Source-to-sink relationship between green leaves and green petals of different ages of the CAM orchid <Emphasis Type="Italic">Dendrobium</Emphasis> cv. Burana Jade

Photosynthetic capacities of green leaves (GL) and green flower petals (GFP) of different ages of the CAM plant Dendrobium cv. Burana Jade were studied through chlorophyll (Chl) content, Chl fluorescence characteristic F_v/F_m, maximal photosynthetic O₂ evolution rates (P _max), and CAM acidities [dawn/dusk fluctuations in titratable acidity (TA)]. All these photosynthetic parameters were higher in GL than in GFP. Among the different ages of GFP, the young GFP had significant higher readings of all photosynthetic parameters than the oldest GFP, indicating that reduced photosynthesis occurred in the senesced GFP. The source-to-sink relationship between GL and GFP was also studied by comparing the diurnal changes in contents of total soluble and insoluble sugars and TA between the fully irradiated (FI) control (with both irradiated GL and GFP) and GL-darkened plants (covering all GL with aluminium foils, leaving only the GFP exposed to radiation). CAM acidities were much lower in GL darkened with aluminium foils compared to those of FI-GL while there were no differences in CAM acidities of their GFP. The contents of total soluble and insoluble sugars and the CAM acidities of GL towards the end of the day were lower in GL-darkened plants compared to that of FI-plants. Hence CAM acidities of GL depended on their saccharide contents. However, diurnal changes of TA in GFP were similar in all GFP regardless of their ages, with or without GL photosynthetic sources. Thus CAM acidities of GFP are independent of GL saccharides. However, lower saccharide content in GFP (especially the oldest GFP) of GL-darkened plants implies that GFP function as sinks and depend on saccharides exported from GL for its development and growth.     

Susceptibility of green leaves and green flower petals of CAM orchid <Emphasis Type="Italic">Dendrobium</Emphasis> cv. Burana Jade to high irradiance under natural tropical conditions

Photosynthetic rates of green leaves (GL) and green flower petals (GFP) of the CAM plant Dendrobium cv. Burana Jade and their sensitivities to different growth irradiances were studied in shade-grown plants over a period of 4 weeks. Maximal photosynthetic O₂ evolution rates and CAM acidities [dawn/dusk fluctuations in titratable acidity] were higher in leaves exposed to intermediate sunlight [a maximal photosynthetic photon flux density (PPFD) of 500–600 μmol m⁻² s⁻¹] than in leaves grown under full sunlight (a maximal PPFD of 1 000–1 200 μmol m⁻² s⁻¹) and shade (a maximal PPFD of 200–250 μmol m⁻² s⁻¹). However, these two parameters of GFP were highest in plants grown under the shade and lowest in full sun-grown plants. Both GL and GFP of plants exposed to full sunlight had lower predawn F_v/F_m [dark adapted ratio of variable to maximal fluorescence (the maximal photosystem 2 yield without actinic irradiation)] than those of shade-grown plants. When exposed to intermediate sunlight, however, there were no significant changes in predawn F_v/F_m in GL whereas a significant decrease in predawn F_v/F_m was found in GFP of the same plant. GFP exposed to full sunlight exhibited a greater decrease in predawn F_v/F_m compared to those exposed to intermediate sunlight. The patterns of changes in total chlorophyll (Chl) content of GL and GFP were similar to those of F_v/F_m. Although midday F_v/F_m fluctuated with prevailing irradiance, changes of midday F_v/F_m after exposure to different growth irradiances were similar to those of predawn F_v/F_m in both GL and GFP. The decreases in predawn and midday F_v/F_m were much more pronounced in GFP than in GL under full sunlight, indicating greater sensitivity in GFP to high irradiance (HI). In the laboratory, electron transport rate and photochemical and non-photochemical quenching of Chl fluorescence were also determined under different irradiances. All results indicated that GFP are more susceptible to HI than GL. Although the GFP of Dendrobium cv. Burana Jade require a lower amount of radiant energy for photosynthesis and this plant is usually grown in the shade, is not necessarily a shade plant.     

Differences in photosynthetic characteristics and accumulation of osmoprotectants in saplings of evergreen plants grown inside and outside a glasshouse during the winter season

Photochemical efficiency, photosynthetic capacity, osmoprotectants, and relative water content (RWC) were recorded in saplings of two evergreen plants (Boehmeria rugulosa Wedd. and Olea glandulifera Wall. ex G. Don) grown inside (GL) and outside (OP) a glasshouse during the winter season. The OP plants experienced 2.0–2.5 °C lower air temperature and dew formation in comparison to GL plants. Diurnal observations indicated no change in RWC in the leaves of GL and OP plants, while significant reduction in both transpiration and net photosynthetic (P _N) rates was observed in OP plants: the reduction in P _N was much more prominent as was also reflected by poor water use efficiency of these plants. Similarly, OP plants also showed decrease in the apparent quantum yield and irradiance-saturated CO₂ assimilation rate. The decrease in P _N was not associated with decreased stomatal conductance. However, a significant reduction in the ratio of variable to maximum chlorophyll (Chl) fluorescence (F_v/F_m) and Chl content was recorded in the OP plants which also contained more total soluble saccharides but less proline contents. The greater enhancement of P _N at 15 °C in comparison to measurements taken at 10 °C in OP plants over GL plants probably indicated an increase in mesophyll capacity of the OP plants’ growth at increased temperature. Hence the enhanced growth and productivity of plants grown in sheltered environments could be associated to their higher photosynthetic activity that may have important bearing on their field establishment and productivity in the long run. The response varied with plant species; reduction in P _N was greater in B. rugulosa than in O. glandulifera. However, the recovery of OP plants in terms of F_v/F_m in the subsequent months revealed that photosynthetic system of these plants is revocable.     

Responses of leaf photosynthesis,pigments and chlorophyll fluorescence within canopy position in a boreal grass (<Emphasis Type="Italic">Phalaris arundinacea</Emphasis> L.) to elevated temperature and CO<Subscript>2</Subscript> under varying water regimes

The effects of elevated growth temperature (ambient + 3.5°C) and CO₂ (700 μmol mol⁻¹) on leaf photosynthesis, pigments and chlorophyll fluorescence of a boreal perennial grass (Phalaris arundinacea L.) under different water regimes (well watered to water shortage) were investigated. Layer-specific measurements were conducted on the top (younger leaf) and low (older leaf) canopy positions of the plants after anthesis. During the early development stages, elevated temperature enhanced the maximum rate of photosynthesis (P _max) of the top layer leaves and the aboveground biomass, which resulted in earlier senescence and lower photosynthesis and biomass at the later periods. At the stage of plant maturity, the content of chlorophyll (Chl), leaf nitrogen (N_L), and light response of effective photochemical efficiency (Φ_PSII) and electron transport rate (ETR) was significantly lower under elevated temperature than ambient temperature in leaves at both layers. CO₂ enrichment enhanced the photosynthesis but led to a decline of N_L and Chl content, as well as lower fluorescence parameters of Φ_PSII and ETR in leaves at both layers. In addition, the down-regulation by CO₂ elevation was significant at the low canopy position. Regardless of climate treatment, the water shortage had a strongly negative effect on the photosynthesis, biomass growth, and fluorescence parameters, particularly in the leaves from the low canopy position. Elevated temperature exacerbated the impact of water shortage, while CO₂ enrichment slightly alleviated the drought-induced adverse effects on P _max. We suggest that the light response of Φ_PSII and ETR, being more sensitive to leaf-age classes, reflect the photosynthetic responses to climatic treatments and drought stress better than the fluorescence parameters under dark adaptation.     

Growth,photosynthesis, and metabolism of sugar beet at an early stage of exposure to elevated CO<Subscript>2</Subscript>

The effects of CO₂ concentration (C _a) on growth, photosynthesis, and the activity of enzymes associated with the translocation and assimilation of CO₂ were studied in sugar beet (Beta vulgaris L. subsp. saccharifera, cv. Ramonskaya) plants. The plants were grown in controlled-climate chamber to the stage of 3–4 leaves and then used in experiments. Experimental plants were exposed in boxes to doubled C _a (700 µl/l, 2C plants), whereas control plants were kept in a chamber with ambient atmosphere (350 µl/l, 1C plants). As compared with 1C plants, in 3 and 8 days, the leaf area of 2C plants increased by 14 and 26%, respectively. The rate of their photosynthesis (P _n) measured in 3, 6, and 8 days increased by 85, 47, and 52%, respectively, whereas in normal air, the values of P _n in 2C plants were by 12, 19, and 15% lower than in 1C plants. After 8-day growth, the content of soluble carbohydrates in the leaves of 2C plants attained 7.2%, being by 80% greater than in 1C plants; the content of starch did not exceed 3%. The total content of chlorophylls a and b in the leaves of 2C plants was by 14% greater than in 1C plants, but their ratio was essentially the same. The level of protein in 2C plants was by 13.4% lower than in 1C plants. The activity and content of Rubisco in 1C and 2C plants were similar. As compared with 1C plants, in 2C plants the activity of soluble carbonic anhydrase (sCA) was lower by 34% in 3 days and by 18% in 8 days; the activity of carbonic anhydrase of membrane preparations (mCA), was lower by 24 and 77%, respectively. Catalase activity in 2C plants became by 8% lower than in 1C plants only after 8 days. A reduction in the photosynthetic ability of 2C plants in ambient atmosphere, a decrease in activity of sCA and, especially, of mCA observed together with invariable activity and content of Rubisco in the leaf extracts are interpreted as early symptoms of acclimation of young plants of sugar beet to elevated CO₂.Translated from Fiziologiya Rastenii, Vol. 52, No. 2, 2005, pp. 184–190.Original Russian Text Copyright © 2005 by Ignatova, Novichkova, Mudrik, Lyubimov, Ivanov, Romanova.This revised version was published online in April 2005 with a corrected cover date.     

Photosynthetic performance and light response of two olive cultivars under different water and light regimes

The olive tree (Olea europaea L.) is commonly grown in the Mediterranean area, where it is adapted to resist periods characterized by severe drought and high irradiance levels. Photosynthetic efficiency (in terms of F_v/F_m and Φ_PSII), photochemical (q_P) and nonphotochemical quenching (NPQ) were determined in two-year-old olive plants (cultivars Coratina and Biancolilla) grown under two different light levels (exposed plants, EP, and shaded plants, SP) during a 21-day controlled water deficit. After reaching the maximum level of drought stress, plants were rewatered for 23 days. During the experimental period, measurements of gas exchange and chlorophyll (Chl) fluorescence were carried out to study the photosynthetic performance of olive plants. The synergical effect of drought stress and high irradiance levels caused a reduction of gas exchange and photosynthetic efficiency and these decreases were more marked in EP. EP showed a higher degree of photoinhibition, a higher NPQ and a lower q_P if compared to SP. Coratina was more sensitive to high light and drought stress but also showed a slower recovery during rewatering, whereas Biancolilla showed a less marked photosynthesis depression during drought and a considerable resilience during rewatering. The results confirm that photoinhibition due to high light intensity and water deficit can be an important factor that affects photosynthetic productivity in this species.     

CO<Subscript>2</Subscript> sequestration in plants: lesson from divergent strategies

Most organisms inhabiting earth feed directly or indirectly on the products synthesized by the reaction of photosynthesis, which at the current atmospheric CO₂ levels operates only at two thirds of its peak efficiency. Restricting the photorespiratory loss of carbon and thereby improving the efficiency of photosynthesis is seen by many as a good option to enhance productivity of food crops. Research during last half a century has shown that several plant species developed CO₂-concentrating mechanism (CCM) to restrict photorespiration under lower concentration of available CO₂. CCMs are now known to be operative in several terrestrial and aquatic plants, ranging from most advanced higher plants to algae, cyanobacteria and diatoms. Plants with C₄ pathway of photosynthesis (where four-carbon compound is the first product of photosynthesis) or crassulacean acid metabolism (CAM) may consistently operate CCM. Some plants however can undergo a shift in photosynthetic metabolism only with change in environmental variables. More recently, a shift in plant photosynthetic metabolism is reported at high altitude where improved efficiency of CO₂ uptake is related to the recapture of photorespiratory loss of carbon. Of the divergent CO₂ assimilation strategies operative in different oraganisms, the capacity to recapture photorespiratory CO₂ could be an important approach to develop plants with efficient photosynthetic capacity.     

A comparative analysis of photosynthetic characteristics of hulless barley at two altitudes on the Tibetan Plateau

To determine the photosynthetic characteristics of C₃ plants and their sensitivity to CO₂ at different altitudes on the Tibetan Plateau, hulless barley (Hordeum vulgare L. ssp. vulgare) was grown at altitudes of 4,333 m and 3,688 m. Using gas-exchange measurements, photosynthetic parameters were simulated, including the maximum net photosynthesis (P _max) and the apparent quantum efficiency (α). Plants growing at higher altitude had higher net photosynthetic rates (P _N), photosynthesis parameters (P _max and α) and sensitivities to CO₂ enhancement than plants growing at lower altitude on the Tibetan Plateau. The enhancements of P _N, P _max, and α for plants growing at higher altitude, corresponding with 10 μmol(CO₂) mol⁻¹ increments, were approximately 0.20∼0.45%, 0.05∼0.20% and 0.12∼0.36% greater, respectively, than for plants growing at lower altitude, respectively, where CO₂ levels rose from 10 to 170 μmol(CO₂) mol⁻¹. Therefore, on the Tibetan Plateau, the changes in the photosynthetic capacities and the photosynthetic sensitivities to CO₂ observed in the C₃ plants grown above 3,688 m are likely to increase with altitude despite the decreasing CO₂ partial pressure.     

Higher photosynthesis,nutrient‐ and energy‐use efficiencies contribute to invasiveness of exotic plants in a nutrient poor habitat in northeast China

The roles of photosynthesis‐related traits in invasiveness of introduced plant species are still not well elucidated, especially in nutrient‐poor habitats. In addition, little effort has been made to determine the physiological causes and consequences of the difference in these traits between invasive and native plants. To address these problems, we compared the differences in 16 leaf functional traits related to light‐saturated photosynthetic rate (P_max) between 22 invasive and native plants in a nutrient‐poor habitat in northeast China. The invasive plants had significantly higher P_max, photosynthetic nitrogen‐ (PNUE), phosphorus‐ (PPUE), potassium‐ (PKUE) and energy‐use efficiencies (PEUE) than the co‐occurring natives, while leaf nutrient concentrations, construction cost (CC) and specific leaf area were not significantly different between the invasive and native plants. The higher PNUE contributed to higher P_max for the invasive plants, which in turn contributed to higher PPUE, PKUE and PEUE. CC changed independently with other traits such as P_max, PNUE, PPUE, PKUE and PEUE, showing two trait dimensions, which may facilitate acclimation to multifarious niche dimensions. Our results indicate that the invasive plants have a superior resource‐use strategy, i.e. higher photosynthesis under similar resource investments, contributing to invasion success in the barren habitat.     

Photosynthesis,chlorophyll fluorescence,and antioxidant enzyme responses of invasive weed <Emphasis Type="Italic">Mikania micrantha</Emphasis> to <Emphasis Type="Italic">Bemisia tabaci</Emphasis> infestation

In a glasshouse, Bemisia tabaci infestation largely reduced response of photosynthesis to irradiance and CO₂ concentration of Mikania micrantha compared with the non-infested control (C) ones. The maximum irradiance-saturated photosynthetic rate (P _max) and saturation irradiance (SI) of the infested M. micrantha were only 21.3 % and 6.5 % of the C-plants, respectively. B. tabaci infestation led to the reduction of contents of chlorophyll and carotenoids in M. micrantha, which was accompanied with the decrease of actual photosystem 2 (PS2) efficiency (Φ_PS2), efficiency of excitation energy capture by open PS2 reaction centres (F_v′/F_m′), electron transport rate (ETR), and photochemical quenching (q_P). Moreover, superoxide dismutase and catalase activities significantly decreased while proline and glutathione contents significantly increased in infested M. micrantha. Hence B. tabaci infestation not only induced direct damage of photosynthetic apparatus but also altered the antioxidant enzymes activities in M. micrantha, which might as consequences accelerate senescence of this weed.     

Effect of panicle removal on photosynthetic acclimation under elevated CO<Subscript>2</Subscript> in rice

To examine the role of sink size on photosynthetic acclimation under elevated atmospheric CO₂ concentrations ([CO₂]), we tested the effects of panicle-removal (PR) treatment on photosynthesis in rice (Oryza sativa L.). Rice was grown at two [CO₂] levels (ambient and ambient + 200 μmol mol⁻¹) throughout the growing season, and at full-heading stage, at half the plants, a sink-limitation treatment was imposed by the removal of the panicles. The PR treatment alleviated the reduction of green leaf area, the contents of chlorophyll (Chl) and Rubisco after the full-heading stage, suggesting delay of senescence. Nonetheless, elevated [CO₂] decreased photosynthesis (measured at current [CO₂]) of plants exposed to the PR treatment. No significant [CO₂] × PR interaction on photosynthesis was observed. The decrease of photosynthesis by elevated [CO₂] of plants was associated with decreased leaf Rubisco content and N content. Leaf glucose content was increased by the PR treatment and also by elevated [CO₂]. In conclusion, a sink-limitation in rice improved N status in the leaves, but this did not prevent the photosynthetic down-regulation under elevated [CO₂].     

Moderate water stress causes different stomatal and non‐stomatal changes in the photosynthetic functioning of Phaseolus vulgaris L. genotypes

The impact of moderate water deficit on the photosynthetic apparatus of three Phaseolus vulgaris L. cultivars, Plovdiv 10 (P10), Dobrudjanski Ran (DR) and Prelom (Prel), was investigated. Water shortage had less impact on leaf hydration, RWC (predawn and midday) and predawn water potential in Prel. RWC and Ψ_p were more reduced in P10, while there was no osmotic adjustment in any cultivar. Although drought drastically reduced stomatal opening in P10 and DR, reduced A_max indicated non‐stomatal limitations that contributed to the negligible P_n. These limitations were on potential thylakoid electron transport rates of PSI and II, pointing to photosystem functioning as a major limiting step in photosynthesis. This agrees with decreases in actual photochemical efficiency of PSII (F_v′/F_m′), quantum yield of photosynthetic non‐cyclic electron transport (?_e) and energy‐driven photochemical events (q_P), although the impact on these parameters would also include down‐regulation processes. When compared to DR, Prel retained a higher functional state of the photosynthetic machinery, justifying reduced need for photoprotective mechanisms (non‐photochemical quenching, zeaxanthin, lutein, β‐carotene) and maintenance of the balance between energy capture and dissipative pigments. The highest increases in fructose, glucose, arabinose and sorbitol in Prel might be related to tolerance to a lower oxidative state. All cultivars had reduced A_max due to daytime stomatal closure in well‐watered conditions. Under moderate drought, Prel had highest tolerance, higher leaf hydration and maintenance of important photochemical use of energy. However, water shortage caused appreciable non‐stomatal limitations to photosynthesis linked to regulation/imbalance at the metabolic level (and growth) in all cultivars. This included damage, as reflected in decreased potential photosystem functioning, pointing to higher sensitivity of photosynthesis to drought than is commonly assumed.     

Innate and evolutionarily increased advantages of invasive <Emphasis Type="Italic">Eupatorium adenophorum</Emphasis> over native <Emphasis Type="Italic">E. japonicum</Emphasis> under ambient and doubled atmospheric CO<Subscript>2</Subscript> concentrations

Both innate and evolutionarily increased ecophysiological advantages can contribute to vigorous growth, and eventually to invasiveness of alien plants. Little effort has been made to explore the roles of innate factors of alien plants in invasiveness and the effects of CO₂ enrichment on alien plant invasions. To address these problems, we compared invasive Eupatorium adenophorum, its native conspecific, and a native congener (E. japonicum) under ambient and doubled atmospheric CO₂ concentrations. Native E. adenophorum from Mexico grew slower than invasive E. adenophorum but faster than native E. japonicum under both CO₂ concentrations. The faster growth rate of invasive E. adenophorum was associated with higher photosynthetic capacity and leaf area ratio. For invasive E. adenophorum, the higher photosynthetic capacity was associated with higher nitrogen (N) allocation to photosynthesis, which was related to lower leaf mass per area; the higher leaf area ratio was due to lower leaf mass per area and higher leaf mass fraction. Tradeoff between N allocations to photosynthesis versus defenses was found. CO₂ enrichment significantly increased relative growth rate and biomass accumulation by increasing actual photosynthetic rate for all studied materials. However, the relative increase in growth was not significantly different among them. CO₂ enrichment did not influence N allocation to photosynthesis, but increased N allocation to cell walls. The reduced leaf N content decreased N content in photosynthesis, explaining the down-regulation of photosynthetic capacity under prolonged elevated CO₂ concentration. Our results indicate that both innate and evolutionary advantages in growth and related ecophysiological traits contribute to invasiveness of invasive E. adenophorum, and CO₂ enrichment may not aggravate E. adenophroum’s invasion in the future.     

Effect of long-term drought stress on leaf gas exchange and fluorescence parameters in C<Subscript>3</Subscript> and C<Subscript>4</Subscript> plants

A field study was performed on triticale, field bean, maize and amaranth, to find differences between studied species in physiological alterations resulting from progressive response as injuries and/or acclimation to long-term soil drought during various stages of plant development. The measurements of leaf water potential, electrolyte leakage, chlorophyll a fluorescence, leaf gas exchange and yield analysis were done. A special emphasis was given to the measurements of the blue, green, red and far-red fluorescence. Beside, different ratios of the four fluorescence bands (red/far-red: F ₆₉₀/F ₇₄₀, blue/red: F ₄₄₀/F ₆₉₀, blue/far-red: F ₄₄₀/F ₇₄₀ and blue/green: F ₄₄₀/F ₅₂₀) were calculated. Based on both yield analysis and measurements of physiological processes it can be suggested that field bean and maize responded with better tolerance to the water deficit in soil due to the activation of photoprotective mechanism probably connected with synthesis of the phenolic compounds, which can play a role of photoprotectors in different stages of plant development. The photosynthetic apparatus of those two species scattered the excess of excitation energy more effectively, partially through its transfer to PS I. In this way, plants avoided irreversible and/or deep injuries to PS II. The observed changes in the red fluorescence emission and in the F _v/F _m for triticale and amaranth could have occurred due to serious and irreversible photoinhibitory injuries. Probably, field bean and maize acclimatized more effectively to soil drought through the development of effective mechanisms for utilising excitation energy in the photosynthetic conversion of light accompanied by the mechanism protecting the photosynthetic apparatus against the excess of this energy.     

Slow fluorescence induction and CO<Subscript>2</Subscript> exchange in bean plants treated with <Emphasis Type="Italic">Reynoutria sachalinensis</Emphasis> extract

Treatment of bean seedlings with a water extract of Reynoutria sachalinensis (giant knotweed) increases visible photosynthesis and the dark respiration of plants. The increase in CO₂ uptake under intense illumination (10 000 lx) correlates with an increase in the fluorescence parameter (F _M − F _T)/F _T.     

Photosynthetic capacity,photochemical efficiency and chlorophyll content of three varieties of <Emphasis Type="Italic">Labisia pumila</Emphasis> Benth. exposed to open field and greenhouse growing conditions

Three varieties (Alata, Pumila and Lanceolata) of Malaysian indigenous medicinal herb Labisia pumila Benth. grown in greenhouse and open field were tested to evaluate and compare their photosynthetic and maximal quantum efficiency of photosystem II (F _v/F _m) reactions. Every variety grown in greenhouse demonstrated higher light-saturated photosynthetic capacity than in the open field. The diurnal net photosynthesis (A) curve in the open field also displayed dual peaks with lower daily average A compared to the greenhouse. Varieties Alata and Pumila were found to acclimatize better under both growing conditions. The diurnal patterns of F _v/F _m indicated that plants grown under greenhouse encountered less photoinhibition than in open field condition. A decrease in chlorophyll (chl) a/b ratio in leaves of greenhouse plants with significant increase in chl b was observed. This study indicates that var. Alata and var. Pumila have the capacity to acclimatize to greenhouse growth condition.     

Photosynthetic responses of radish (<Emphasis Type="Italic">Raphanus sativus</Emphasis> var. <Emphasis Type="Italic">longipinnatus</Emphasis>) plants to infection by turnip mosaic virus

Plant growth, chlorophyll (Chl) content, photosynthetic gas exchange, ribulose-1,5-bisphosphate carboxylase (RuBPCO) enzyme activity, and Chl fluorescence in radish (Raphanus sativus var. longipinnatus) plants were examined after turnip mosaic virus (TuMV) infection. Plant fresh mass, dry mass, Chl content, net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), and RuBPCO activity were significantly lower in infected plants after 5 weeks of virus infection as compared to healthy plants. The 5-week virus infection did not induce significant differences in intercellular CO₂ concentration (C _i, photochemical efficiency of photosystem 2, PS2 (F_v/F_m), excitation capture efficiency of open PS2 reaction centres (F_v'/F_m'), effective quantum efficiency of photosystem 2 (ΔF/F_m'), and photochemical quenching (q_P), but non-photochemical quenching (q_N) and alternative electron sink (AES) were significantly enhanced. Thus the decreased plant biomass of TuMV-infected plants might be associated with the decreased photosynthetic activity mainly due to reduced RuBPCO activity.     

Activity of the photosynthetic apparatus at periodic elevation of CO<Subscript>2</Subscript> concentration

We studied the influence of prolonged (a few weeks) and short-term (a few hours) periodical elevation of the ambient CO₂ concentration ([C_a]) on the photosynthetic apparatus and carbohydrate content in the third leaf of three-week-old cucumber (Cucumis sativus L.) plants. On the basis of experimental data and subsequent modeling, we revealed the limiting processes in the photosynthetic apparatus functioning: Rubisco activity, the rate of ribulose bisphosphate (CO₂ acceptor) regeneration, the rate of triose phosphate utilization in the Calvin cycle, and the influence of stomata on the photosynthesis rate. An increase in soluble carbohydrate content and a decrease in starch accumulation at a short-term [C_a] elevation indicate an important role of carbohydrate accumulation and their partition between organs in the regulation of the photosynthesis. We concluded that periodic [C_a] elevation can be used to improve plant productivity.     

Light‐induced systemic signalling down‐regulates photosynthetic performance of soybean leaves with different directional effects

• When plants are exposed to a heterogeneous environment, photosynthesis of leaves is not only determined by their local condition, but also by certain signals from other parts of the same plant, termed systemic regulation. Our present study was conducted to investigate the effects of light‐dependent systemic regulation on the photosynthetic performance of soybean (Glycine max L. Merr.) under heterogeneous light conditions.
• Soybean plants were treated with heterogeneous light. Then gas exchange characteristics were measured to evaluate the photosynthetic performance of leaves. Parameters related to photosynthetic pigments, chlorophyll fluorescence, Rubisco and photosynthates were examined to study the mechanisms of light‐dependent systemic regulation on photosynthesis.
• Light‐induced systemic signalling by illuminated leaves reduced the P_n of both upper and lower non‐illuminated leaves on the same soybean plant. The decrease in g_s and increase in C_i in these non‐illuminated leaves indicated restriction of carbon assimilation, which was further verified by the decline in content and activity of Rubisco. However, the activation state of Rubisco decreased only in upper non‐illuminated leaves. Quantum efficiency of PSII (ΦPSII) and ETR also decreased only in upper non‐illuminated leaves. Moreover, the effects of light‐induced systemic signalling on carbohydrate content were also detectable only in upper non‐illuminated leaves.
• Light‐induced systemic signalling by illuminated leaves restricts carbon assimilation and down‐regulates photosynthetic performance of non‐illuminated leaves within a soybean plant. However, effects of such systemic regulation differed when regulated in upward or downward direction.

     

Functional responses of Acer species to two simulated forest gap environments: leaf-level properties and photosynthesis

Seedlings of eight forest maple (Acer L.) species were grown outdoors through a full season under two irradiation treatments: (a) “gap edge” with a photosynthetic photon flux density of 30 μmol m-² s-¹ and a red:far-red ratio of 0.55, and (b) “gap centre” with 400 μmol m-² s-¹ and a red:far-red ratio of 1.12. Area-based leaf nitrogen concentration was greater in gap centre-grown seedlings, whereas, except for A. saccharum, area-based chlorophyll (Chl) (a+b) was higher in gap edge-grown plants. There was also a significantly lower Chl a/b ratio in gap edge-grown plants. Maximum photosynthetic rate (P _max ) was 60 % higher in the gap-centre treatment. These results are consistent with the functional expectation that shade-acclimated plants will increase their radiant-energy harvesting capacity as a result of limited photon input while gap-acclimated plants will operate more efficiently under bright irradiance by increasing their carboxylation capacity. This inverse relationship between the capacity of the light-harvesting component and the carboxylation component is, however, only partially supported by Chl fluorescence measurements of intact leaves. Compared to gap centre-grown plants, the lower total fluorescence quenching in gap edge-grown plants indicated a lower carboxylation capacity that was in accord with the observed P _max . However, edge-grown seedlings did not show the expected improvement in light-harvesting efficiency and reduction in electron transport of photosystem 2 inferred from their marginally greater t_1/2 and lower F_v/F_m, respectively. Hence while maples acclimated to different irradiation levels by adjusting leaf N and Chl contents, they showed limited acclimation potential at the photosystem level. Variations in the leaf traits examined had only minor effect on low irradiance photosynthesis and sunfleck utilization. This revised version was published online in August 2006 with corrections to the Cover Date.