Strong light elevates thermotolerance of photosynthetic apparatus and the content of membranes and polar lipids in wheat leaves

The influence of excess irradiance on resistance of wheat (Triticum aestivum L.) photosynthetic apparatus to heating in darkness and in the light was investigated and compared with changes in leaf cell ultra-structure and composition of cell lipids and fatty acids. The leaves of 14- to 16-day-old plants grown at low irradiance (about 20 W/m²) were exposed for 1 h to irradiance of 370 or 600 W/m² PAR. Using infrared gas analysis, we found that the preexposure of leaves to excess irradiation elevated resistance of apparent photosynthesis to 10-min heat treatment at 40–45°C. The rate of Hill reaction (reduction of 2,6-dichlorophenolindophenol by isolated chloroplasts) was higher for leaves heated at high irradiance than for leaves heated in darkness. During illumination of leaves with strong light, mesophyll cells became more abundant in mitochondria and peroxysomes, as well as in cisternae of endoplasmic reticulum and Golgi complex. The chloroplast thylakoids and grana became more extensive and numerous. At the same time, the leaf content of main classes of membrane glycerolipids increased in parallel with the increase in the phospholipid/glycolipid and lipid/chlorophyll ratios. The unsaturation index of fatty acids of membrane lipids increased because of the elevated content of linolenic acid. Thus, excessive light (not fully utilized in photosynthesis) induced in wheat leaves a series of nonspecific adaptive changes that were similar to those occurring under the action of other environmental factors, such as heat shock, cooling, salinity, and osmotic stresses.     

Comparative ecophysiology of leaf and canopy photosynthesis

Leaves and herbaceous leaf canopies photosynthesize efficiently although the distribution of light, the ultimate resource of photosynthesis, is very biased in these systems. As has been suggested in theoretical studies, if a photosynthetic system is organized such that every photosynthetic apparatus photosynthesizes in concert, the system as a whole has the sharpest light response curve and is most adaptive. This condition can be approached by (i) homogenization of the light environment and (ii) acclimation of the photosynthetic properties of leaves or chloroplasts to their local light environments. This review examines these two factors in the herbaceous leaf canopy and in the leaf. Changes in the inclination of leaves in the canopy and differentiation of mesophyll into palisade and spongy tissue contribute to the moderation of the light gradient. Leaf and chloroplast movements in the upper parts of these systems under high irradiances also moderate light gradients. Moreover, acclimation of leaves and chloroplasts to the local light environment is substantial. These factors increase the efficiency of photosynthesis considerably. However, the systems appear to be less efficient than the theoretical optimum. When the systems are optically dense, the light gradients may be too great for leaves or chloroplasts to acclimate. The loss of photosynthetic production attributed to the imperfect adjustment of photosynthetic apparatus to the local light environment is most apparent when the photosynthesis of the system is in the transition between the light-limited and light-saturated phases. Although acclimation of the photosynthetic apparatus and moderation of light gradients are imperfect, these markedly raise the efficiency of photosynthesis. Thus more mechanistic studies on these adaptive attributes are needed. The causes and consequences of imperfect adjustment should also be investigated.     

Acclimation of mesophyll and bundle sheath chloroplasts of maize to different irradiances during growth

The regulation by light of the photosynthetic apparatus, and composition of light-harvesting complexes in mesophyll and bundle sheath chloroplasts was investigated in maize. Leaf chlorophyll content, level of plastoquinone, PSI and PSII activities and Lhc polypeptide compositions were determined in plants grown under high, moderate and low irradiances. Photochemical efficiency of PSII, photochemical fluorescence quenching and non-photochemical fluorescence quenching over a range of actinic irradiances were also determined, using chlorophyll a fluorescence analysis. Acclimation of plants to different light conditions caused marked changes in light-harvesting complexes, LHCI and LHCII, and antenna complexes were also reorganized in these types of chloroplasts. The level of LHCII increased in plants grown in low light, even in agranal bundle sheath chloroplasts where the amount of PSII was strongly reduced. Irradiance also affected LHCI complex and the number of structural polypeptides, in this complex, generally decreased in chloroplasts from plants grown under lower light. Surprisingly moderate and low irradiances during growth do not affect the light reaction and fluorescence parameters of plants but generated differences in composition of light-harvesting complexes in chloroplasts. On the other hand, the changes in photosynthetic apparatus in plants acclimated to high light, resulted in a higher efficiency of photosynthesis. Based on these observations we propose that light acclimation to high light in maize is tightly coordinated adjustment of light reaction components/activity in both mesophyll and bundle sheath chloroplasts. Acclimation is concerned with balancing light utilization and level of the content of LHC complexes differently in both types of chloroplasts.     

Acclimation of mesophyll and bundle sheath chloroplasts of maize to different irradiances during growth

The regulation by light of the photosynthetic apparatus, and composition of light-harvesting complexes in mesophyll and bundle sheath chloroplasts was investigated in maize. Leaf chlorophyll content, level of plastoquinone, PSI and PSII activities and Lhc polypeptide compositions were determined in plants grown under high, moderate and low irradiances. Photochemical efficiency of PSII, photochemical fluorescence quenching and non-photochemical fluorescence quenching over a range of actinic irradiances were also determined, using chlorophyll a fluorescence analysis. Acclimation of plants to different light conditions caused marked changes in light-harvesting complexes, LHCI and LHCII, and antenna complexes were also reorganized in these types of chloroplasts. The level of LHCII increased in plants grown in low light, even in agranal bundle sheath chloroplasts where the amount of PSII was strongly reduced. Irradiance also affected LHCI complex and the number of structural polypeptides, in this complex, generally decreased in chloroplasts from plants grown under lower light. Surprisingly moderate and low irradiances during growth do not affect the light reaction and fluorescence parameters of plants but generated differences in composition of light-harvesting complexes in chloroplasts. On the other hand, the changes in photosynthetic apparatus in plants acclimated to high light, resulted in a higher efficiency of photosynthesis. Based on these observations we propose that light acclimation to high light in maize is tightly coordinated adjustment of light reaction components/activity in both mesophyll and bundle sheath chloroplasts. Acclimation is concerned with balancing light utilization and level of the content of LHC complexes differently in both types of chloroplasts.     

Heat shock increases thermotolerance of photosynthetic electron transport and the content of chloroplast membranes and lipids in wheat leaves

Preliminary heating of 15-16-day-old wheat (Triticum aestivum L.) plants for 3 h at 37–38°C (heat shock, HS) increased the tolerance of photosynthetic electron transport (determined as the reduction of 2,6-dichlorophenol indophenol by isolated chloroplasts) toward heating of leaves at 42–48°C in high light (100 klx). At the same time, HS did not affect the activity of the xanthophyll cycle reactions in the 30–48°C temperature range. HS exposure induced an increase in the thylakoid length, the number of grana, and the average number of thylakoids per granum. The volume of the thylakoid system increased 1.4-fold. Such indices as the total content of chlorophylls (a + b), the chlorophyll a/b ratio, as well as the contents of individual carotenoids, chloroplast membrane proteins, and the soluble leaf proteins remained unchanged. The de novo photosynthetic membrane formation was accompanied by the 1.5-fold increase in major chloroplast lipids. It was concluded that, in mature wheat chloroplasts, HS induced the formation of thylakoids characterized by a changed molecular structure and by increased lipid/protein and lipid/chlorophyll ratios.     

Photosynthetic acclimation in relation to nitrogen allocation in cucumber leaves in response to changes in irradiance

Leaves deep in canopies can suddenly be exposed to increased irradiances following e.g. gap formation in forests or pruning in crops. Studies on the acclimation of photosynthesis to increased irradiance have mainly focused on the changes in photosynthetic capacity (A_max), although actual irradiance often remains below saturating level. We investigated the effect of changes in irradiance on the photosynthesis irradiance response and on nitrogen allocation in fully grown leaves of Cucumis sativus. Leaves that fully developed under low (50 µmol m^?2 s^?1) or moderate (200 µmol m^?2 s^?1) irradiance were subsequently exposed to, respectively, moderate (LM‐leaves) or low (ML‐leaves) irradiance or kept at constant irradiance level (LL‐ and MM‐leaves). Acclimation of photosynthesis occurred within 7 days with final A_max highest in MM‐leaves, lowest in LL‐leaves and intermediate in ML‐ and LM‐leaves, whereas full acclimation of thylakoid processes underlying photosystem II (PSII) efficiency and non‐photochemical quenching occurred in ML‐ and LM‐leaves. Dark respiration correlated with irradiance level, but not with A_max. Light‐limited quantum efficiency was similar in all leaves. The increase in photosynthesis at moderate irradiance in LM‐leaves was primarily driven by nitrogen import, and nitrogen remained allocated in a similar ratio to Rubisco and bioenergetics, while allocation to light harvesting relatively decreased. A contrary response of nitrogen was associated with the decrease in photosynthesis in ML‐leaves. Net assimilation of LM‐leaves under moderate irradiance remained lower than in MM‐leaves, revealing the importance of photosynthetic acclimation during the leaf developmental phase for crop productivity in scenarios with realistic, moderate fluctuations in irradiance that leaves can be exposed to.     

CHANGES IN PHOTOSYNTHESIS IN RESPONSE TO COMBINED IRRADIANCE AND TEMPERATURE STRESS IN CYANOBACTERIAL SURFACE WATERBLOOMS1

Buoyant cyanobacteria, previously mixed throughout the water column, float to the lake surface and form a surface waterbloom when mixing subsides. At the surface, the cells are exposed to full sunlight, and this abrupt change in photon irradiance may induce photoinhibition; at the same time, temperature rises as well. This study investigated the damaging effects of this increase in temperature as well as the ecologically more relevant combination of both an increased temperature and a high photon irradiance. Analysis of surface blooms with oxygen microelectrodes showed that integrated oxygen contents that are dependent on the balance of photosynthetic oxygen evolution and respiratory oxygen uptake decreased when temperature was raised above the lake temperature. Gross rates of photosynthesis were unaffected by temperatures up to of 35°C; hence, a moderate increase in temperature mainly stimulated oxygen uptake. Preincubation of cells of the cyanobacterium Anabaena flos-aquae (Lyngb.) de Brébisson at temperatures up to 35°C did not affect the subsequent measurement of rates of net photosynthesis. Another 5°C rise in temperature severely damaged the photosynthetic apparatus. Failure to restore net rates of photosynthesis was coupled to a strong quenching of the ratio of variable to maximum fluorescence, F_v/F_m, that was the result of a rise in F_o. A combination of high temperature and high photon irradiance was more damaging than high temperature alone. In contrast, low photon irradiances offered substantial protection against heat injury of the photosynthetic apparatus. I conclude from this study that because cyanobacteria usually are acclimated to low average irradiance prior to bloom formation, there is a reasonable risk of chronic photoinhibition. The increase in temperature will enhance the photodamage of cells in the top layer of the bloom. Low photon irradiances in subsurface layers will offer protection against heat injury. If the high temperatures extend to the deepest, dark layers of the bloom, damage in those layers is likely to occur.     

The Control of the Light Acclimation of Photosynthesis in Glycine max: Dependence on Import as Modified by Intraplant Shading

The dependence of photosynthetic capacity on imported and locally-assimilatedsupplies of carbon during leaf development under different irradianceswas investigated in Glycine max. The potential export of carbonto the developing, mainstem trifoliate leaf (source-potential)was restricted non-destructively by shading all lower, sourceleaves (source-shading), while local photosynthesis was modifiedconcurrently by exposing the young leaf to different light levelsduring development. When source-shading was applied below the2nd mainstem trifoliate leaf at the bud stage of development,photosynthetic capacity was unaffected in leaves which had developedunder moderate and low irradiances (500 and 250 µmol PARm ^–2 s^–1 respectively), but was reduced significantlyin leaves developed under a high irradiance (900 µmolPAR m ^–2 s^–1). If source-shading was applied beneaththe 2nd leaf at unfolding, the reduction of photosynthetic capacityunder the high irradiance was relatively minor. The photosyntheticcapacity attained by the 2nd leaf during development under differentirradiances was influenced by the previous light environmentof the whole plant. In contrast to the 2nd leaf, the photosyntheticcapacities of the 1st and 4th mainstem leaves were relativelyunaffected by source-shading, even under the highest light regime.While photosynthetic capacity showed a widespread insensitivityto the light level of the lower region of the canopy, source-shadingreduced final leaf size irrespective of node position or localirradiance during leaf development. These effects were not relatedto differences in daily photosynthesis by the expanding leaf,and are discussed in terms of the source/sink balance of thedeveloping leaf. Key words: Glycine max, source-shading, photosynthetic capacity     

Light-Induced Adaptive Responses under Greenhouse and Controlled Conditions in the Fern Pteris cretica var. ouvrardii

The photosynthetic capabilities of the fern Pteris cretica var. ouvrardii were analysed by means of the light response curves of CO₂ exchange. In control growth conditions (greenhouse, low-light: 20–32 W m^?2); photosynthesis was shown to be saturated for low irradiance (20–25 W m^?2); the saturating photosynthetic rate, very low as compared to higher plants, was due to an extremely high intracellular resistance. When irradiance during the photosynthesis measurement was higher than 60–80 W m^?2, a constant decline of net CO₂ exchange as a function of time was observed. When irradiance during growth was enhanced, whether in greenhouse (20–250 W m^?2) or controlled (62 W m^?2) conditions, the first fronds that had developed in the new condition from the crosier stage exhibited decreased net maximal photosynthesis and a decreased efficiency in low light, but saturating irradiance was unmodified. However, the fronds whose entire differentiation (from meristem) occurred under these moderate irradiances (plants defoliated of all fronds and crosiers at the time of transfer), possessed more efficient photosynthetic characteristics than control plants. Pteris is able to grow under extreme shade conditions (4–8 W m^?2); light saturating photosynthesis and efficiency are higher under extreme shade than under control conditions. These adaptive characteristics indicate that Pteris is a well-adapted shade species.     

Effects of irradiance on growth,photosynthesis, and water use efficiency of seedlings of the chaparral shrub,Ceanothus megacarpus

Summary The effects of irradiance during growth on biomass allocation, growth rates, leaf chlorophyll and protein contents, and on gas exchange responses to irradiance and CO₂ partial pressures of the evergreen, sclerophyllous, chaparral shrub, Ceanothus megacarpus were determined. Plants were grown at 4 irradiances for the growth experiments, 8, 17, 25, 41 nE cm^-2 sec^-1, and at 2 irradiances, 9 and 50 nE cm^-2 sec^-1, for the other comparisons.At higher irradiances root/shoot ratios were somewhat greater and specific leaf weights were much greater, while leaf area ratios were much lower and leaf weight ratios were slightly lower than at lower irradiances. Relative growth rates increased with increasing irradiance up to 25 nE cm^-2 sec^-1 and then leveled off, while unit leaf area rates increased steeply and unit leaf weight rates increased more gradually up to the highest growth irradiance.Leaves grown at 9 nE cm^-2 sec^-1 had less total chlorophyll per unit leaf area and more per unit leaf weight than those grown at 50 nE cm^-2 sec^-1. In a reverse of what is commonly found, low irradiance grown leaves had significantly higher chlorophyll a/b than high irradiance grown leaves. High irradiance grown leaves had much more total soluble protein per unit leaf area and per unit dry weight, and they had much higher soluble protein/chlorophyll than low irradiance grown leaves.High irradiance grown leaves had higher rates of respiration in very dim light, required higher irradiances for photosynthetic saturation and had higher irradiance saturated rates of photosynthesis than low irradiance grown leaves. CO₂ compensation irradiances for leaves of both treatments were very low, <5 nE cm^-2 sec^-1. Leaves grown under low and those grown under high irradiances reached 95% of their saturated photosynthetic rates at 65 and 85 nE cm^-2 sec^-1, respectively. Irradiance saturated rates of photosynthesis were high compared to other chaparral shrubs, 1.3 for low and 1.9 nmol CO₂ cm^-2 sec^-1 for high irradiance grown leaves. A very unusual finding was that leaf conductances to H₂O were significantly lower in the high irradiance grown leaves than in the low irradiance grown leaves. This, plus the differences in photosynthetic rates, resulted in higher water use efficiencies by the high irradiance grown leaves. High irradiance grown leaves had higher rates of photosynthesis at any particular intercellular CO₂ partial pressure and also responded more steeply to increasing CO₂ partial pressure than did low irradiance grown leaves. Leaves from both treatments showed reduced photosynthetic capability after being subjected to low CO₂ partial pressures (100 bars) under high irradiances. This treatment was more detrimental to leaves grown under low irradiances.The ecological implications of these findings are discussed in terms of chaparral shrub community structure. We suggest that light availability may be an important determinant of chaparral community structure through its effects on water use efficiencies rather than on net carbon gain.     

Light acclimation, CO2 response and long-term capacity of underwater photosynthesis in three terrestrial plant species

To characterize underwater photosynthetic performance in some terrestrial plants, we determined (i) underwater light acclimation (ii) underwater photosynthetic response to dissolved CO₂, and (iii) underwater photosynthetic capacity during prolonged submergence in three species that differ in submergence tolerance: Phalaris arundinacea, Rumex crispus (both submergence-tolerant) and Arrhenatherum elatius (submergence-intolerant). None of the species had adjusted to low irradiance after 1 week of submergence. Under non-submerged (control) conditions, only R. crispus displayed shade acclimation. Submergence increased the apparent quantum yield in this species, presumably because of the enhanced CO₂ affinity of the elongated leaves. In control plants of the grass species P. arundinacea and A. elatius, CO₂ affinities were higher than for R. crispus. The underwater photosynthetic capacity of R. crispus increased during 1 month of submergence. In P. arundinacea photosynthesis remained constant during 1 month of submergence at normal irradiance; at low irradiance a reduction in photosynthetic capacity was observed after 2 weeks, although there was no tissue degeneration. In contrast, underwater photosynthesis of the submergence-intolerant species A. elatius collapsed rapidly under both irradiances, and this was accompanied by leaf decay. To describe photosynthesis versus irradiance curves, four models were evaluated. The hyperbolic tangent produced the best goodness-of-fit, whereas the rectangular hyperbola (Michaelis-Menten model) gave relatively poor results.     

Are red leaves photosynthetically active?

At irradiances close to those representing a sunny day, red and green leaves of poinsettia (Euphorbia pulcherrima) showed only minor differences in their photosynthetic capacities despite the strong differences in their pigment composition. However, contrarily to green leaves, red leaves did not show inhibition of photosynthesis at high irradiances, because anthocyanins protected chloroplasts from photoinhibition.     

INFLUENCE OF AGE AND MICROCLIMATE ON THE PHOTOCHEMISTRY OF RHODODENDRON MAXIMUM LEAVES II. CHLOROPLAST STRUCTURE AND PHOTOSYNTHETIC LIGHT RESPONSE

The influence of age on chloroplast structure and photosynthetic light response of Rhododendron maximum L. was studied in three different microhabitats. The three microhabitats constituted a gradient of low, intermediate, and high irradiance levels. The most dramatic change in chloroplast structure with increasing age was the proliferation of the number and size of plastoglobuli. The magnitude and age specific rate of chloroplast occlusion by plastoglobuli increased in habitats with higher irradiance. Photosynthetic responses to light differed among the age categories of leaves. Light saturated photosynthesis and quantum yield decreased as leaves aged. However, in high light environments the rate of reduction of quantum yield or light saturated photosynthetic rate was more rapid than in the low light environment. The quantity of plastoglobuli increased in association with reduced light reaction capacity. The presence and abundance of plastoglobuli in R. maximum chloroplasts and their association with reduced photosynthetic performance indicates that the photosynthetic apparatus of the R. maximum chloroplast is sensitive to photodestruction by high irradiance: commonly a winter phenomenon in these environments.     

Inclination of sun and shade leaves influences chloroplast light harvesting and utilization

Light harvesting and utilization by chloroplasts located near the adaxial vs the abaxial surface of sun and shade leaves were examined by fluorometry in two herbaceous perennials that differed in their anatomy and leaf inclination. Leaves of Thermopsis montana had well-developed palisade and spongy mesophyll whereas the photosynthetic tissue of Smilacina stellata consisted of spongy mesophyll only. Leaf orientation depended upon the irradiance during leaf development. When grown under low-light levels, leaves of S. stellata and T. montana were nearly horizontal, whereas under high-light levels, S. stellata leaves and T. montana leaves were inclined 60⁰ and 30⁰, respectively. Leaf inclination increased the amount of light that was intercepted by the lower leaf surfaces and affected the photosynthetic properties of the chloroplasts located near the abaxial leaf surface. The slowest rates of quinone pool reduction and reoxidation were found in chloroplasts located near the adaxial leaf surface of T. montana plants grown under high light, indicating large quinone pools in these chloroplasts. Chloroplasts near the abaxial surface of low-light leaves had lower light utilization capacities as shown by photochemical quenching measurements. The amount of photosystem II (PSII) down regulation, measured from each leaf surface, was also found to be influenced by irradiance and leaf inclination. The greatest difference between down regulation monitored from the adaxial vs abaxial surfaces was found in plants with horizontal leaves. Different energy dissipation mechanisms may be employed by the two species. Values for down regulation in S. stellata were 2–3 times higher than those in T. montana, while the portion of the PSII population which was found to be Q_B nonreducing was 4–6 times lower in high light S. stellata leaves than in T. montana. All values of Stern-Volmer type nonphotochemical quenching (NPQ) from S. stellata leaves were similar when quenching analysis was performed at actinic irradiances that were higher than the irradiance to which the leaf surface was exposed during growth. In contrast, with T. montana, NPQ values from the abaxial leaf surface were up to 45% higher than those from the adaxial leaf surface regardless of growth conditions. The observed differences in chloroplast properties between species and between the adaxial and abaxial leaf surfaces may depend upon a complex interaction among light, leaf anatomy and leaf inclination.     

A new paradigm in leaf-level photosynthesis: direct and diffuse lights are not equal

Global-change scenarios suggest a trend of increasing diffuse light due to expected increases in cloud cover. Canopy-level measurements of plant-community photosynthesis under diffuse light show increased productivity attributed to more uniform distribution of light within the forest canopy, yet the effect of the directional quality of light at the leaf level is unknown. Here we show that leaf-level photosynthesis in sun leaves of both C(3) and C(4) plants can be 10-15% higher under direct light compared to equivalent absorbed irradiances of diffuse light. High-light-grown leaves showed significant photosynthetic enhancement in direct light, while shade-adapted leaves showed no preference for direct or diffuse light at any irradiance. High-light-grown leaves with multiple palisade layers may be adapted to better utilize direct than diffuse light, while shade leaf structure does not appear to discriminate light based on its directionality. Based upon our measurements, it appears that leaf-level and canopy-level photosynthetic processes react differently to the directionality of light, and previously observed increases in canopy-level photosynthesis occur even though leaf-level photosynthesis decreases under diffuse light.     

Mesostructure and activity of photosynthetic apparatus for three crassulacean species grown in cold climate

A comparative study of leaf anatomy and morphology and of CO₂ exchange was conducted with Rhodiola rosea L., Hylotelephium triphyllum (Haw.) Holub., and Sedum acre L. as representative Crassulacean species occurring in the northeast European Russia. The leaf mesophyll in R. rosea was clearly differentiated into the palisade and spongy tissues, whereas the mesophyll of stonecrops (H. triphyllum and S. acre) was composed of round-shaped cells. The leaves of S. acre featured the largest volume of mesophyll cells and possessed water-retaining cells located around conducting bundles. The chloroplast volume in S. acre (50 μm³) was three times smaller and the number of chloroplasts per cell (170 cell^?1) was three times higher than in R. rosea and H. triphyllum (50–55 cell^?1). The content of chlorophylls (5–7 mg/g dry wt) and carotenoids (1.5–2.0 mg/g dry wt) in R. rosea leaves was 2–3 times higher than in leaves of stonecrops. The rate of CO₂ net uptake in Crassulacean species depended on mesostructure and correlated with the content of pigments and soluble carbohydrates. The photosynthetic rate in R. rosea under optimal irradiance and temperature attained the value of 40 mg/(g dry wt), which is 3 and 8 times higher than in H. triphyllum and S. acre, respectively. The temperature optimum for photosynthesis of R. rosea was observed at 8–18°C, while the optimum for stonecrops was shifted towards higher temperatures by 3–5°C. At chilling temperatures (5–7°C), the leaves of R. rosea retained 50% of their maximal photosynthetic rate, while photosynthetic rates in H. triphyllum and S. acre leaves lowered to 25–30% of the maximal rate. The increase in temperature to 25–30°C led to depression of CO₂ net uptake in leaves of Crassulacean species. In R. rosea and H. triphyllum, the rate of photosynthetic electron flow was depressed at high irradiances and temperatures that were supraoptimal for net photosynthesis. It is concluded that the photosynthetic apparatus of Crassulacean species is well adapted to moderate and chilling temperatures, which adjusts the plant metabolism to “life strategies” under conditions of cold climate.     

Low temperature effects on photosynthesis and growth of grapevine

Growth and photosynthesis of grapevine (Vitis vinifera L.) planted on two sloping cool climate vineyards were measured during the early growth season. At both vineyards, a small difference in mean minimum air temperature (1–3 °C) between two microsites accumulated over time, producing differences in shoot growth rate. The growth rates of the warmer (upper) microsite were 34–63% higher than the cooler (lower) site. Photosynthesis measurements of both east and west canopy sides revealed that the difference in carbon gain between the warmer and cooler microsites was due to low temperatures restricting the photosynthetic contribution of east‐facing leaves. East‐facing leaves at the warmer microsite experienced less time at suboptimal temperature while being exposed to high irradiance, contributing to an average 10% greater net carbon gain compared to the east‐facing leaves at the cooler microsite. This chilling‐induced reduction in photosynthesis was not due to net photo‐inhibition. Further analysis revealed that CO₂‐ and light‐saturated photosynthesis of grapevines was restricted by stomatal closure from 15 to 25 °C and by a limitation of RuBP regeneration and/or end‐product limitation from 5 to 15 °C. Changes in photosynthetic carboxylation efficiency implied that Rubisco activity may also play a regulatory role at all temperatures. This restriction of total photosynthetic carbon gain is proposed to be a major contributor to the temperature dependence of growth rate at both vineyards during the early season growth period.     

Photosynthetic acclimation to simultaneous and interacting environmental stresses along natural light gradients: optimality and constraints

There is a strong natural light gradient from the top to the bottom in plant canopies and along gap-understorey continua. Leaf structure and photosynthetic capacities change close to proportionally along these gradients, leading to maximisation of whole canopy photosynthesis. However, other environmental factors also vary within the light gradients in a correlative manner. Specifically, the leaves exposed to higher irradiance suffer from more severe heat, water, and photoinhibition stresses. Research in tree canopies and across gap-understorey gradients demonstrates that plants have a large potential to acclimate to interacting environmental limitations. The optimum temperature for photosynthetic electron transport increases with increasing growth irradiance in the canopy, improving the resistance of photosynthetic apparatus to heat stress. Stomatal constraints on photosynthesis are also larger at higher irradiance because the leaves at greater evaporative demands regulate water use more efficiently. Furthermore, upper canopy leaves are more rigid and have lower leaf osmotic potentials to improve water extraction from drying soil. The current review highlights that such an array of complex interactions significantly modifies the potential and realized whole canopy photosynthetic productivity, but also that the interactive effects cannot be simply predicted as composites of additive partial environmental stresses. We hypothesize that plant photosynthetic capacities deviate from the theoretical optimum values because of the interacting stresses in plant canopies and evolutionary trade-offs between leaf- and canopy-level plastic adjustments in light capture and use.     

Response of Tradescantia albiflora to growth irradiance: Change versus changeability

Most chloroplasts undergo changes in composition, function and structure in response to growth irradiance. However, Tradescantia albiflora, a facultative shade plant, is unable to modulate its light-harvesting components and has the same Chl a/Chl b ratios and number of functional PS II and PS I reaction centres on a Chl basis at all growth irradiances. With increasing growth irradiance, Tradescantia leaves have the same relative amount of chlorophyll—proteins of PS II and PS I, but increased xanthophyll cycle components and more zeaxanthin formation under high light. Despite high-light leaves having enhanced xanthophyll cycle content, all Tradescantia leaves acclimated to varying growth irradiances have similar non-photochemical quenching. These data strongly suggest that not all of the zeaxanthin formed under high light is necessarily non-covalently bound to major and minor light-harvesting proteins of both photosystems, but free zeaxanthin may be associated with LHC II and LHC I or located in the lipid bilayer. Under the unusual circumstances in light-acclimated Tradescantia where the numbers of functional PS II and PS I reaction centres and their antenna size are unaltered during growth under different irradiances, the extents of PS II photoinactivation by high irradiances are comparable. This is due to the extent of PS II photoinactivation being a light dosage effect that depends on the input (photon exposure, antenna size) and output (photosynthetic capacity, non-radiative dissipation) parameters, which in Tradescantia are not greatly varied by changes in growth irradiance.This revised version was published online in October 2005 with corrections to the Cover Date.     

Stimulation of chlororespiration by heat and high light intensity in oat plants

High irradiance and moderate heat inhibit the activity of the photosynthetic apparatus of oat (Avena sativa L.) leaves. The incubation of oat leaves under high light intensity in conjunction with high temperatures strongly decreased the maximal quantum yield of photosystem (PS) II, indicating the close synergistic effect of both stress factors on PS II inhibition and the subsequent irreversible damage to the photosynthetic apparatus. The PS I A/B protein levels remained similar to control values in leaves incubated under high light intensity or moderate heat, and decreased only when both stress factors were simultaneously applied. Immunoblot analysis of thylakoid membranes using specific antibodies raised against the NDH-K subunit of the thylakoidal NADH dehydrogenase complex (NADH DH) and against plastid terminal oxidase (PTOX) revealed an increase in the amount of both proteins in response to high light intensity and/or heat treatments. In addition, these stress treatments were seen to stimulate the activity of electron donation by NADPH and ferredoxin to plastoquinone, the PTOX activity in plastoquinone oxidation and the NADH DH activity in thylakoid membranes. Incubation with n-propyl gallate (an inhibitor of PTOX) inhibited the increase of NDH-K and PTOX levels under high light intensity and heat, and slightly stimulated the activity of electron donation by NADPH and ferredoxin to plastoquinone. Antimycin A (an inhibitor of cyclic electron flow) increased the NADH DH activity and preserved the levels of NDH-K and PTOX in thylakoid membranes from leaves incubated under high light intensity and heat. The up-regulation of the PTOX and the thylakoidal NADH DH complex under these stress conditions supports a role for chlororespiration in the protection against high irradiance and moderate heat.