Quantum Efficiency Of Photosynthesis Research Articles

Exogenous application of salicylic acid mitigates salt stress in rice seedlings by regulating plant water status and preventing oxidative damage

Salicylic acid (SA) is a hormone participating in the acclimation of plants to biotic and abiotic stresses, including salinity. We investigated the possible underlying mechanism of mitigating salt stress by SA using NaCl-treated rice plants sprayed twice with exogenous SA at different concentrations. SA application resulted in increased growth, relative water content, proline accumulation, the quantum efficiency of photosynthesis and activity of superoxide dismutase in NaCl-treated plants. Application of SA decreased Na+ concentration and increased K+ concentration, thus increasing the K+/Na+ ratio. The application of SA mitigated the effect of NaCl by improving plant water status, ion homeostasis and decreased oxidative damage. Foliar application of 0.5 mM SA was more effective in mitigating the salt stress while 2 mM SA was inhibitory, and the second spray of SA showed no significantly enhanced ameliorating effect over the first spray.

Trocas gasosas e requerimento em área foliar para rendimento de bananeiras Cavendish e Gros Michel

Abstract The objective was to evaluate gas exchange and the leaf area requirement for yield in Cavendish and Gros Michel bananas. The cultivars Grande Naine, Calipso, Bucaneiro, FHIA-17 and FHIA-23, and two production cycles, were arranged in a completely randomized experimental design, in a factorial scheme, with five replicates. For the physiological characteristics, a 5x9x2 factorial arrangement was considered, with cultivars, evaluation periods and reading times. Cultivars Bucaneiro and Calipso require larger leaf area per unit produced and FHIA-23, smaller, which suggests a greater photosynthetic efficiency per leaf unit for FHIA-23, confirmed by the higher masses of bunches and hands and greater numbers of hands and fruits. Gas exchange varies with cultivars, evaluation periods and reading times. Cultivars FHIA-17 and Calipso, Gros Michel type, respectively showed higher photosynthesis and transpiration rates than Grande Naine, Cavendish type. Net photosynthesis, stomatal conductance, quantum efficiency of photosynthesis, instantaneous water use efficiency and carboxylation efficiency are higher at 8:00 h than at 14:00 h. The month of June allowed the cultivars to express the highest values of photosynthesis rates, incident radiation on leaves, carboxylation efficiency and stomatal conductance.

English

The aim of this paper is to evaluate gas exchanges and yield of Prata-type banana plants subjected to fertilizer sources for organic management in soil with improved fertility. Two cultivars (‘Dwarf-Prata’ and ‘BRS Platina’), 5 K2O rates (0, 200, 400, 600, and 800 kg ha-1 year-1) supplied by cattle manure and Naturalplus® rock powder, and 2 evaluation times (8:00 am and 2:00 pm) were arranged in a randomized complete block design with 15 replicates. As for production, a randomized block design with 4 cycles and 3 replicates was used. In fertile soils, gas exchanges are little influenced by fertilization. Fertilization with increasing application rates determines a quadratic variation in stomatal conductance and internal CO2 concentration in ‘Dwarf-Prata’ and a linear variation for leaf temperature and instantaneous water-use efficiency in ‘BRS Platina’. Quantum efficiency of photosynthesis is higher at 8:00 am, whereas at 2:00 pm, for leaf temperature. Photosynthesis and leaf transpiration directly correlate with stomatal conductance, instantaneous water-use efficiency, and leaf temperature, while instantaneous water-use efficiency and photosynthesis inversely associate with transpiration and leaf temperature. Cattle manure and rock powder fertilizations do not increase yield in banana plants grown in soils with improved fertility. Key words: Musa species, manure, rock powder, productivity, physiological variables.

Gas exchange in ‘BRS Princesa’ banana (Musa spp.) under partial rootzone drying irrigation in the north of Minas Gerais, Brazil

This research aimed to evaluate physiological characteristics of 'BRS Princesa' banana under Partial Rootzone Drying irrigation, PRD. The five treatments, 1 - 50% of reduction in water depth (WD) with a frequency alternation of irrigation side (FA) of seven days during the whole production cycle; 2 - 50% of WD and FA of 14 days in the cycle; 3 - 50% of WD and FA of 21 days; 4 - 50% of WD with a fixed irrigation (only one lateral line on one side of the plant); and 5 - full irrigation (two lateral lines with open shutoff valve) throughout the cycle, they were arranged in a randomized block design with five replicates. There is a reduction in photosynthetic rates and water use efficiency as a result of enzymatic impairment or stomatal closure, due to the increased of leaf temperature. The stomatal conductance - gs was lower in plants irrigated with 50% of ETc with fixed irrigation in one side of the plant. Even with water deficit in the soil, using the PRD with 50% of ETc, Photosynthesis - A Transpiration - E; Leaf temperature - Tleaf, internal concentration of CO2 - Ci; stomatal conductance - gs; photosynthetically active radiation - Qleaf; carboxylation efficiency - A/Ci; Quantum efficiency of Photosynthesis - A/Qleaf; and instantaneous water use efficiency - WUE 'BRS Princesa' banana were similar among strategies, highlighting the possibility of use of the irrigation management technique with PRD, saving water, and no physiological changes in the plants.

Photosynthesis versus irradiance relationships in the Atlantic sector of the Southern Ocean

Eleven incubation experiments were conducted in the South Atlantic sector of the Southern Ocean to investigate the relationship between new production (ρNO–3), regenerated production (ρNH+4), and total carbon production (ρC) as a function of varying light. The results show substantial variability in the photosynthesis–irradiance (P vs E) parameters, with phytoplankton communities at stations that were considered iron (Fe)-limited showing low maximum photosynthetic capacity (PBmax) and low quantum efficiency of photosynthesis (αB) for ρNO3, but high PBmax and αB for ρNH4, with consequently low export efficiency. Results at stations likely relieved of Fe stress (associated with shallow bathymetry and the marginal ice zone) showed the highest rates of PBmax and αB for ρNO3 and ρC. To establish the key factors influencing the variability of the photosynthetic parameters, a principal components analysis was performed on P vs E parameters, using surface temperature, chlorophyll-a concentration, ambient nutrients, and an index for community size structure. Strong covariance between ambient nitrate (NO3) and αB for ρNO3 suggests that Fe and possibly light co-limitation affects the ability of phytoplankton in the region to access the surplus NO3 reservoir. However, the observed relationships between community structure and the P vs E parameters suggest superior performance by smaller-sized cells, in terms of resource acquisition and Fe limitation, as the probable driver of smaller-celled phytoplankton communities that have reduced photosynthetic efficiency and which require higher light intensities to saturate uptake. A noticeable absence in covariances between chlorophyll-a and αB, between PBmax and αB, and between temperature and αB may have important implications for primary-production models, although the absence of some expected relationships may be a consequence of the small dataset and low range of variability. However, significant relationships were observed between ambient NO3 and αB for ρNO3, and between the light-saturation parameter Ek for ρNO3 and the phytoplankton community’s size structure, which imply that Fe and light co-limitation drives access to the surplus NO3 reservoir and that larger-celled communities are more efficient at fixing NO3 in low light conditions. Although the mean PBmax results for ρC were consistent with estimates of global production from satellite chlorophyll measurements, the range of variability was large. These results highlight the need for more-advanced primary-production models that take into account a diverse range of environmental and seasonal drivers of photosynthetic responses.

IRRIGATION STRATEGIES WITH WATER DEFICIT IN ‘TOMMY ATKINS’ MANGO TREE

ABSTRACT With the limited availability of water in semi-arid regions, it is necessary that irrigation is accurate and there is high water use efficiency. This study aimed to evaluate the regulated deficit irrigation (RDI) and partial rootzone drying (PRD) in yield, water use efficiency and gas exchange of ‘Tommy Atkins’ mango tree in semi-arid conditions. The experimental design was a randomized block with seven treatments for RDI under micro sprinkler and five treatments for the PRD under drip. Treatments were applied in phases PI - early blooming to early fruit expansion, PII - early expansion to early physiologic ripening and in phase III - physiologic ripening of fruits, with application of the RDI of 100, 75 and 50 % of ETc in different combinations of phases and application of PRD 100, 80, 60 and 40 % of ETc in three phases with the partial rootzone druing at 15 days. Photosynthesis, carboxylation efficiency and quantum efficiency of photosynthesis are lower in PRD 40% ETc at 8 a.m. compared to full irrigation and PRD 80 % of ETc. Phases of expansion and fruit ripening are more appropriate for application of RDI with 50 % and 75% of ETc without loss to the crop yield of ‘Tommy Atkins’ mango tree and greater water use efficiency. The strategy of partial rootzone drying, PRD, every 15 days with 40 % of ETc, provides greater WUE.

Iron plaque decreases cadmium accumulation in Oryza sativa L. and serves as a source of iron.

Cadmium (Cd) contamination occurs in paddy soils; hence it is necessary to reduce Cd content of rice. Application and mode of action of ferrous sulphate in minimizing Cd in rice was monitored in the present study. Pot culture with Indian rice variety Swarna (MTU 7029) was maintained in Cd-spiked soil containing ferrous sulphates, which is expected to reduce Cd accumulation in rice. Responses in rhizosphere pH, root surface, metal accumulation in plant and molecular physiological processes were monitored. Iron plaque was induced on root surfaces after FeSO4 application and the amount of Fe in plaque reduced with increases in Cd in the soil. Rhizosphere pH decreased during plaque formation and became more acidic due to secretion of organic acids from the roots under Cd treatment. Moreover, iron chelate reductase activity increased with Cd treatment, but in the absence of Cd, activity of this enzyme increased in plaque-induced plants. Cd treatment caused expression of OsYSL18, whereas OsYSL15 was expressed only in roots without iron plaque. Fe content of plants increased during plaque formation, which protected plants from Cd-induced Fe deficiency and metal toxicity. This was corroborated with increased biomass, chlorophyll content and quantum efficiency of photo-synthesis among plaque-induced plants. We conclude that ferrous sulphate-induced iron plaque prevents Cd accumulation and Fe deficiency in rice. Iron released from plaque via organic acid mediated dissolution during Cd stress.

Yield, Water Use Efficiency and Physiological Characteristic of “Tommy Atkins” Mango under Partial Rootzone Drying Irrigation System

This study was conducted to evaluate the yield, water use efficiency and physiologic characteristics of “Tommy Atkins” mango under partial rootzone drying. The experiment was carried out in an orchard of mango in the irrigated perimeter of Ceraíma, BA. Five treatments and six repetitions were used with one tree per experimental plot: 01—Full Irrigation, 100% of ETc, 02—100% of ETc with frequency of alternance (FA) of 15 days, 03—80% ETc with FA of 15 days, 04—60% of ETc with FA of 15 days, 05—40% ETc with FA of 15 days. The treatments were conductedfrom the beginning of flowering to the harvest of mango in an orchard irrigated by drip. There was no significant reduction in productivity of “Tommy Atkins” mango under PRD up to 40% of ETc and PRD with 40% ETc resulted in greater water use efficiency. There was no significant difference among the averages of photosynthetically active radiation, Qleaf; leaf temperature, Tl; internal CO2 concentration, Ci; transpiration, E; stomatal conductance, gs; photosynthesis, A; carboxylation efficiency, A/Ci; quantum efficiency of photosynthesis, A/Qleaf and instantaneous water use efficiency, A/E between full irrigation and PRD with different water depth. The increase in leaf temperature causes reduction in A/Qleaf and A/E, however, it increases the transpiration. Increased gs results in higher rates of E, A and A/Ci. The use of the PRD with 40% of ETc causes no interference in gas exchange, maintains the yield and increases water use efficiency in “Tommy Atkins” mango tree.

CHARACTERIZATION OF THE PHYSIOLOGICAL RESPONSE OF THE HIGHLY-TOLERANT TOMATO CV. ‘PONCHO NEGRO’ TO SALINITY AND EXCESS BORON

The variety of tomato (Solanum lycopersicum) called ‘Poncho Negro’ by farmers represents an important source of genetic resources of agricultural interest, because it has managed to thrive at levels of salinity and excess B that other varieties of the same species find impossible. This work was conducted under controlled growth conditions in a greenhouse, evaluating and obtaining plant material that served for physiological, chemical, and biochemical determinations. Stress conditions were supplied by irrigation, using different concentrations of sodium chloride (NaCl; 75 and 150 mM) and excess boron (B; 5 and 20 mg L−1). The rate of net carbon dioxide (CO2) assimilation, the quantum efficiency of photosynthesis and the relative water content (RWC) were measured. In addition, B, sodium (Na+), potassium (K+), and calcium (Ca2+), soluble sugars, and proline were quantified. The results suggest that ‘Poncho Negro’ accumulated Na+ mainly in the roots, partly limiting its entry into the aerial parts. In addition, it should be noted that the interaction of B and salinity reduced the movement of Na+ to the leaves. The ability of cv. ‘Poncho Negro’ to minimize leaf Na+ accumulation, accumulate more leaf B than control plants, and maintain its K+ level, when grown with an excess of B, possibly allowed the observed increase in the rate of photosynthesis. In addition, these tomato plants used proline and soluble sugars as osmo-regulators under high-B and saline conditions. Under all stress conditions studied, this variety of tomato was able to regulate its water content, with RWC values of approximately 86%.

Prolonged sensitivity of immobilized thylakoid membranes in cross-linked matrix to atrazine

Freshly prepared pea thylakoid membranes were immobilized in bovine serum albumin–glutaraldehyde cross-linked matrix (BSA–GA matrix) and their stability under long term storage was analyzed by Pulse-Amplitude-Modulated (PAM) chlorophyll fluorescence and photosynthetic oxygen evolution measured by oxygen rate electrode. The thylakoid membranes stored at 4 °C showed prolonged stability in BSA–GA matrix and additional adsorption on nitrocellulose membrane filters gave them more stability. The sensitivity of the parameters of the oxygen evolution of thylakoid membranes to atrazine increased with immobilization. The half-inhibition time for oxygen evolution and quantum efficiency of photosynthesis could be prolonged to more than 15 days. These results suggest that the immobilized thylakoid membranes in BSA–GA matrix can be used as biological receptor in biosensors for a long period of time (up to 25 days) applying the proposed new method for atrazine detection by using polarographic oxygen rate electrode. This method is more sensitive, faster and easier to use than other methods for detection of herbicides based on determination of the photochemical activity of photosystem II.

Climate‐ameliorating measures influence photosynthetic gas exchange of apple leaves

AbstractSunburn has become one of the major threats to apple fruit production in South Africa and other countries with Mediterranean climate. Some climate‐ameliorating measures have been developed to control sunburn in apples. Effects of the climate‐ameliorating measures, viz. evaporative cooling, Surround® WP and shade net, on leaf gas exchange of a 5‐year‐old orchard of ‘Cripps’ Pink’ apple were investigated during hot summer days in Stellenbosch, South Africa. Evaporative cooling increased net photosynthetic rate (A) and stomatal conductance (gs) because of its lowering of leaf temperature and leaf‐to‐air vapour pressure difference (VPD). Shade net also reduced leaf temperature because of reduction in photosynthetic photon flux density (PPFD). Quantum efficiency of photosynthesis was increased under shade net to compensate for reduced PPFD. Shade net also reduced transpiration rate more than A, resulting in increased midday water‐use efficiency. The diurnal trends of A and gs in the Surround WP and control treatments were similar, indicating limited ameliorative impact of Surround WP. Furthermore, Surround WP typically reduced maximum rate of carboxylation and the light‐saturated rate of electron transport. In all treatments, A decreased by 70% when leaf temperature increased from 35°C to 40°C. In conclusion, all treatments affected leaf photosynthetic gas exchange. Evaporative cooling enhanced leaf A and gs because of distinct ameliorative effects on leaf temperature and VPD. Shade net reduced leaf temperature with no consistent effects on leaf gas exchange attributes. Surround WP had limited or no impact on leaf temperature and negatively affected leaf gas exchange attributes.

Pigment composition and photosynthetic activity of pea chlorophyll mutants

Pea chlorophyll mutants chlorotica 2004 and 2014 have been studied. The mutants differ from the initial form (pea cultivar Torsdag) in stem and leaf color (light green in the mutant 2004 and yellow-green in the mutant 2014), relative chlorophyll content (approximately 80 and 50%, respectively), and the composition of carotenoids: the mutant 2004 contains a significantly smaller amount of carotene but accumulates more lutein and violaxanthine; in the mutant 2014, the contents of all carotenoids are decreased proportionally to the decrease in chlorophyll content. It is shown that the rates of CO2 assimilation and oxygen production in the mutant chlorotica 2004 and 2014 plants are reduced. The quantum efficiency of photosynthesis in the mutants is 29-30% lower than in the control plants; in their hybrids, however, it is 1.5-2 higher. It is proposed that both the greater role of dark respiration in gas exchange and the reduced photosynthetic activity in chlorotica mutants are responsible for the decreased phytomass increment in these plants. On the basis of these results, the conclusion is drawn that the mutations chlorotica 2004 and 2014 affect the genes controlling the formation and functioning of various components of the photosynthetic apparatus.

Photosynthetic parameters in water masses in the vicinity of the Chatham rise, south pacific ocean, during late summer

The relationship between photosynthesis (P) and light (E) was determined for phytoplankton from the mixed layer and the deep chlorophyll maximum (DCM) at 23 stations off the east coast of New Zealand during February‐March 1995. The stations encompassed three water masses, Sub‐tropical, Sub‐antarctic and the Sub‐tropical Convergence Zone. The light‐saturated rate of photosynthesis (Pmax) and slope of the light‐limited portion of the curve (a) were determined using a light gradient incubator. The absorption spectra of the phytoplankton assemblages were used to calculate the maximum quantum efficiency of photosynthesis (ϕmax)‐ A higher mixed‐layer Pmax in Sub‐tropical water over Sub‐antarctic water (4.26 compared to 5.85 mg C (mg Chl.a)−1 h−1) was consistent with the temperature difference of 4°C, although there was also a taxonomic composition difference between these waters. The highest variability of P max and α, as well as the highest P max values, were seen within the STC, but ranges for all water masses overlapped. Physical conditions in the water masses were similar and no correlations between any measured physical or chemical feature of the stations and any P‐E parameter could be identified.

Carotenoid-sensitized photosynthesis: Quantum efficiency, fluorescence and energy transfer

The observation in the early 1940s that the quantum efficiency of photosynthesis in a diatom was almost the same whether incident light was absorbed by chlorophyll a or by fucoxanthol sparked subsequent investigations of the variety of chloroplast pigments and in a diversity of photosynthetic organisms. Subsequent fluorimetric measurements provided the first relevant observation on the existence of excitation energy transfer in photosynthesis. These and some other experiments prior to the classical work of Arnold and Oppenheimer [(1950) J Gen Physiol 33: 423–435] and of Duysens [(1952) Doctoral thesis, State University of Utrecht, the Netherlands] are reviewed here.

Effects of repeated parasitism byRhinanthus minoron the growth and photosynthesis of a perennial grass,Poa alpina

SUMMARYPlantlets ofPoa alpinaL. taken from a single viviparous mother plant in August 1991 were pot‐grown in an experimental garden in Abisko, Swedish Lapland. Half of the pots were infected with the annual hemiparasitic angiospermRhinanthus minorL. A proportion of the plants were harvested in August 1992: the infected grasses were less than half the mass of uninfected controls and had a higher proportion of brown (dead) tissue, but a similar partitioning of biomass into roots, stems and leaves. The grasses remaining after this harvest were divided into four groups: control (infected neither in 1992 nor in 1993 (−−)), newly infected (1993 only (−+)), not re‐infected (+–) and re‐infected (++). Total biomass accumulation by August 1993 was greatest in control grasses (−−) and least in those plants which had been infected for two consecutive years (++). Plants infected for the first time in their second year of growth (−+) showed fewer visible effects than those infected in their first year of growth (+–), but the relative biomass accumulation in 1993, when expressed per gram of dry matter existing at the end of the 1992 growing season, was least in those plants infected for the first time in 1993 ((‐ +) 6·41 g g−1) and greatest in those which had previously been infected but were relieved of infection in the second year ((+−) 12·65 g g−1). Unlike that in plants harvested in the first year of the investigation, the pattern of biomass partitioning differed between the groups of 2‐yr‐old plants depending on the year of infection. Uninfected plants (−−), and those plants infected for the first time in 1993 (−+) produced many viviparous plantlets. Those plants infected in 1992 ((+−) and (++)) showed negligible plantlet production, but had more numerous and larger leaves, which senesced more slowly and had higher nitrogen concentrations than those of uninfected controls. The maximum rate of host photosynthesis, photosynthetic light compensation point and quantum efficiency of photosynthesis, measured on a unit green‐leaf area basis in July 1993, did not differ significantly between the four groups.Parasites which were grown on 6‐month‐old grass (1992 harvest) were only two thirds of the mass of those growing on older grasses (1993 harvest), and those on grass parasitized for two years (++, i.e. those without plantlets) accumulated twice as much biomass as those growing on grasses infected for the first time in 1993 (–+, i.e. those with plantlets). Parasites which were growing on previously parasitized hosts also had greater foliar nitrogen concentrations than those growing on hosts not parasitized until 1993 (−+).

Temporal expression of effects of varying nitrogen supply on canopy growth, photosynthesis and fruit production for Actinidia deliciosa vines in the field

The effects of varying nitrogen supply on canopy leaf area, response of leaf net photosynthesis (An) to quantum flux density (Q), and fruit yields of kiwifruit vines (Actinidia deliciosa var. deliciosa) were examined in a two‐year field experiment. Vines were grown with 0, 250 or 750 kg N ha−1 year−1. The responses to nitrogen supply were compared with responses to shade, to examine the impact of reduced carbon assimilation on canopy leaf area and fruit yields. Nitrogen supply did not affect significantly any of the measured variables during the first season of the experiment. In the second season, canopy leaf area was reduced significantly where nitrogen supply was limited. The quantum efficiency of photosynthesis (φq) increased from 0. 03 mol CO2 mol−1 Q soon after leaf emergence to more than 0. 05 mol CO2 mol−1 Q during the middle of the growing season. The quantum saturated rate of An (Asat) also increased during the season, from 7–10 μmol CO2 m−2 s−1 soon after leaf emergence, to 15–20 (μmol CO2 m−2 s−1 during the middle of the growing season. φq and Asat increased significantly with nitrogen supply at all measurement times during the second season. For vines with high nitrogen, fruit yields in both seasons were similar, averaging 3. 05 kg m−2. Fruit yields in the second season were reduced significantly where nitrogen supply was limited, due to reduced fruit numbers. The relative effects of reduced leaf area and reduced leaf photosynthesis for carbon assimilation by nitrogen deficient vines were examined using a mathematical model of canopy photosynthesis for kiwifruit vines. Simulations of canopy photosynthesis indicated that effects on leaf area and on leaf photosynthesis were of similar importance in the overall effects of nitrogen deficiency on carbon assimilation. The effects of nitrogen supply on fruit numbers (i. e. flower development) preceded the measured effects on carbon assimilation, indicating that the nitrogen supply affected carbon partitioning to reserves in the first season.

Interaction of Salt Stress and Photoinhibition on Photosynthesis in Barley and Sorghum

Summary The effect of salinity (short term), high light and interaction of the two was investigated in leaves and isolated chloroplasts from these leaves of barley (C3) and sorghum (C4) plants. The light saturated rate of CO 2 uptake and maximum quantum yield decreased with increasing salt concentrations. Stomatal conductance was more sensitive to salt stress than photoinhibition. Photoinhibition treatment of salt stressed plants resulted in a further decrease of the CO 2 assimilation rate and quantum efficiency of photosynthesis. The recovery of the CO 2 assimilation rate was considerably lower in salt and high light stressed plants compared to plants which were given a photoinhibition treatment only (no salt). It is concluded that salt stress itself does not exert adverse effects on the primary photosynthesis process since the F v /F m ratio and electron transport activity did not decrease with higher salt concentrations. However, salinity stress enhanced substantially the susceptibility to photoinhibition; this may be due to the predisposing of the photosynthetic apparatus to damage under salt stress, resulting in lower light saturated rates of photosynthesis a decreased ability to recover from photodamage.

Photophosphorylation after Chilling in the Light

The response of in situ photophosphorylation in attached cucumber (Cucumis sativus L. cv Ashley) leaves to chilling under strong illumination was investigated. A single-beam kinetic spectrophotometer fitted with a clamp-on, whole leaf cuvette was used to measure the flash-induced electrochromic absorbance change at 518 minus 540 nanometers (DeltaA(518-540)) in attached leaves. The relaxation kinetics of the electric field-indicating DeltaA(518-540) measures the rate of depolarization of the thylakoid membrane. Since this depolarization process is normally dominated by proton efflux through the coupling factor during ATP synthesis, this technique can be used, in conjuction with careful controls, as a monitor of in situ ATP formation competence. Whole, attached leaves were chilled at 5 degrees C and 1000 microeinsteins per square meter per second for up to 6 hours then rewarmed in the dark at room temperature for 30 minutes and 100% relative humidity. Leaf water potential, chlorophyll content, and the effective optical pathlength for the absorption measurements were not affected by the treatment. Light- and CO(2)-saturated leaf disc oxygen evolution and the quantum efficiency of photosynthesis were inhibited by approximately 50% after 3 hours of light chilling and by approximately 75% after 6 hours. Despite the large inhibition to net photosynthesis, the measurements of DeltaA(518-540) relaxation kinetics showed photophosphorylation to be largely unaffected by the chilling and light exposure. The amplitude of the DeltaA(518-540) measures the degree of energization of the photosynthetic membranes and was reduced significantly by chilling in the light. The cause of the decreased energization was traced to impaired turnover of photosystem II. Our measurements showed that the chilling of whole leaves in the light caused neither an uncoupling of photophosphorylation from photosynthetic electron transport nor any irreversible inhibition of the chloroplast coupling factor in situ. The sizeable inhibition in net photosynthesis observed after chilling in the light cannot, therefore, be attributed to any direct effect on photophosphorylation competence.

Shade adaptation of photosynthesis in Coffea arabica

The effect of irradiance on the rate of net photosynthesis was measured for mature leaves of coffee grown under five levels of radiation from 100% to 5% daylight. The rate of light-saturated photosynthesis per unit leaf area (PNmax) increased from 2 μmol CO2 m(-2) s(-1) under 5% daylight to 4.4 μmol CO2 m(-2) s(-1) under 100% daylight. The photon flux density (PAR, photosynthetically active radiation) needed for 50% saturation of photosynthesis, as well as the light compensation point, also increased with increasing levels of irradiation during growth. The quantum efficiency of photosynthesis (α), measured by the initial slope of the photosynthetic response to increasing irradiance, was greater under shaded growth conditions. The rate of dark respiration was greatest for plants grown in full daylight. On the basis of the increase in the quantal efficiency of photosynthesis and the low light compensation point when grown under shaded conditions, coffee shows high shade adaptation. Plants adjusted to shade by an increased ability to utilize short-term increases in irradiance above the level of the growth irradiance (measured by the difference between photosynthesis at the growth irradiance, PNg, and PNmax).

Effect of fertilization level on some physiological, morphological and growth characteristics of Ficus benjamina

Young Ficus benjamina L. (weeping fig) plants were grown in perlite under low photon flux density regime in the greenhouse. Influence of seven fertilization levels – ranging from nutrient deficiency to saline conditions – on photosynthetic, morphological and growth characteristics was studied. The compensation point for photosynthetic photon flux density was minimal and the quantum efficiency of photosynthesis was maximal at a fertilization level with an electrical conductivity of 1.75 mS. Dark respiration was not affected by the level of fertilization. Chlorophyll content per unit leaf area increased while stomatal conductance slowly decreased with increasing fertilization level. Growth expressed in terms of either height, total dry weight or total leaf area attained an optimum at 1.75 mS, i.e. at the level where the compensation point for photon flux density and the quantum efficiency of photosynthesis were optimal. Leaf size and visible quality decreased at higher fertilizer levels.