a total intensity of less than 1/6 foot-candle;it must not be assumed that the technique would be satisfactoryfor seedlings grown in total darkness.     

Oak Seedlings Grown in Different Light Qualities

Seedlings of oak (Quercus robur) were germinated in darkness for 3 weeks and then given continuous light or short pulses of light (5–8 min every day). The morphological development was followed during 25 days. In continuous white, blue, and red light the stem growth terminated after about 10 days by formation of a resting bud. At that time the seedlings were about 100 mm high. In con tinuous long wavelength farred light (wavelength longer than 700 nm) the stem growth including leaf formation was continuous without the formation of resting buds, and the stem length was about 270 mm after 25 days. The number of nodes developed became twice that of the seedlings grown in while light. The leaves became well developed in all light colours, but leaf areas were largest in plants cultivated in white light. Compared to dark grown seedlings the mean area per leaf was increased about five times in continuous long wavelength far red light. A supplement with short (5 min) pulses of red light each day increased the leaf area up to 20 times. The stem elongation showed a high energy reaction response, i.e. the stem length increased only in continuous long wavelength far-red light but was not influenced by short pulses of red light or far-red light. The leaf expansion, however, was increased by short pulses of red light with a partial reversion of the effect by a subsequent pulse of far-red light. The fraction of the plant covered with periderm was higher in plants given continuous light. In respect to periderm inhibition continuous long wavelength far red light was the most effective. The transfer of seedlings from darkness to continuous white light gave anthocyanin formation in the stem 10–20 mm below the apex. This formation took place in the cortex and was evident in plants grown in darkness or under short pulses of light. Plants grown in continuous red, blue or long wavelength Far red light showed only traces of anthocyanin.     

Phytochrome mediated regulation of sucrose phosphate synthase activity in maize

The extractable activity of sucrose phosphate synthase was determined in etiolated seedlings of maize (Zea mays L.), soybean (Glycine max [L.] Merr.), and sugar beet (Beta vulgaris L.) following treatments of changing light quality. A 30-minute illumination of 30 microeinsteins per square meter per second white light produced a three-fold increase in sucrose phosphate synthase activity at 2 hours postillumination when compared to seedlings maintained in total darkness. Etiolated maize seedlings treated with 3.6 microeinsteins per square meter per second of red and far-red light showed a 50% increase and a 50% decrease in sucrose phosphate synthase activity, respectively, when compared to etiolated maize seedlings treated with white light. Maize seedlings exposed for 30 minutes to red followed by 30 minutes to far-red showed an initial increase in sucrose phosphate synthase activity followed by a rapid decrease to control level. Neither soybean or sugar beet sucrose phosphate synthase responded to the 30-minute illumination of white light. Phytochrome is involved in sucrose phosphate synthase regulation in maize, whereas it is not responsible for changes in sucrose phosphate synthase activity in soybean or sugar beet.     

Evidence for a phytochrome-mediated phototropism in etiolated pea seedlings

Entirely etiolated pea seedlings (Pisum sativum, L. cv Alaska) were tested for a phototropic response to short pulses of unilateral blue light. They responded with small curvatures resembling in fluence-dependence and kinetics of development a phytochrome-mediated phototropic response previously described in maize mesocotyls. Irradiations from above with saturating red or far-red light, either immediately before or after the unilateral phototropic stimulus, strongly reduced or eliminated subsequent positive phototropic curvature. Only blue light from above, however, entirely eliminated curvature at all fluences of stimulus. It is concluded that the phototropism is primarily a result of phytochrome action.     

Mesocotyl growth of etiolated seedlings ofAvena sativa andZea mays in relation to light and diffusible auxin

Diffusible auxin levels were measured in coleoptiles and mesocotyls of dark-grown seedlings ofavena sativa (cv. Spear) andZea mays (cv. Golden Cross Bantam) using theAvena curvature bioassay. The coleoptile tip was confirmed as the major auxin source in etiolated seedlings. Auxin levels were found to decrease basipetally in sequent sections of theAvena coleoptile but not to decrease in apical sections of increasing length. An inhibitor capable of inducing positive curvatures ofAvena test coleoptiles was discovered in diffusates from the mesocotyls of oat and corn seedlings. The amount of this inhibitor was correlated with the cessation of mesocotyl growth of oat seedlings grown in darkness, and with the inhibition of mesocotyl growth of corn seedlings exposed to red light.     

Effect of red light on the phototropic sensitivity of corn coleoptiles

The effect of red light in alteration of the phototropic sensitivity of corn coleoptiles (Zea mays L., cultivar Burpee Barbecue Hybrid) is investigated. Phototropic dosage-response curves for etiolated coleoptiles are compared with those for coleoptiles receiving 1 hour of continuous red light immediately prior to phototropic induction. In the former case, only curvature comparable to the first positive curvature of oat coleoptiles is obtained. There is no evidence for first negative curvature and only minimal second positive curvature. The reciprocity law proved valid for all curvatures obtained. With red light, the sensitivity of the first positive curvature was decreased over ten-fold and there was clear appearance of second positive curvature for which the reciprocity law was not valid. Once again there was no evidence for negative curvature. Time course studies indicated that within 1 hour of the beginning of red light treatment at 25°, reactions leading to the decrease in phototropic sensitivity of the first positive component had gone to completion whether the red light was continuous or consisted of a single 1 second exposure followed by a 1 hour dark period. An action spectrum for the red-induced change in phototropic sensitivity showed a marked peak near 660 mμ with a small broad shoulder between 610 and 630 mμ, characteristic of phytochrome-mediated responses. The effect of red light could be fully reversed by low dosages of far-red light, but longer doses of far red were less effective. Large dosages of far-red light alone induced the same alteration in phototropic sensitivity as did red light.     

An Endogenous 24-hour Rhythm in the Growth Rate of the Avena Coleoptile

The rates of growth of coleoptiles of intact Avena seedlingswere studied by means of time-lapse photography, using infra-redradiation. When the seedlings are germinated in red light and subsequentlytransferred to darkness, a growth rhythm is established in whichthe first peak in the growth-rate curve occurs about 16–17hours after the transfer, and the second peak 24 hours later.When the transfer is made sufficiently early, three peaks mayoccur before growth ceases. The occurrence of the peaks andthe emergence of the primary leaf are independent of one another. Alteration of the point in the life-history at which the seedlingsare transferred from light to darkness changes the times ofoccurrence of the peaks, but does not affect the period of therhythm. The incidence of the rhythm shows no correlation withtime of day; therefore the rhythm is not due to diurnal changesin external conditions. Interruption of the dark period by several hours' exposure tored light causes the suppression of a previously induced rhythmand the establishment of a new one which commences at the timethe seedlings are restored to darkness. When they are grownunder continuous red light no rhythm is induced. Within the range 16 to 28 C., temperature has little or noeffect on the period of the rhythm. When seedlings of Triticum are grown under the same conditionsas those which induce a rhythm in Avena, no rhythmical variationin the growth rate can be detected.     

The Geotropic Curvature in Roots of Norway Spruce (Picea abies) Containing Anthocyanins

Seedlings of Norway spruce (Picea abies L.) have been found to synthesize anthocyanins in the root tips as well as in the hypocotyls upon irradiation with white light when kept at 4°C for 6–8 days. In addition, it has also been found that the elongation and the geotropic curvature of spruce roots are dependent on the light conditions. The course of the geotropic curvature in spruce roots containing anthocyanins has been followed during a period of 5 h, in which the seedlings were geotropically stimulated continuously in the horizontal position. When the stimulation was performed in white light and in darkness at 21°C, significantly larger curvatures were observed in the roots pretreated at 4°C in darkness than in the roots containing anthocyanins. The specific curvature (curvature in degrees per mm elongation), however, was approximately the same in both types of roots stimulated in white light. This was due to a retarded elongation of the roots pretreated with light at 4°C and containing anthocyanins. A smaller difference in elongation rate between roots with and without anthocyanins was observed in the dark than in the light, but even in the dark the anthocyanin-containing roots grew more slowly than roots without anthocyanins. In order to find out if it is the anthocyanin content or the illumination which affects the elongation and geotropic curvature in the roots, a series of similar experiments was performed using cress seedlings grown at 4°C in light or darkness. Roots of cress seedlings cultivated under conditions which would induce anthocyanin formation in spruce roots exhibited the highest geotropic responses both in light and darkness as compared to cress seedlings grown at 4°C in darkness. As in the case of spruce roots an increase in elongation was observed in cress roots illuminated during the geotropic stimulation. These similarities in the behaviour made it relevant to compare the development of the geotropic curvature in cress and spruce roots.     

Impact of weak permanent magnetic field on antioxidant enzyme activities in radish seedlings

The impact of weak permanent magnetic field (PMF) with magnetic flux density of 185–650 μT on activities of antioxidant enzymes in 5-day-old radish (Raphanus sativus L. var. radicula D.C.) seedlings, cv, Rosovo-krasnyi s belym konchikom was demonstrated. In the range of 185–325 μT PMF suppressed superoxide dismutase (SOD) activity in seedling grown in darkness and catalase (CAT) grown in both darkness and light. At the same values of magnetic flux density, all fractions of guaiacol peroxidase were activated and MDA accumulation was enhanced. At the higher values of magnetic flux density, SOD was activated in seedlings grown in darkness and CAT was activated in seedlings grown in both darkness and light. Other indices decreased or remained unchanged. It was concluded that PMF action depends on its intensity: at its low values, antioxidant enzymes are inhibited and at high values — activated.     

Response of seedlings of a dwarf and a normal strain of watermelon to gibberellins

Hypocotyl and root elongation in a dwarf and a normal strain of watermelon (Citrullus lanatus [Thunb.] Matsu.) in the absence or presence of different gibberellins was investigated in seedlings grown under gold fluorescent light or in darkness. The normal strain, “Sugar Baby,” responded only slightly to the gibberellic acids employed. At appropriate concentrations all of the gibberellic acids were capable of normalizing growth in the monorecessive dwarf strain, WB-2, in darkness or in light. Gibberellins A₄₊₇ and A₇ were effective in stimulating hypocotyl elongation at concentrations 10 to 15 times lower than that needed for a response to GA₁ or GA₃. Dark-grown dwarfs responded to about a 3-fold lower concentration of GA₄₊₇ than those grown in light.     

Photoperiod and light quality effects on rate of dark respiration and on photosynthetic capacity in developing seedlings of Chenopodium rubrum

Development and acclimation of energy transduction were studied in seedlings of Chenopodium rubrum L. ecotype selection 184 (50° 10' N; 105° 35' W) in response to photomorphogenic and photoperiodic treatments. Dark respiration and photosynthetic capacity [nmol O₂ (pair of cotyledons)⁻¹ h⁻¹] were measured with an oxygen electrode. Changes in chloroplast ultrastructure were analyzed concomitantly. After germination, seedlings were grown at constant temperature either in darkness or in continuous light (white, red, far-red and blue) or were subjected to diurnal cycles of light/dark or changes in light quality. Dark respiration was low in far-red light treated seedlings. In red light treated seedlings dark respiration was high and the mean value did not depend on fluence rate or photoperiod. Blue light stimulated transitorily and modulated dark respiration in photoperiodic cycles. Photosynthetic capacity was reduced by far-red light and increased by red light. In response to blue light photosynthetic capacity increased, with indications of a requirement for continuous energy input. Phytochrome and a separate blue light receptor seemed to be involved. In continuous red light a clear cut circadian rhythm of dark respiration was observed. Blue light had a specific effect on chloroplast structure.     

Cytokinin, acting through ethylene, restores gravitropism to Arabidopsis seedlings grown under red light.

Cytokinin replaces light in several aspects of the photomorphogenesis of dicot seedlings. Arabidopsis thaliana seedlings grown under red light have been shown to become disoriented, losing the negative hypocotyl gravitropism that has been observed in seedlings grown in darkness or white light. We report here that cytokinin at micromolar concentrations restores gravitropism to seedlings grown under red light. Cytokinin cancels the effect of red light on the gravity-sensing system and at the same time replaces light in the inhibition of hypocotyl elongation. Furthermore, application of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid acts similarly to cytokinin. Cytokinin cannot restore gravitropism under red light to an ethylene-insensitive mutant that is defective at the EIN2 locus. Stimulation of ethylene production, therefore, can explain the action of cytokinin in restoring negative gravitropism to the hypocotyls of Arabidopsis seedlings grown under continuous red light.     

Analysis of phytochrome kinetics in light-grown Avena sativa L. seedlings

The phytochrome content, the rate of phytochrome accumulation after a light/dark transition and the rate of phytochrome destruction after a 1.5 d reaccumulation period in darkness were measured in light grown Avena sativa L. seedlings. The results using spectrophotometrical methods (Norflurazon treated seedlings) and the radio-immunoassay (RIA) (green seedlings) were almost identical. The rate of phytochrome synthesis was analysed by measuring the activity of poly(A⁺)-RNA coding for the phytochrome apoprotein. It was demonstrated that the rate of phytochrome synthesis is different in light and in dark. These results were confirmed by measuring the incorporation of radioactive label in vivo. Five minutes red (and 5 min far-red) light strongly reduces the rate of phytochrome synthesis. Even after prolonged dark periods only 50% of the initial rate of phytochrome synthesis is recovered for light and dark grown seedlings which received one red light pulse.     

GROWTH PATTERNS OF PEA SEEDLINGS IN DARKNESS AND IN RED AND WHITE LIGHT

Thomson , Betty F., and Pauline Monz Miller . (Connecticut Coll., New London.) Growth patterns of pea seedlings in darkness and in red and white light. Amer. Jour. Bot. 48(3): 256–261. Illus. 1961.—Seedlings of peas were grown in vermiculite at 22°C. and exposed 16 hr. daily to red or white light or kept in darkness. Others were grown in soil in the greenhouse. Samples harvested daily to 16 days were dissected, the length of each internode and leaf measured and the total number of leaves and leaf primordia counted. The form of the stem apex and youngest primordia and interrodes is the same in light as in darkness. Leaf production is accelerated very slightly and the growth of leaves and internodes is decidedly accelerated by light. Leaf-leaf, leaf-internode and internode-internode correlations indicate that the morphogenetic effect of light is limited to later stages of organ growth. Dry weight is consumed more rapidly in light than in darkness, probably because of more rapid growth and slightly greater amounts of respiring tissue in light-grown plants.     

Control of Flowering of Xanthium pensylvanicum by Red and Far-red Light

A study was made of the effects of various durations, intensities and combinations of red and far-red light interruptions on the flowering responses of Xanthium pensylvanicum Wallr. A dual response to treatments of far-red light was observed. In short dark periods, far-red light alone did not greatly affect flowering but was able to overcome the inhibition of flowering caused by red light. In dark periods longer than 15 hours, far-red inhibited flowering and added to rather than overcame the inhibition by red light. The dark period length required for far-red inhibition remained the same whether far-red was given at the start or at the eighth hour of darkness.

In 48-hour dark periods Xanthium showed 3 responses to additions of red and far-red light breaks: A) response to red light; B) response to far-red light; and C) response to red followed by far-red light. Red light given any time in the first 30 hours of darkness overcame the inhibitory effect of far-red light given at either the start or the eighth hour of darkness. Red light given later than the thirtieth hour did not overcome the far-red effect.

Approximately the same energy of red light was required to overcome the inhibitory effect of far-red at the second hour of darkness as was required to produce maximum red light inhibition at the eighth hour. Although far-red light was most inhibitory when given early in a long dark period, approximately the same energy of far-red light was required to saturate the far-red response at the fourth, eighth and sixteenth hours.

The results are discussed in relation to other reports of far-red inhibition of flowering in short-day plants.

     

Photophosphorylation and Ultrastructural Development in Pinus nigra Chloroplasts, Grown under Different Spectral Composition of Light

Developed pine seedlings synthesize chlorophyll in darkness. Their photosystem II reducing capacity is very low. The development of chloroplast structure and of photophosphorylation ability has been studied in plastids isolated from Pinus nigra (var. austriaca) developed for 14 days under different spectral compositions of light as compared with chloroplasts isolated from seedlings grown under white light or in darkness. Chloroplast structure was studied by electron microscopy. Cyclic and non-cyclic photophosphorylation were studied under white light. The spectral bands which are efficient for the development of granal structures are different from those needed to make photosystem II functional: red light makes photosystem II functional but does not permit the formation of granal structure, and under yellow light the granal structure develops but photosystem II is not fully functional. Orange light alone fulfils both these purposes. The spectral band around 650 nm seems effective in making the photosystem II functional.     

Boron-induced Bioelectric Field Change in Mung Bean Hypocotyl

Low concentrations of boron were found to affect the bioelectric field potentials of hypocotyls excised from 4-day-old mung bean (Phaseolus aureus cv. Oklahoma 612) seedlings grown in darkness and irradiated with red light. The significance of this effect is discussed in relation to the possible role of boron in some membrane function.     

Phytochrome A enhances the promotion of hypocotyl growth caused by reductions in levels of phytochrome B in its far-red-light-absorbing form in light-grown Arabidopsis thaliana.

We sought to determine if phytochrome B (phyB)-mediated responses to the red light (R)/far-red light (FR) ratio are affected by phytochrome A (phyA) activity in light-grown seedlings of Arabidopsis thaliana. Pulses of FR delayed into the dark period were less effective than end-of-day (EOD) FR in promoting hypocotyl growth over a given period in darkness. White light minus blue light interposed instead of darkness between the end of the white-light photoperiod and the FR pulse was sufficient to maintain responsivity to the decrease in phyB in FR-light-absorbing form in wild-type (WT) seedlings, but not in the phyA mutant. Compared with EOD R, hourly R+FR pulses provided throughout the night caused a stronger promotion of stem growth than a single EOD R+FR pulse in WT Arabidopsis, cucumber, mustard, sunflower, tobacco, and tomato, but not in phyA Arabidopsis or in the aurea mutant of tomato. WT seedlings of Arabidopsis responded to a range of high EOD R/FR ratios, whereas the phyA mutant required stronger reductions in the EOD R/FR ratio. In sunlight, phyA seedlings of Arabidopsis showed no response to the "early warning" signals of neighboring vegetation, and hypocotyl-growth promotion occurred at higher plant densities than in the WT. Thus, under a series of light conditions, the sensitivity or responsivity to reductions in the R/FR ratio were larger in WT than in phyA seedlings. A product of phyA is therefore proposed to enhance the hypocotyl-growth response to decreases in phyB in FR-light-absorbing form in light grown seedlings.