Nitrogen allocation in response to partial shading of a plant: Possible mechanisms

The significance of photosynthetic and transpiration rates for the perception by plants of light gradients in leaf canopies has been investigated with regard to nitrogen allocation and re-allocation. A gradient of photon flux density (PFD) over a plant's foliage was simulated by shading one leaf of a pair of primary leaves of bean ( Phaseolus vulgaris L. cv. Rentegever). Photosynthetic rate was manipulated independently of PFD and, to some extent, also of transpiration, by subjecting the leaf to different CO₂ concentrations. Transpiration rate was changed independently of PFD and photosynthetic rate by subjecting the leaf to different vapour pressure differences (VPD). A reduced partial pressure of CO₂ reduced specific leaf mass (SLM) as did a decreased PFD, but did not change leaf N per unit area (N_LA) and light saturated rate of photosynthesis (A_max). A reduced VPD caused several effects consistent with the effect of PFD. It decreased N_LA and A_max and increased the chlorophyll to N ratio in old and young leaves. Furthermore, it decreased the chlorophyll a to b ratio and inhibited leaf growth in young leaves. The transpiration stream is partitioned among the leaves of a plant according to their transpiration rates. The results suggest that relative rates of import of xylem sap into leaves of a plant play an important role in the perception of partial shading of a plant, a situation normally found in dense vegetations. The possible role of cytokinin influx into leaves as controlled by transpiration rate, is discussed.     

Nitrogen allocation in response to partial shading of a plant: Possible mechanisms

The significance of photosynthetic and transpiration rates for the perception by plants of light gradients in leaf canopies has been investigated with regard to nitrogen allocation and re-allocation. A gradient of photon flux density (PFD) over a plant's foliage was simulated by shading one leaf of a pair of primary leaves of bean ( Phaseolus vulgaris L. cv. Rentegever). Photosynthetic rate was manipulated independently of PFD and, to some extent, also of transpiration, by subjecting the leaf to different CO₂ concentrations. Transpiration rate was changed independently of PFD and photosynthetic rate by subjecting the leaf to different vapour pressure differences (VPD). A reduced partial pressure of CO₂ reduced specific leaf mass (SLM) as did a decreased PFD, but did not change leaf N per unit area (N_LA) and light saturated rate of photosynthesis (A_max). A reduced VPD caused several effects consistent with the effect of PFD. It decreased N_LA and A_max and increased the chlorophyll to N ratio in old and young leaves. Furthermore, it decreased the chlorophyll a to b ratio and inhibited leaf growth in young leaves. The transpiration stream is partitioned among the leaves of a plant according to their transpiration rates. The results suggest that relative rates of import of xylem sap into leaves of a plant play an important role in the perception of partial shading of a plant, a situation normally found in dense vegetations. The possible role of cytokinin influx into leaves as controlled by transpiration rate, is discussed.     

Light dependent activation of CO2 assimilation and the ratio of Rubisco activase to Rubisco during leaf aging of rice (Oryza sativa)

The effects of the ratio of Rubisco activase to Rubisco (activase/Rubisco ratio) on light dependent activation of CO₂ assimilation were investigated during leaf aging of rice. Changes of photosynthetic CO₂ gas exchange rates in relation to step increases of light intensity from two photon flux densities of 60 µmol m⁻² s⁻¹ (low initial PFD) and 500 µmol m⁻² s⁻¹ (high initial PFD) to saturated PFD of 1 800 µmol m⁻² s⁻¹ were measured. These photosynthetic activation processes were considered to be limited by the Rubisco activation rate when analyzed by the relaxation method. The relaxation time of low initial PFD gradually declined from 3 to 33 days after leaf emergence and showed high and negative correlation to the activase/Rubisco ratio. The initial rate of Rubisco activation under low initial PFD linearly correlated to the amounts of Rubisco activase, whereas these were almost constant from 3 to 23 days after leaf emergence. But these correlations could not be recognized in the case of high initial PFD. Moreover, the relaxation times were more sensitive to intercellular CO₂ concentration (C_i) under high initial PFD than under low initial PFD, especially, at C_i below 300 µl l⁻¹. These results suggest the involvement of the activase/Rubisco ratio in the photosynthetic activation under relatively low initial PFD, and the limitation of photosynthetic activation under relatively high initial PFD by Rubisco carbamylation during leaf aging of rice.     

SUN-TRACKING AND RELATED LEAF MOVEMENTS IN A DESERT LUPINE (LUPINUS ARIZONICUS)

Both diaheliotropic (sun-tracking) and paraheliotropic (cupping) leaf movements are described for the Arizona Lupine [Lupinus arizonicus (Wats.) Wats.]. The leaf movements are shown to be non-circadian in nature. Evidence is presented that an active K⁺ ion transport mechanism is involved in these turgor-related leaf movements. Increasing concentrations of lanthanum, a known ion transport inhibitor, showed increasing inhibition of both leaf movements. Increasing concentrations of other salts did not inhibit either leaf movements, instead there was an increase in the cupping leaf movement (elevation of the leaflets) which is shown to be a water stress response.     

Short period leaf movements in Oxalis regnellii

Oxalis regnellii Mig. is a trifoliate plant, and the three leaflets usually show synchronized up and down movements with a circadian period of 26–27 h. The three leaflets can also perform desynchronized ultradian oscillations, and we report on such rhythms under different conditions. A study of the occurrence of ultradian leaf movement rhythms as a function of irradiance is presented. At an irradiance of approximately 1 μW cm⁻², the occurrence was maximal and ca 30%. The periods varied from 5 to 15 h. Four other cases of ultradian rhythms in different conditions are also presented. In one case spontaneous ultradian rhythms occurred, and in another, two of the leaflets showed ultradian rhythms when the third leaflet had received a light pulse. In two more cases, the three leaflets on a leaf were separated by physical cuts along the petiole between the pulvini; in both cases the period was approximately 5 h. Possible mechanisms to explain the ultradian rhythms in Oxalis regnelli are discussed.     

The utilization of lightflecks for growth in four Australian rain-forest species

1. The ability of rain-forest plants to utilize sunflecks for growth was investigated using the following species: Alocasia macrorrhiza, Diploglottis diphyllostegia, Micromelum minutum and Omalanthus novo-guinensis.
2. Growth analysis and gas-exchange measurements were used to assess performance of the four species when exposed to either constant or fluctuating light.
3. Final biomass (g dry wt) in D. diphyllostegia and M. minutum grown under the lightfleck regime (total daily PFD = 7·02 mol m^–2 day^–1) was significantly greater than in the same species grown under constant low PFD (total daily PFD = 4·86 mol m^–2 day^–1). In contrast, final biomass in lightfleck O. novo-guinensis and A. macrorrhiza was significantly reduced in comparison with the same species grown under constant low PFD.
4. When grown under either constant or fluctuating light but with the same total daily PFD, A. macrorrhiza and O. novo-guinensis had significantly lower final biomass in fluctuating light as compared to constant light. Final biomass in D. diphyllostegia was not significantly different in either regime, while M. minutum had significantly higher final biomass in the fluctuating light regime.
5. Responses of the four species to fluctuating or constant light appeared to be the result of physiological rather than morphological acclimation as net assimilation rate was more closely correlated with relative growth rate than was leaf area ratio.     

Response of net photosynthesis in bean (Phaseolus vulgaris) leaves to the elevation of the partial pressures of oxygen and carbon dioxide

Bean ( Phaseolus vulgaris L. cv. Golden Saxa) plants were grown under low artificial light or under natural daylight. The rate of net photosynthesis (P_N) was measured at: CO₂ partial pressure, p(CO₂), of 0.03, 0.09 or 0.15 kPa; O₂ partial pressure, p(O₂), of 2, 21 or 31 kPa and at light intensities of 350 or 1000 μmol m⁻² s⁻¹ (photosynthetically active radiation). In plants which had been grown under natural light, stimulation of P_N at 21 kPa p(O₂) was found only at elevated p(CO₂) and high light. It is proposed that this phenomenon is dependent on a high capacity of the photosynthetic apparatus to regenerate ribulose 1.5-bisphosphate.     

Imipramine affects circadian leaf movements in Oxalis regnellii

The sensitivity of the circadian leaf movement of Oxalis regnellii Mig. to imipramine (a tricyclic dibenzazepine) was investigated. Imipramine, like Li⁺, is used as a therapeutic agent against depressive disorders in man. The therapeutic effects of the two substances might be mediated via effects on basic circadian rhythms and the cellular level. It was indeed possible to influence the circadian movement of Oxalis by imipramine; pulses (10⁻³ M , 4h) phase shifted the rhythms and caused advances. A phase response curve is presented. No period change of the movements was caused by permanent presence of imipramine (5 - 10⁻⁵ or 10⁻⁵ M ). The nature of the imipramine-induced phase shift is discussed and compared with Li⁻¹ effects on the same circadian system.     

Chloroplast movement in Alocasia macrorrhiza

Chloroplast movements in a rainforest understory plant Alocasia macrorrhiza (L.) G. Don are striking, creating changes in leaf transmittance that are visible to the naked eye. We have characterized the light requirements for these changes and the resulting changes in light penetration to different cell layers within the leaf and through the entire thickness of the leaf. Plants were grown either in a relatively constant, growth-chamber environment or in a variable, greenhouse environment. Irradiance-response curves for chloroplast movement were the same for both groups of plants, saturating at about 1 000 μmol m⁻² s⁻¹, though only the greenhouse-grown plants normally encountered light sufficient to drive the movement. Chloroplast movement caused changes in whole-leaf transmittance on the order of a few percent across the entire visible spectrum. Transmittance changes were larger within the leaf, especially directly under the palisade layer. Chloroplast movement could be manipulated experimentally by removing blue wavelengths from the spectrum of incident light or by treating with cytochalasin D.     

Contribution of current photosynthates to root respiration of non-nodulated Medicago sativa: effects of light and nitrogen supply

Abstract: The effects of light (PFD) and nitrogen (N) supply on root respiration of new C (currently assimilated carbon, R _new) and old C ( R _old) were analysed in non-nodulated Medicago sativa . Plants were pre-treated with high/low PFD and high/low N supply with a regular 16/8 h light/dark cycle. Five to eight weeks after planting current photosynthates were labelled with ¹³C and their contribution to root respiration was continuously measured during a 24 h day/night cycle. PFD conditions during labelling were either those of the pre-treatments (control, 25 or 6 mol m^-2 d^-1) or, for high PFD plants, 6 mol m^-2 d^-1 by shortening the photoperiod or reducing irradiance. The fraction of new C in the respiratory CO₂ increased during the light period, but remained constant in the dark period. In control plants, R _new contributed 40 % to the daily root respiration in high PFD/high N conditions. Continuously low PFD increased (50 %) and low N decreased (26 %) the contribution of R _new. Exposing plants from high PFD pre-treatments to a short photoperiod or to low PFD stimulated R _old, indicating mobilisation of reserve C. This stimulation was more pronounced in plants with high N supply than in those with low N supply. Comparison with other legumes suggested that R _new in root respiration was mainly defined by the ratio between the assimilatory capacity of the shoots and the maintenance costs of roots with a short-term capacity of buffering respiratory demand by mobilisation of reserves in situations of fluctuating PFD.     

Multiplication of Pseudomonas syringae pv. phaseolicola "in planta"

Multiplication of Pseudomonas phaseolicola was determined in 17 different bean cultivars ( Phaseolus vulgaris ) and 9 other plant species, and the effect of different inoculation methods and conditions was also studied.
In susceptible leaves, a generation time of 2.1 h was determined in the early phase (2 days after inoculation). Different multiplication rates in susceptible and resistant leaves were clearly observed 4 days after inoculation. At this time the first small water-soaked spots were visible in the susceptible cultivars. Bacteria multiplied up to the 7th day after inoculation with a maximum of 10⁹ cells per cm² leaf (equal to ca. 4 × 10¹⁰ bacterial cells/cm³). At the same time, the water-soaked spots had reached their maximum size in most cases. Thus, bacterial multiplication and development of water-soaked spots paralleled each other.
In resistant leaves, no water-soaked spots appeared, and the final bacterial concentration was 1/1000–1/100 of that in susceptible leaves. Gomparison of races 1 and 2 in several bean cultivars indicated the non-existence of a gene-for-gene relationship with this disease. Old leaves were less susceptible to infection. Some bacterial multiplication was also observed in non-host plants. There was a general correlation between bacterial multiplication in the non-host plants and their botanical relation to Phaseolus vulgaris .     

Response to directional light by leaves of a sun-tracking lupine (Lupinus succulentus)

Experiments were done to examine the phototrophic response of sun-tracking leaves of Lupinus succulentus Dougl. to fixed beams of white and broad band light. Upon irradiation with 15 W m⁻² white light that struck the laminae at an angle of 45°, there was a 45–60 min lag period prior to leaf movement. The greatest rate of movement was 15° h⁻¹, and reorientation ceased when leaves attained a position within 15° of perpendicular to the light beam. Laminar movement was largely pulvinar, and a 60 min inductive light treatment was sufficient to activate a maximum pulvinar response in subsequent darkness. Light striking the lamina at angles between 20 and 70° induced similar maximum pulvinar responses and only light that struck the upper (adaxial) leaf surface was effective. Leaf tracking was fully activated by blue light but not by red or yellow light.     

Paraheliotropic leaf movement in Siratro as a protective mechanism against drought-induced damage to primary photosynthetic reactions: damage by excessive light and heat

Damage to primary photosynthetic reactions by drought, excess light and heat in leaves of Macroptilium atropurpureum Dc. cv. Siratro was assessed by measurements of chlorophyll fluorescence emission kinetics at 77 K (-196°C). Paraheliotropic leaf movement protected waterstressed Siratro leaves from damage by excess light (photoinhibition), by heat, and by the interactive effects of excess light and high leaf temperatures. When the leaves were restrained to a horizontal position, photoinhibition occurred and the degree of photoinhibitory damage increased with the time of exposure to high levels of solar radiation. Severe inhibition was followed by leaf death, but leaves gradually recovered from moderate damage. This drought-induced photoinhibitory damage seemed more closely related to low leaf water potential than to low leaf conductance. Exposure to leaf temperatures above 42°C caused damage to the photosynthetic system even in the dark and leaves died at 48°C. Between 42 and 48°C the degree of heat damage increased with the time of exposure, but recovery from moderate heat damage occurred over several days. The threshold temperature for direct heat damage increased with the growth temperature regime, but was unaffected by water-stress history or by current leaf water status. No direct heat damage occurred below 42°C, but in water-stressed plants photoinhibition increased with increasing leaf temperature in the range 31–42°C and with increasing photon flux density up to full sunglight values. Thus, water stress evidently predisposes the photosynthetic system to photoinhibition and high leaf temperature exacerbates this photoinhibitory damage. It seems probable that, under the climatic conditions where Siratro occurs in nature, but in the absence of paraheliotropic leaf movement, photoinhibitory damage would occur more frequently during drought than would direct heat damage.Abbreviations and symbols PFD photon flux area density - PSI, PSII photosyntem I, II - F _M, F _O, F _V maximum, instantaneous, variable fluorescence emission - PLM paraheliotropic leaf movement; all data of parameter of variation are mean ± standard error     

The effects of nitrogen, light and water availability on tropic leaf movements in soybean (Glycine max)

Abstract. Rapid, tropic leaf movements and photo-synthetic responses of the heliotropic plant, soybean, Glycine max cv. Cumberland, grown under two different nitrogen, three different light and two different water treatments were examined. Measurements of leaf orientation during midday periods outdoors, and tropic reorientation of leaflets in response to vertical illumination indoors, revealed a positive, linear relationship between leaf water potential and the cosine of the angle of incidence between the leaf and the direct beam of the excitation light. This relationship was altered by nitrogen availability, such that a lower cosine of incidence (lower leaf irradiance) for a given leaf water potential was measured for plants grown under low nitrogen compared to those grown under high nitrogen. Additionally, plants grown under low nitrogen and low water availability showed more rapid rates of leaf movement compared to plants receiving high levels of these resources. Light regime during growth had no effect on the relationship between the cosine of incidence and leaf water potential. Reduced water and nitrogen availabilities during growth resulted in lower photosaturated rates of photosynthesis and stomatal conductance, as well as alterations in the relationship between these parameters. Thus, higher values for the ratio of intercellular CO₂/ambient CO₂ were measured for low-N grown plants (higher nitrogen use efficiencies) and lower values of this ratio for water stressed plants (higher water use efficiencies). The results show that environmental growth conditions other than water availability have the potential to modify leaf orientation responses to vectorial light in heliotropic legumes such as soybean. This has implications for the potential of heliotropic movements to minimize environmental stress-induced damage to the photosynthetic apparatus, and to modulate leaf-level resource use efficiencies.     

Effect of desiccation on potassium and anion currents from young root hairs: Implication on tip growth

Little is known about the early response of roots to desiccation. Young growing root hairs of Arabidopsis thaliana , Vigna unguiculata and Phaseolus vulgaris were used to study the early response of roots to desiccation since they behave like sensors that are able to perceive environmental signals. In control conditions, root hairs were polarized around −120 mV and displayed inward rectifying K⁺ currents. When submitted to short-term desiccation, root hairs stopped their tip growth and their membrane became depolarized. Under these conditions, the K⁺ influx carried by the inward rectifying K⁺ channels was not maintained and instead slow deactivating anion channels were recorded. The inhibition of K⁺ influx and the large anion efflux due to the activation of slow anion currents could participate in the inhibition of tip growth.     

Stomatal sensing of the environment

The effects of environmental factors on stomatal behaviour are reviewed and the questions of whether photosynthesis and transpiration eontrol stomata or whether stomata themselves control the rates of these processes is addressed. Light affects stomata directly and indirectly. Light can act directly as an energy source resulting in ATP formation within guard cells via photophosphorylation, or as a stimulus as in the case of the blue light effects which cause guard cell H⁺ extrusion. Light also acts indirectly on stomata by affecting photosynthesis which influences the intercellular leaf CO₂ concentration ( C _i). Carbon dioxide concentrations in contact with the plasma membrane of the guard cell or within the guard cell acts directly on cell processes responsible for stomatal movements. The mechanism by which CO₂ exerts its effect is not fully understood but, at least in part, it is concerned with changing the properties of guard cell plasma membranes which influence ion transport processes. The C _i may remain fairly constant for much of the day for many species which is the result of parallel responses of stomata and photosynthesis to light. Leaf water potential also influences stomatal behaviour. Since leaf water potential is a resultant of water uptake and storage by the plant and transpirational water loss, any factor which affects these processes, such as soil water availability, temperature, atmospheric humidity and air movement, may indirectly affect stomata. Some of these factors, such as temperature and possibly humidity, may affect stomata directly. These direct and indirect effects of environmental factors interact to give a net opening response upon which is superimposed a direct effect of stomatal circadian rhythmic activity.     

An automated greenhouse sand culture system suitable for studies of P nutrition

Abstract. Maintenance of realistically low solution P concentrations under controlled conditions is a major difficulty in studies of P nutrition. In this report, we describe a relatively simple and economical sand culture system capable of sustaining plant growth to maturity under controlled yet realistic P regimes. The system uses Al₂O₃ as a solid-phase P buffer, and modern process control technology to control irrigation and addition of other mineral nutrients. Aspects of the design, use and potential applications of automated solid-phase systems are discussed. The system was used to grow Phaseolus vulgaris to matarity at 0.4 mmol m³, 1.0 mmol m³ and 27 mmol m³ P with and without mycorrhizal inoculation. At flowering, low solution P concentrations were associated with reduced leaf concentrations of P in nonmycorrhizal plants, and reduced leaf concentrations of Ca in both mycorrhizal and nonmycorrhizal plants. Mycorrhizal inoculation increased leaf P, K, Mg and Mn concentrations, but reduced leaf N concentration. Low P regimes reduced final seed yield by diminishing both the number of pods per plant and the number of seeds per pod. Mycorrhizal inoculation significantly enhanced seed yield under low P regimes by increasing seed weight, the number of pods per plant, and the number of seeds per pod.     

Basic properties of the circadian leaf movements of Oxalis regnellii, and period change due to lithium ions

The circadian leaf movement of Oxalis regnellii Mig, has been investigated. The three leaflets of a stalk were normally synchronized, and under the experimental conditions chosen they showed a period of 26.2 ± 0.1 h. Cutting off one or two leaflets led to a successive decrease of the period length (25.7 ± 0.1 and 25.1 ± 0.3 h resp.). It was possible to phase shift the leaf movements by mechanical means (advance of 1.6 ± 0.3 h).
Lithium ions, added permanently to the transpiration stream, increased the period length of the leaf movements by more than one hour (with 10 m M Li⁺). A 24 h pulse of 20 m M LiCl caused a permanent 2–3 h phase delay of the circadian rhythm. Four-h pulses, on the other hand, provoked only transient phase delays, the magnitude being dependent on the phase of application. Lithium concentrations were determined for different regions of leaves and pulvini. It was shown that leaf segments had considerably lower concentrations than pulvini. No significant difference in the lithium concentration was observed between the upper and lower part of pulvini.
In the light leaf position was strongly correlated with water uptake and the consequences for applications of substances to the circadian system via the transpiration stream is discussed. A simple model of the oscillatory system and reactions connected to it is discussed.     

Mechanism of copper-enhanced photoinhibition in thylakoid membranes

The effect of copper on photoinhibition of photosystem II (PSII) in vitro was studied in bean ( Phaseolus vulgaris L. cv. Dufrix) and pumpkin ( Cucurbita pepo L.) thylakoids. The thylakoids were illuminated at 200–2 000 μmol photons m⁻² s⁻¹ in the presence of 70–1 830 added Cu²⁺ ions per PSII. Three lines of evidence show that the irreversible damage of PSII caused by illumination of thylakoids in the presence of Cu²⁺ was mainly due to donor-side photoinhibition resulting from inhibition of the PSII donor side by Cu²⁺. First, addition of an artificial electron donor partially restored PSII activity of thylakoids that had been illuminated in the presence of Cu²⁺. Second, already moderate light was enough to cause rapid inhibition of PSII, and the inhibition could be saturated by light. Third, the extrinsic polypeptides of the oxygen-evolving complex were found to become oxidized by the combined effect of Cu²⁺ and light. The presence of oxygen was not necessary for the copper-induced enhancement of photoinhibition of PSII. When the illumination was prolonged, copper caused a gradual collapse of the thylakoid structure by increasing degradation of thylakoid proteins.     

Long-term effects of low levels of SO2 on bean plants (Phaseolus vulgaris). I. Immission-response pattern of net photosynthesis and transpiration during life-long, continuous measurements

The immission-response effect of five low levels of sulfur dioxide on net photosynthesis and transpiration was studied during continuous measurements in near-complete life cycles of whole bean plants ( Phaseolus vulgaris L. cv. Processer) grown in a controlled environment. Sixteen plants were grown in individual water cultures in each of five 100 1 glass assimilation chambers with a new type of exposure system with separate root aeration. SO₂ immission ranged from 10 μg m⁻³ to 950 μg m⁻³ during 12-h day-time exposure periods, five days a week, while a low, natural background of NO_x was accepted.
The SO₂-induced photosynthetic reductions were in the short term, but in particular on the long-term level very closely related with stomatal conductance (significance level better than 0.0005). However, a causal coherence was not inferred. Physiological inhibitions were composed of: (1) A reversible component (night and week-end recovery) and (2) an irreversible component (related to reduced green leaf area). The pattern of leaf growth was studied, with the conclusion that SO₂ reduced leaf area by promoting senescence, rather than by interfering with leaf emergence and development.