Effects of Photoperiodic Induction and Debudding in Xanthium pennsylvanicum and of Partial Defoliation in Phaseolus vulgaris on Rates of Net Photosynthesis and Stomatal Conductances

Xanthium pennsylvanicum plants received four treatments in thefactorial experiment (a) debudded v. not debudded, and (b) longdays . photoinductive short days. Rates of net photosynthesis,carbon dioxide compensation points (), and stomatal conductanceswere assessed after 8 days and before leaf growth or stomatalsize were appreciably affected. Leaf size, stomatal frequencies,and lengths of guard cells were estimated at this time and again22 flays after treatment. Debudding alone slightly Increased stomatal conductance; inductionalone had a similar but larger effect. Debudding and inductiontogether caused a more than additive increase in net photosynthesisat 8 days, with marked decreases in . At 22 days this combinationcaused more than additive increases in leaf size and guard-celllengths while stomatal frequencies had decreased. Induction alone directly increased stomatal conductance andthis may be responsible for the increase in photosynthesis;but debudding alone may directly affect photosynthesis by increasingthe supply of cytokinins to the leaves. The positive interactionof these factors in photosynthesis could not be explained interms of stomatal conductance and a synergism between cytokininsand a photoperiodically induced hormone is suggested. In Phaseolus vulgaris plants, 4 days after partial defoliation,stomatal conductances and rates of net photosynthesis increasedgreatly in the remaining leaflets.     

Effect of Photoperiodic Induction on the Transpiration Rate and Stomatal Behaviour of Debudded Xanthium Plants

In controlled-environment studies with debudded Xanthium plants,appreciable changes in stomatal activity and attendant ratesof transpiration were found to be associated with photoperiodicinduction. Leaves of plants kept on a 10-h inductive photoperiodafter removal of the apical and axillary buds grew at ratescomparable to those of similarly debudded plants kept on a 10-h-interruptednon-inductive photoperiod (consisting of an 8-h photoperiodand a 2-h interruption of the dark period). There was a large effect of leaf age on stomatal behaviour:the minimum stomatal resistance of leaves of non-induced plantsdecreased from about 5 s cm^–1 when the leaf was 25 percent of its final size to 1.6 s cm^–1 when almost fullyexpanded. Thereafter, it slowly increased with time. Superimposedon this age response was a marked effect of photoperiodic induction.The stomata on induced leaves opened more widely than thoseon non-induced leaves, the response being greatest with theleaves which were youngest at the time induction commenced.However, this ‘opening’ tendency was maintainedfor only a few weeks; thereafter, the stomata failed to openas widely. This later ‘closing’ tendency of stomataon leaves of induced plants progressed rapidly and in a basipetalsequence and presaged a necrotic form of leaf senescence whichdeveloped in the same sequence. The closing tendency on leavesof non-induced plants progressed slowly in an acropetal direction;leaves senesced in the same sequence with the familiar yellowingsymptoms. It is suggested that flower induction sets in train a sequenceof events which influence stomatal movement (and other processes)and inevitably leads to the death of the induced axis. Transpiration rates calculated from measurements of the physicalenvironments and stomatal resistances agreed well with thosemeasured.     

The Comparative Effects of Blue and Red Light on the Stomata of Allium cepa L. and Xanthium pennsylvanicum

Stomatal responses to blue and red light were compared in leavesof Xanthium pennsylvanicum (which contain starch in their guardcells) and in onion leaves (which are devoid of starch). Bluelight was found to be more effective than red in opening stomatain both species. However, a significant difference in the ratiosof blue to red light required to produce equal stomatal openingwas found between Xanthium pennsylvanicum and onion. It is concludedthat blue light may promote stomatal opening by its effect onenzymes controlling the starch and soluble polysaccharide contentof guard cells.     

Effect of cytokinin on the distribution of 14C-glyphosate in Xanthium pennsylvanicum

Cytokinin N⁶-(δ²-isopentenyl) adenosine (i⁶A) was applied five times to runoff on the first leaf of Xanthium pennsylvanicum Wallr. After a 24 h application period the second leaf was treated with the herbicide methyl-¹⁴C-glyphosate, [N-(phosphonomethyl)glycine]. The distribution of radioactivity was determined after 12 and 24 h. At no time after treatment did ¹⁴C-glyphosate move preferentially into the i⁶A treated leaf. Radioactivity accumulated in all plant parts, but in i⁶A-treated plants, there were more counts in the actively growing areas after 12 h. The primary effect of i⁶A after 24 h was enhanced uptake of ¹⁴C-glyphosate into the plant. To determine if the increased uptake and altered movement of ¹⁴C-glyphosate was due to a cytokinin-induced transpiration increase, leaf diffusive resistance was measured. Leaf diffusive resistances were unaffected by i⁶A during the 48 h after application. By 12 h after the application of glyphosate, the herbicide was found not to affect the cytokinin content in the Xanthium root tips.     

Effect of Intercalated Long Days and Light Interruption of Dark Period on Flowering, Extension Growth and Senescence of impatiens balsamina

Plants of Impatiens balsamina L. grown under long days were divided into 5 lots to receive 1, 2, 4, 8 and 16 consecutive short day (SD) cycles respectively. Each lot was divided into 5 groups to receive 1, 2, 4, 8 and 16 long day (LD) cycles subsequent to SD regime and the cycles were repeated till the end. Observations on the number, position and time of emergence of floral buds, flowers and extension growth were recorded. The floral buds are initiated and these develop into flowers even when Individual SDs are intercalated with 16 LD cycles, showing that the sub-threshold stimulus is not wiped off but becomes effectively summated through a long non-inductive period. The floral bud initiation in lots receiving less than 4 and flowering in those receiving less than 8 consecutive SD cycles are delayed with decreasing number of consecutive SDs and increasing number of intercalating LDs. This progressive delay is probably due to the delay that is caused by these treatments in the completion of requisite number of SD cycles. The first node to show floral bud initiation is shifted up with increasing intercalated LDs only in plants receiving less than 4 SD cycles and not in those receiving more. Some of the lower floral buds in plants receiving less than 8 consecutive SD cycles either abort or revert to vegetative growth. The first node to flower is, therefore, shifted up. The number of such buds increases either with a decrease in the number of consecutive SDs or an increase in the number of intercalated LDs. The number of floral buds produced in plants receiving 2 or more and flowers in those receiving 4 or more consecutive SD cycles does not differ much with the number of intercalated LDs, but decreases in those receiving less number of SDs. Some nodes bear more than one floral bud and flower. Such nodes are observed in plants receiving individual SD cycles only when intercalated with individual LDs but in all groups in plants receiving 16 consecutive SD cycles. The rate of extension growth increases with an increase in the number of consecutive SDs. The rate in plants receiving individual SDs closely resembles that of plants grown under continuous LDs and that of consecutive 16 SDs with that of control SD plants. The attainment of maximum and the consequent steep fall preceding senescence is successively delayed with an increase in the number of intercalated LDs in plants receiving 16 consecutive SD cycles. Light interruption of the dark period inhibits both the initiation of floral buds and their development Into flowers. showing that in this plant. short days are necessary both for the initiation of floral buds and their development into flowers.     

Studies on Dark Fixation of Carbon Dioxide in Kalanchoe: II. Effect of Interaction of Photoperiodic Induction with Water Stress and Growth Retardants

In Kalanchoë blossfeldiana von Poellnitz cv. Tom Thumband cv. Feuer Blute interaction of CO₂ fixation with photoperiodicinduction and water stress was examined. It was found that TomThumb raised in dilute culture solution and kept in photoinductivecycles (8 h light and 16 h dark) flowered but failed to showa net dark CO₂ fixation. A net dark fixation was observed whenthe concentration of the culture solution was increased or plantswere sprayed with 300 or 750 mg 1^–1 CCC or 300 or 2000mg 1^–1 B-9. In a non-inductive photoperiod no net darkfixation was observed with these treatments although there wasa tendency for dark fixation to increase. Feuer Blute did not flower in inductive photoperiods when keptin half strength solution. It is suggested that in Tom Thumbboth photoperiodic induction and water stress are required forinitiation of net CO₂ dark fixation. In Feuer Blute where CAMis occurring normally photo-induction is sufficient to induceflowering. In half strength solution CO2 dark fixation is disturbedand floral induction also does not occur.     

Effect of Changing the Light/Dark Schedule, the Time of Onset of the Light or Dark Period, or the Daylength, on Rhythms of Epidermal Cell Proliferation

Rhythms of labeling and mitotic indices were studied in the hindlimb epidermis of the anuran tadpole Rana pipiens under different light/dark (LD) cycles and daylengths in order to examine the role of the various parameters of the lighting regimen in setting the periods of the rhythms and the timing of the cell proliferation peaks. Altering the time of, or inverting, the 12 h light period on a 24 h day resulted in phase shifting of basically bimodal circadian rhythms with peaks in the light and dark. Thus the cell proliferation rhythms were entrained to the LD cycle. These rhythms also entrained to noncircadian schedules since they lengthened on a 15L : 15D cycle and shortened on a 9L : 9D cycle, although the bimodal characteristic of a peak in the light and a peak in the dark remained. Studies of 18L: 6D and 6L : 18D cycles in which either the time of onset of light or dark was changed relative to the 12L: 12D control indicated that the onset of dark may regulate the timing of the labeling index peaks while the onset of light may determine the time of occurrence of mitotic index peaks. Control of the timing of labeling and mitotic index peaks by different parameters of the LD cycle suggests a mechanism for cell cycle regulation by the environmental lighting schedule. Analysis of the rhythms on all the cycles studied suggested that labeling index rhythms equal the length of, or twice the length of, the dark period. Mitotic index rhythms equal the daylfength or a multiple of the length of the dark period.     

The Effects of Light Breaks in the Dark Period on Flower-bud Development in Varieties of Phaseolus vulgaris L.

A light break imposed in the middle of, or more effectively,two-thirds of the way through a 13 h dark period inhibited thedevelopment of the first initiated flower buds and reduced theproduction of open flowers in two daylength-sensitive varietiesof Phaseolus vulgaris. The effects were similar to those ofa long photoperiod applied continuously and therefore provideclear evidence of the importance of the dark period in mediatingthe effects of daylength on flower-bud development in the twovarieties. Phaseolus vulgaris, bean flower-bud development, photopcriodism     

Effect of Light Quality on Stomatal Opening in Leaves of Xanthium strumarium L

Flux response curves were determined at 16 wavelengths of light for the conductance for water vapor of the lower epidermis of detached leaves of Xanthium strumarium L. An action spectrum of stomatal opening resulted in which blue light (wavelengths between 430 and 460 nanometers) was nearly ten times more effective than red light (wavelengths between 630 and 680 nanometers) in producing a conductance of 15 centimoles per square meter per second. Stomata responded only slightly to green light. An action spectrum of stomatal responses to red light corresponded to that of CO₂ assimilation; the inhibitors of photosynthetic electron transport, cyanazine (2-chloro-4[1-cyano-1-methylethylamino]-6-ethylamino-s-triazine) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea, eliminated the response to red light. This indicates that light absorption by chlorophyll is the cause of stomatal sensitivity to red light. Determination of flux response curves on leaves in the normal position (upper epidermis facing the light) or in the inverted position (lower epidermis facing the light) led to the conclusion that the photoreceptors for blue as well as for red light are located on or near the surfaces of the leaves; presumably they are in the guard cells themselves.     

Contrasted Effects of CO2 on the Regulation of Dormancy and Germination in Xanthium pennsylvanicum and Setaria faberi Seeds

The effects of CO₂ on dormancy and germination were examinedusing seeds of cocklebur (Xanthium pennsylvanicum Wallr.) andgiant foxtail (Setaria faberi Herrm.). The rate of germinationof the giant foxtail seeds as well as cocklebur was promotedby exogenously applied CO₂ at a concentration of 30 mmol mol^-1regardless of the sowing conditions. However, seeds which failedto germinate in the presence of CO₂, entered a secondary phaseof dormancy under unfavourable germination conditions. If CO₂was applied to seeds under conditions such as water stress imposedwith a 200 mol m^-3 mannitol solution, a hypoxic atmosphere of100 mmol mol^-1 O₂ or a treatment of 0·1 mol m^-3 ABA,development of secondary dormancy was accelerated. These contrastedeffects of CO₂ were observed in ecological studies. Under naturalfield conditions germination of buried giant foxtail seeds respondedpositively to CO₂ during a period of release from primary dormancyfrom Feb. to May, but CO₂ accelerated secondary dormancy commencingin early Jun. In other words, in the presence of CO₂, both theenvironmental conditions and the germination states of the seedsclearly showed secondary dormancy-inducing effects. Thus, itseems that CO₂ has contrasted effects on regulation of dormancyand germination of seeds depending on the germination conditions.Copyright1995, 1999 Academic Press Xanthium pennsylvanicum, cocklebur, Setaria faberi, giant foxtail, CO₂, water stress, hypoxia, ABA, germination, secondary dormancy     

Spectral Dependence of Night-break Effect on Photoperiodic Floral Induction in Lemna paucicostata 441

A single dark period of longer than 8 hr induced flowering inLemna paucicostata 441 cultured in E medium. Monochromatic lightsof 450, 550, 650 and 750 nm with a half-power bandwidth of 9nm given for 10 min at the 8th hour of a 14-hr dark period inhibitedflowering. The fluence rates required for 50% inhibition were10, 0.5, 0.1 and 3 µmol m^–2. sec^–1, respectively.When applied between the 4th and the 10th hour of the dark period,lights of 450, 550 and 650 nm were inhibitory showing a maximumeffect at the 8th hour. But 750-nm light completely inhibitedflowering when applied at any time during the first 8 hr ofthe dark period. The inhibitory effect of 750-nm light givenat the beginning of the dark period was totally reversed bya subsequent exposure to 650-nm light, and the fluence-responsecurves for the effect of 750-nm light given at the 0, 4th and8th hour were essentially the same. This suggests that the presenceof P_FR is crucial for the floral initiation throughout the first8 hr of the inductive dark period. The role of phytochrome inthe photoperiodic flower induction of L. paucicostata is discussed. (Received January 4, 1982; Accepted March 19, 1982)     

Induction of Phenylalanine Ammonia-lyase in Xanthium Leaf Disks. Photosynthetic Requirement and Effect of Daylength

A cycloheximide-sensitive increase in the activity of phenylalanine ammonia-lyase (EC 4.3.1.5) occurs in Xanthium leaf disks exposed to light. Radioactive ammonia-lyase has been isolated by means of sucrose density gradient centrifugation and starch gel electrophoresis from disks fed l-isoleucine-U-(14)C or l-arginine-U-(14)C. The incorporation of radioactive amino acids into phenylalanine ammonia-lyase together with the inhibitory effects of cycloheximide indicate that the observed increase in enzyme activity involves the induction of lyase synthesis.The light-dependent synthesis of the ammonia-lyase is completely inhibited by 50 mum 3-(4-chlorophenyl)-1,1-dimethylurea (CMU) indicating that photosynthesis is involved. Only a trace quantity of some photosynthetic product must be needed because half light saturation occurs at very low intensity (ca. 30 ft-c). Exogenous carbohydrate is also required for continuing enzyme synthesis over a 72 hr period. But carbohydrate does not replace the photosynthetic requirement in darkness.Enzyme formed in light disappears rapidly from disks placed in the dark. The decay of ammonia-lyase activity follows first order kinetics. The half-life of the lyase ranged from 6 to 15 hr in leaf material used. Cyoloheximide inhibits the decay of lyase activity. Thus the maintenance of turnover in Xanthium leaf disks requires de novo synthesis of protein. That turnover, i.e., degradation as well as synthesis of lyase protein occurs is suggested by the apparent loss of radioactive ammonia-lyase from leaf disks placed in darkness. Light-induced synthesis coupled with rapid turnover can produce a diurnal fluctuation of ammonia-lyase activity in Xanthium leaf disks. Alternating periods of enzyme synthesis and degradation were observed in disks exposed to 24 hr cycles of light and dark. The average level of enzyme activity maintained in the tissue was directly related to the length of the light period. Induction of lyase synthesis was also observed in excised leaves and to a lesser extent in leaves of whole plants.     

Effects of Moonlight on Flower Induction in Pharbitis nil, Using a Single Dark Period

Flowering of Pharbitis nil plants was slightly inhibited byexposure to the light of the full moon for 8 or more hours witha single dark period of 16, 14 or 13 h. It is suggested thatin the natural environment moonlight may have at most only aslight delaying effect on the time of flower induction in short-dayplants. Flowering, moonlight, night-break, Pharbitis nil, photoperiodism, short day plants     

Studies on the Role of Photosynthesis in the Photoperiodic Induction of Flowering in the Short-Day Plants Kalanchoe blossfeldiana Poellniz and Xanthium pensylvanicum Wallr: II. THE EFFECT OF CHEMICAL INHIBITORS OF PHOTOSYNTHESIS

The role of photosynthesis in flower induction in the short-dayplants Kalanchoe blossfeldiana and Xanthium pensylvanicum wasinvestigated by chemical suppression of photosynthesis and preventionof chlorophyll formation in the induced leaf. ‘Bleaching’leaves with streptomycin completely prevented flowering in X.pensylvanicum at concentrations shown to reduce the chlorophylland carotenoid content of the leaf significantly. Such leaveswere unable to induce flowering even when supplied with sugarsand other photosynthetic products. Photosystem II inhibitors,DCMU and cadmium ion, inhibited induction in both species aswell as suppressing photosynthesis (as tested by O₂ evolutionand starch production) whereas the photosystem I inhibitor,metronidazole, had no effect. Antimycin A inhibited floweringin K. blossfeldiana and may have a similar site of action toDCMU. Neither ammonium ion nor DBMIB, which acts upon plastoquinone(i.e. between PS I and PS II in the ‘Z scheme’),had any effect on floral induction and it is argued that theinductive process is independent of photosynthetic phosphorylationbut a step in the electron transport pathway between the sitesof action of DCMU and DBMIB may be crucial. DSPD and its hydrolysisproduct, salicylaldehyde, suppressed flowering in K. blossfeldianabut the uncertainty regarding their chemistry precludes anyfirm conclusions regarding the nature of their action.