Regulation of Transpiration in Avena. Responses to Red and Blue Light Steps

The transpiration responses of primary Arena leaves to red and blue light steps were investigated. The response to a red light step was a so-called slow response (with a rise time of about 8 min). The response to a blue light step consisted of both a slow, and a rapid response (with a rise time of about 2 min). CO₂-free air outside the leaf eliminated only the slow response, i.e. in CO₂-free ait the plant responded to blue light steps but not to red ones. A short exposure of red light prior to a blue light step enhanced the rapid response. The same enhancement of the rapid response could be achieved by means of a temporary pretreatment with CO₂-free air. The magnitude of the rapid response was dependent on the blue light irradiance and no threshold effects could be detected. — The experimental results are discussed by means of a model, based on stomatal regulation by, ion transport between the subsidiary cells and the guard cells. It is suggested that the slow transpiration response is due to CO₂-regulation of the stomatal aperture and that the rapid response reflect a CO₂-independent blue light sensitive process, which acts directly on the ion transport through the subsidiary and guard cell membranes.     

Oscillatory Transpiration and Water Uptake of Avena Plants

The transpiration rate of oat plants, 6 days old, has been investigated. Dependent on the irradiance level of the white light used in the experiments, the transpiration rate oscillated with different period times. In darkness or at low irradiances the period was about 100–110 min. At higher irradiances the period was about 40 min. At intermediate irradiances autocorrelation analysis was used to find the period content of the transpiration rate. It was concluded that two oscillatory systems were present in the plants, characterized by their different periods. When plants cultivated in a light/dark cycle were used, the transpiration oscillations were influenced by a circadian rhythm. Oscillations in darkness were then most pronounced in the mornings. Plants cultivated in continuous light did not show such a circadian rhythm, but the oscillations died out after about 20 h. Kinetin induced transpiration oscillations in darkness and made them sustain for a longer time.     

Circadian rhythms in Kalanchoë: effects of irradiance and temperature on gas exchange and carbon metabolism

Gas exchange in K. blossfeldiana shows a circadian rhythm in net CO₂ uptake and transpiration when measured under low and medium irradiances. The period length varies between 21.4 h at 60 W m^-2 and 24.0 h at 10 W m^-2. In bright light (80 W m^-2) or darkness there are no rhythms. High leaf temperatures result in a fast dampening of the CO₂-uptake rhythm at moderate irradiances, but low leaf temperatures can not overcome the dampening in bright light. The rhythm in CO₂ uptake is accompanied by a less pronounced and more rapidly damped rhythm in transpiration and by oscillations in malate levels with the amplitude being highly reduced. The oscillations in starch content, usually observed to oscillate inversely to the acidification in light-dark cycles, disappear after the first cycle in continuous light. The balance between starch and malate levels depends in continuous light on the irradiance applied. Leaves show high malate and low starch content at low irradiance and high starch and low malate in bright light. During the first 12 h in continuous light replacing the usual dark period, malate synthesis decreases with the increasing irradiance. Up to 50 W m^-2 starch content decreases; at higher irradiances it increases above the values usually measured at the end of the light period of the 12:12 h light-dark cycle.Abbreviations CAM Crassulacean acid metabolism - FW fresh weight - PEP phosphoenolpyruvate     

Rapid, Blue-Light-Induced Transpiration Response Restricted to Plants with Grass-like Stomata

It is shown that the response of transpiration to light is different in species from Gramineae and Cyperaceae compared to species from other families. Blue light (380–500 nm) causes both a rapid response (rise time 2 min) and a slow response (rise time 5 to 10 min) in species from these two families but induces only a slow response in other families. Red light results only in a slow response in all plants investigated. It is hypothesized that the rapid response to blue light is due to the special anatomy and/or function of the so-called grass-like (graminaceous) stomata of plants belonging to Gramineae or Cyperaceae.     

Circadian Rhythms in Stomatal Responsiveness to Red and Blue Light

Stomata of many plants have circadian rhythms in responsiveness to environmental cues as well as circadian rhythms in aperture. Stomatal responses to red light and blue light are mediated by photosynthetic photoreceptors; responses to blue light are additionally controlled by a specific blue-light photoreceptor. This paper describes circadian rhythmic aspects of stomatal responsiveness to red and blue light in Vicia faba. Plants were exposed to a repeated light:dark regime of 1.5:2.5 h for a total of 48 h, and because the plants could not entrain to this short light:dark cycle, circadian rhythms were able to "free run" as if in continuous light. The rhythm in the stomatal conductance established during the 1.5-h light periods was caused both by a rhythm in sensitivity to light and by a rhythm in the stomatal conductance established during the preceding 2.5-h dark periods. Both rhythms peaked during the middle of the subjective day. Although the stomatal response to blue light is greater than the response to red light at all times of day, there was no discernible difference in period, phase, or amplitude of the rhythm in sensitivity to the two light qualities. We observed no circadian rhythmicity in net carbon assimilation with the 1.5:2.5 h light regime for either red or blue light. In continuous white light, small rhythmic changes in photosynthetic assimilation were observed, but at relatively high light levels, and these appeared to be attributable largely to changes in internal CO2 availability governed by stomatal conductance.     

Phase resetting of the circadian rhythm of carbon dioxide assimilation inBryophyllum leaves in relation to their malate content following brief exposure to high and low temperatures,darkness and 5% carbon dioxide

Leaves ofBryophyllum fedtschenkoi show a persistent circadian rhythm in CO₂ assimilation when kept in continuous illumination and normal air at 15°C. The induction of phase shifts in this rhythm by exposing the leaves for four hours at different times in the circadian cycle to 40° C, 2° C, darkness and 5% CO₂ have been investigated. Exposure to high temperature has no effect on the phase at the apex of the peak but is effective at all other times in the cycle, whereas exposure to low temperature, darkness or 5% CO₂ is without effect between the peaks and induces a phase shift at all other times. The next peak of the rhythm occurs 17 h after a 40° C treatment and 7–10 h after a 2° C, dark or 5% CO₂ treatment regardless of their position in the cycle. When these treatments are given at times in the cycle when they induce maximum phase shifts, they cause no change in the gross malate status of the leaf. The gross malate content of the leaf in continuous light and normal air at 15% shows a heavily damped circadian oscillation which virtually disappears by the time of the third cycle, but the CO₂ assimilation rhythm persists for many days. The generation of the rhythm, and the control of its phase by environmental factors are discussed in terms of mechanisms that involve the synthesis and metabolism of malate in specific localised pools in the cytoplasm of the leaf cells.     

Influence of carbon supply on the circadian rhythmicity of photosynthesis and its stimulation by blue light in Ectocarpus siliculosus: clues to the mechanism of inorganic carbon acquisition in lower brown algae

Stimulation or light-saturated rates of photosynthesis in Ectocarpus siliculosus (Dillwyn) Lyngb. by blue light was eliminated by increasing dissolved inorganic carbon (DIC) or by lowering pH in natural seawater. The amplitude of the circadian rhythm of photosynthesis was also diminished under these conditions, and the pH compensation points in a closed system were higher in the presence of blue light and during the circadian day. These observations suggest that blue light and the circadian clock regulate the activity of a carbon acquisition system in these plants. The inhibitor of external carbonic anhydrase, acetazolamide, reduced overall rates of photosynthesis by only about 30%, but ethoxyzolamide suppressed the circadian rhythm of photosynthesis almost completely and markedly reduced the duration of responses to blue light pulses. Similar patterns were obtained when photosynthesis was measured in strongly limiting DIC concentrations (0–0.5 mol m^?3). Since blue light stimulated photosynthesis under these conditions of strong carbon limitation, we suggest that blue light activates the release of CO₂ from an internal CO₂ store. We propose a metabolic pathway with similarities to that of CAM plants. Non-photosynthetic fixation leads to the accumulation of a storage metabolite. The circadian clock and blue light control the mobilization of CO₂ at the site of decarboxylation of this metabolite. In the presence of continuous blue light the pathway is proposed to cycle and act as a pump for CO₂ into the chloroplasts. This hypothesis helps to explain a number of previously reported peculiarities of brown algal photosynthesis.     

Photosynthetic induction times in shade-tolerant species with long and short-lived leaves

In the understory of a tropical rainforest, light flecks can contribute 10–80% of the total light flux. We investigated the capacity of eight shade-tolerant species to use light flecks by examining the time required for full induction of photosynthesis during an artificial light fleck. CO₂ fixation rates were measured with a portable LiCor gas-exchange system for plants growing in the field on Barro Colorado Island, Panama. In all species induction to 50% of maximum CO₂ fixation occurred quickly, from 1 to 3 min. In species with short leaf lifetimes (1 year), induction to 90% of maximum also occurred quickly, in 3–6 min. In contrast, the species with longer lived leaves (>4 years) required 11–36 min for induction to 90% of maximum. Induction times for leaves from gap and understory plants of the same species were indistinguishable. Elevated CO₂ did not eliminate the slow induction phase of long-lived leaves. This suggests that slow induction did not result from stomatal limitation. O₂ evolution, measured on excised leaf disks, induced in 3–4 min in species with short-lived leaves, and 4–8 min in species with long-lived leaves. The rapid induction of O₂ evolution indicates that the slower induction of CO₂ fixation in long-lived leaves was not caused by a delay in the induction of electron transport. Activation of rubisco may be the major factor limiting response times in species with long-lived leaves. Species from Panama with short-lived leaves had remarkably rapid induction times that are comparable to those of algae or higher plant chloroplasts. We also found that understory forest plants induced two to seven times more quickly than did potted plants.     

Crassulacean acid metabolism in the epiphytic ferns Drymoglossum piloselloides and Pyrrosia longifolia: studies on responses to environmental signals

Abstract The paper reports the results of the comprehensive study of crassulacean acid metabolism in two epiphytic tropical ferns, Drymoglossum piloselloides and Pyrrosia longifolia. The plants were investigated under different light, temperature and water status. It was found that both species are obligate CAM plants. The diurnal acidity rhythm is due to the fluctuation in malic acid concentration, which accounts for the change in titratable acidity. Besides malic acid, shikimate and oxalate are found to be present, but not contributing to the CAM acid rhythm. The diurnal rhythm of malic acid content results in a corresponding rhythm in leaf water relations. Both Φ_Φ and Φ_total, were lowest at the end of the night, i.e. when the level of malic acid was highest. The effects of temperature on CO₂ exchange were inverse to those observed in other CAM plants. In both ferns studied, dark CO₂ fixation increased when the night temperature was increased. Increase in day temperature reduced CO₂ uptake during phase IV and during the following night. The observed responses of the ferns to temperature changes suggest that the in situ environmental conditions are optimal for their CAM performance. In weak light, the plants showed net CO₂ output during the midday deacidification period. Increases in light intensity reduced such CO₂ output. Under drought conditions, the CO₂ exchange in the ferns was reduced to zero within 5–6 d, indicating that the ferns studied are more susceptible to water deficiency than other CAM plants. This could be due to a higher cuticular conductance for water. The results are discussed, in particular, in relation to CAM performance of epiphytes growing in the wet tropics.     

Can CO2 enrichment modify the effect of water and high light stress on biomass allocation and relative growth rate of cork oak seedlings?

To test whether the impact of an enriched-CO₂ environment on the growth and biomass allocation of first-season Quercus suber L. seedlings can modify the drought response under shade or sun conditions, seedlings were grown in pots at two CO₂ concentrations × two watering regimes × two irradiances. Compared to CO₂, light and water treatment had greater effects on all morphological traits measured (height, stem diameter, number of leaves, leaf area, biomass fractions). Cork oak showed particularly large increases in biomass in response to elevated CO₂ under low-watered (W−) and high-illuminated conditions (L+). Allocation shifted from shoot to root under increasing irradiance (L+), but was not affected by CO₂. Changes in allocation related to water limitation were only modest, and changed over time. Relative growth rate (RGR) and net assimilation rate (NAR) were significantly greatest in the L+/W+ treatment for both CO₂ concentrations. Changes in RGR were mainly due to NAR. Growth responses to increased light, water or CO₂ were strongest with light, medium with water availability and smallest for CO₂, in terms of RGR. The rise in NAR for light and water treatments was counterbalanced by a decrease in SLA (specific leaf area) and LMF (leaf mass fraction). Results suggest that elevated CO₂ caused cork oak seedlings to improve their performance in dry and high light environments to a greater extent than in well-irrigated and low light ones, thus ameliorating the effects of soil water stress and high light loads on growth.     

Spontaneous action potentials and circumnutation in <Emphasis Type="Italic">Helianthus annuus</Emphasis>

Action potentials (APs) in plants are involved in fast leaf or trap closure as well as elongation, respiration, photosynthesis, and fertilization regulation. Here, spontaneous APs (SAPs) in relation to endogenous stem movement named circumnutation (CN) have been investigated in Helianthus annuus in different light conditions in freely circumnutating and immobilized plants. Extracellular electrical measurements and time-lapse photography were carried out simultaneously. The parameters of CN (trajectory length, period, and direction) and the number and transmission direction of SAPs were analysed. In continuous light (25–40 μmol m^?2 s^?1), all plants circumnutating vigorously in a regular elliptical manner and no SAPs were observed. In light/dark conditions, the plants circumnutated in a daily pattern, most SAPs were observed in the dark and freely circumnutated sunflowers had two times more SAPs (10 SAPs/24 h/plant) than the immobilized plants (5 SAPs/24 h/plant). In continuous very low light (5 μmol m^?2 s^?1), the plants circumnutated weakly and irregularly and SAPs appeared without the circadian pattern. 3–5 SAPs/24/plant occurred in the freely circumnutating and immobilized plants. In light/dark and continuous very low light conditions, an ultradian rhythm of SAPs was observed and the mean spacing between SAPs was approx. 121–271 min in the freely circumnutating and immobilized plants. Under all light conditions, more SAPs were transmitted basipetally than acropetally. One-hour lasting series of 3–4 min spaced SAPs locally propagated were observed as well in very low light. Basipetal and acropetal SAPs passing along the stem motor region accompany irregularity, changes in the CN trajectory direction, and stem torsion. These results demonstrate that APs and CN changes play a role in plant adaptation to light conditions and that there is an ultradian rhythm of SAPs beside ultradian CN rhythm.     

Effects of irradiance and leaf water deficit on net carbon dioxide assimilation and mesophyll and transport resistances

Rate of net CO₂ assimilation by soil-grown soybean plants were studied over a range of relative leaf water contents at each of four levels of irradiance. There was a large interaction between light level and leaf water deficit on the rate of CO₂ assimilation. The effect of leaf water deficit on assimilation became larger as irradiance increased. Both stomatal resistance to CO₂ transport and mesophyll resistance to CO₂ assimilation increased as leaf-water deficit increased. The increase in both resistance with changing leaf-water content was largest at high irradiance and became smaller as irradiance decreased. Relief of soil-moisture stress by watering induced large oscillations of CO₂ assimilation, stomatal resistance, and mesophyll resistance. The oscillation of the mesophyll resistance occurred in the absence of changes in relative water content and appeared to be related to oscillations in leaf temperature. The observed increase in mesophyll resistance with decreasing leaf-water content under nonoscillative conditions may be caused by changes in leaf temperature rather than leaf water content.     

Carbon gain and photosynthetic response of chrysanthemum to photosynthetic photon flux density cycles

Most models of carbon gain as a function of photosynthetic irradiance assume an instantaneous response to increases and decreases in irradiance. High- and low-light-grown plants differ, however, in the time required to adjust to increases and decreases in irradiance. In this study the response to a series of increases and decreases in irradiance was observed in Chrysanthemum × morifolium Ramat. “Fiesta” and compared with calculated values assuming an instantaneous response. There were significant differences between high- and low-light-grown plants in their photosynthetic response to four sequential photosynthetic photon flux density (PPFD) cycles consisting of 5-minute exposures to 200 and 400 micromoles per square meter per second (μmol m⁻²s⁻¹). The CO₂ assimilation rate of high-light-grown plants at the cycle peak increased throughout the PPFD sequence, but the rate of increase was similar to the increase in CO₂ assimilation rate observed under continuous high-light conditions. Low-light leaves showed more variability in their response to light cycles with no significant increase in CO₂ assimilation rate at the cycle peak during sequential cycles. Carbon gain and deviations from actual values (percentage carbon gain over- or underestimation) based on assumptions of instantaneous response were compared under continuous and cyclic light conditions. The percentage carbon gain overestimation depended on the PPFD step size and growth light level of the leaf. When leaves were exposed to a large PPFD increase, the carbon gain was overestimated by 16 to 26%. The photosynthetic response to 100 μmol m⁻² s⁻¹ PPFD increases and decreases was rapid, and the small overestimation of the predicted carbon gain, observed during photosynthetic induction, was almost entirely negated by the carbon gain underestimation observed after a decrease. If the PPFD cycle was 200 or 400 μmol m⁻² s⁻¹, high- and low-light leaves showed a carbon gain overestimation of 25% that was not negated by the underestimation observed after a light decrease. When leaves were exposed to sequential PPFD cycles (200-400 μmol m⁻² s⁻¹), carbon gain did not differ from leaves exposed to a single PPFD cycle of identical irradiance integral that had the same step size (200-400-200 μmol m⁻² s⁻¹) or mean irradiance (200-300-200 μmol m⁻² s⁻¹).     

Control of the circadian rhythm of carbon dioxide assimilation in Bryophyllum leaves by exposure to darkness and high carbon dioxide concentrations

The circadian rhythm of CO₂ assimilation in detached leaves of Bryophyllum fedtschenkoi at 15° C in normal air and continuous illumination is inhibited both by exposure to darkness, and to an atmosphere enriched with 5% CO₂. During such exposures substantial fixation of CO₂ takes place, and the malate concentration in the cell sap increases from about 20 mM to a constant value of 40–50 mM after 16 h. On transferring the darkened leaves to light, and those exposed to 5% CO₂ to normal air, a circadian rhythm of CO₂ assimilation begins again. The phase of this rhythm is determined by the time the transfer is made since the first peak occurs about 24 h afterwards. This finding indicates that the circadian oscillator is driven to, and held at, an identical, fixed phase point in its cycle after 16 h exposure to darkness or to 5% CO₂, and it is from this phase point that oscillation begins after the inhibiting condition is removed. This fixed phase point is characterised by the leaves having acquired a high malate content. The rhythm therefore begins with a period of malate decarboxylation which lasts for about 8 h, during which time the malate content of the leaf cells must be reduced to a value that allows phosphoenolpyruvate carboxylase to become active. Inhibition of the rhythm in darkness, and on exposure to 5% CO₂ in continuous illumination, appears to be due to the presence of a high concentration of CO₂ within the leaf inhibiting malic enzyme which leads to the accumulation of high concentrations of malate in the leaf cells. The malate then allosterically inhibits phosphoenolpyruvate carboxylase upon which the rhythm depends. The results give support to the view that malate synthesis and breakdown form an integral part of the circadian oscillator in this tissue.Abbreviations B. Bryophyllum - PEPCase phosphoenolpyruvate carboxylase     

Photosynthetic induction in C4 leaves

Simultaneous measurements of CO₂ uptake, transpiration rate, and chlorophyll a fluorescence in leaf strips of C₄ plants during the induction phase of photosynthesis are described. The timecourse of CO₂ fixation is biphasic with the initial phase occurring within the first 1 to 5 min and the secondary phase consisting of a slow rise to the steady-state rate of photosynthesis. Transpiration rate follows the CO₂-fixation timecourse closely but the intercellular CO₂ concentration never falls below saturation for C₄ plants. Chlorophyll a fluorescence quenching occurs exclusively during the initial fast phase of the CO₂-fixation timecourse. The effect of duration of dark pretreatment of leaves on these parameters and the effects of light intensity and CO₂ concentration are examined. These results are discussed with respect to the C₄ cycle and photochemical and non-photochemical chlorophyll fluorescence quenching.Abbreviations IRGA infra-red gas analyser - NADP-ME, NAD-ME and PEP-CK the three groups of C₄ plants utilising the enzymes NADP-malic enzyme, NAD-malic enzyme and phosphoenolpyruvate carboxykinase, respectively, for C₄-acid decarboxylation - PEP phosphoenolpyruvate - 3-PGA 3-phosphoglyceric acid     

Photoperiodic Entrainment Patterns in the CO(2) Output of Lemna perpusilla 6746 and of Several Other Lemnaceae

The CO₂ output of Lemna perpusilla 6746 in “skeleton photoperiods” consisting of alternating 10½-hour and 13-hour dark periods separated by ¼-hour illuminations was recorded under stable high and low nitrate conditions. The phase relationship finally attained between light schedule and output is the same regardless of which dark period is given first, but entrainment is more rapid (as is flowering) with an initial 13-hour dark period. In all respects other than bistability—the assumption of two different stable phase relationships depending on the initial dark period—both flowering and the course of CO₂ output conform to Pittendrigh's model derived from Drosophila eclosion rhythms, confirming the view that an endogenous circadian rhythm, or biological clock, underlies the photoperiodic control of flowering in this plant. Experiments with rigorous temperature control show that earlier results with long light exposures were in part due to temperature changes; in consequence, it is clear that entrainment patterns with high nitrate differ even more from those in low nitrate than was previously evident, and not simply by the addition of a “nitrate peak.” Other Lemnaceae tested with a few simple light-dark schedules in both types of media show a variety of responses, with no obvious correlation to photoperiodic response type.     

Induction and Regulation of Expression of a Low-CO2-Induced Mitochondrial Carbonic Anhydrase in Chlamydomonas reinhardtii

The time course of and the influence of light intensity and light quality on the induction of a mitochondrial carbonic anhydrase (CA) in the unicellular green alga Chlamydomonas reinhardtii was characterized using western and northern blots. This CA was expressed only under low-CO₂ conditions (ambient air). In asynchronously grown cells, the mRNA was detected 15 min after transfer from air containing 5% CO₂ to ambient air, and the 21-kD polypeptide was detected on western blots after 1 h. When transferred back to air containing 5% CO₂, the mRNA disappeared within 1 h and the polypeptide was degraded within 3 d. Photosynthesis was required for the induction in asynchronous cultures. The induction increased with light up to 500 μmol m⁻² s⁻¹, where saturation occurred. In cells grown synchronously, however, expression of the mitochondrial CA was also detected in darkness. Under such conditions the expression followed a circadian rhythm, with mRNA appearing in the dark 30 min before the light was turned on. Algae left in darkness continued this rhythm for several days.     

Biochemical Correlates of the Circadian Rhythm in Photosynthesis in Phaseolus vulgaris

A circadian rhythm in photosynthesis occurs in Phaseolus vulgaris after transfer from a natural or artificial light:dark cycle to constant light. The rhythm in photosynthesis persists even when intercellular CO₂ partial pressure is held constant, demonstrating that the rhythm in photosynthesis is not entirely due to stomatal control over the diffusion of CO₂. Experiments were conducted to attempt to elucidate biochemical correlates with the circadian rhythm in photosynthesis. Plants were entrained to a 12-hour-day:12-hour-night light regimen and then monitored or sampled during a subsequent period of constant light. We observed circadian oscillations in ribulose-1,5-bisphosphate (RuBP) levels, and to a lesser extent in phosphoglyceric acid (PGA) levels, that closely paralleled oscillations in photosynthesis. However, the enzyme activity and activation state of the enzyme responsible for the conversion of RuBP to PGA, ribulose-1,5-bisphosphate carboxylase/oxygenase, showed no discernible circadian oscillation. Hence, we examined the possibility of circadian effects on RuBP regeneration. Neither ribulose-5-phosphate kinase activity nor the level of ATP fluctuated in constant light. Oscillations in triose-phosphate levels were out of phase with those observed for RuBP and PGA.