Acclimation of photosynthesis to lightflecks in tomato leaves: interaction with progressive shading in a growing canopy

Plants in natural environments are often exposed to fluctuations in light intensity, and leaf‐level acclimation to light may be affected by those fluctuations. Concurrently, leaves acclimated to a given light climate can become progressively shaded as new leaves emerge and grow above them. Acclimation to shade alters characteristics such as photosynthetic capacity. To investigate the interaction of fluctuating light and progressive shading, we exposed three‐week old tomato (Solanum lycopersicum ) plants to either lightflecks or constant light intensities. Lightflecks of 20 s length and 1000 μmol m^?2 s^?1 peak intensity were applied every 5 min for 16 h per day, for 3 weeks. Lightfleck and constant light treatments received identical daily light sums (15.2 mol m^?2 day^?1). Photosynthesis was monitored in leaves 2 and 4 (counting from the bottom) during canopy development throughout the experiment. Several dynamic and steady‐state characteristics of photosynthesis became enhanced by fluctuating light when leaves were partially shaded by the upper canopy, but much less so when they were fully exposed to lightflecks. This was the case for CO₂‐saturated photosynthesis rates in leaves 2 and 4 growing under lightflecks 14 days into the treatment period. Also, leaf 2 of plants in the lightfleck treatment showed significantly faster rates of photosynthetic induction when exposed to a stepwise change in light intensity on day 15. As the plants grew larger and these leaves became increasingly shaded, acclimation of leaf‐level photosynthesis to lightflecks disappeared. These results highlight continuous acclimation of leaf photosynthesis to changing light conditions inside developing canopies.     

Light acclimation in leaves of the juvenile and adult life phases of ivy (Hedera helix)

Hoflacher, H. and Bauer, H. 1982. Light acclimation in leaves of the juvenile and adult life phases of ivy (Hedera helix). – Physiol. Plant. 56: 177–182. Light acclimation was investigated during the juvenile and adult life phases of the whole-plant-development in Hedera helix L. For this purpose, cuttings of the juvenile and adult parts of one single parent plant were grown under low-light (PAR 30–50 μmol photons m^?2 s^?1) and high-light (PAR 300–500 μmol m^?2 s^?1) conditions: CO₂ exchange, chloroplast functions, and specific anatomy of fully developed leaves differentiated under these conditions were determined. In juvenile plants the leaves formed under low and high light had light-saturated rates of net photosynthesis of 6.5 and 11.1 mg CO₂ (dm leaf area)^?2 h^?1, respectively. In adult plants the rates were 9.4 and 22.2 mg dm^?2 h^?1, indicating a more pronounced capacity for acclimation to strong light in the adult life phase. Higher photosynthetic capacities were accompanied by higher conductances for the CO₂ transfer through the stomata, leading to almost the same CO₂ concentration in the intercellular spaces. Thus, stomatal conductances were not primarily responsible for the different photo-synthetic capacities. The higher rates in adult and high-light grown leaves were mainly the result of formation of thicker leaves with more chloroplasts per unit leaf area. Expressed per chloroplast, the photosynthetic capacity, the Hill reaction, and the activity of ribulose bisphosphate carboxylase were almost identical in plants grown in low-light and high-light. Measurements of photosynthetic capacity and thickness of leaves of Hedera sampled from field habitats with contrasting light regimes confirm the results of growth chamber studies. It is, therefore, concluded that both life phases of Hedera are capable of acclimating to strong light, but that during the juvenile phase this capacity is not fully developed.     

The interaction of light irradiance with ethylene in regulating growth of Helianthus annuus shoot tissues

Shade light found in ecological niches where plants are growing under a canopy or in proximity of taller neighbouring vegetation consist mainly of two separate light signals: low red to far-red ratio and low photosynthetically active radiation (PAR). The effect of the latter on the growth of 7-day old sunflower shoots was examined by assessing hypocotyl, cotyledon and leaf tissue growth under three varying PAR levels: near-normal of 1,000 μmol m^?2 s^?1, low of 100 μmol m^?2 s^?1 and very low of 10 μmol m^?2 s^?1. Then, the possible interaction between PAR signaling and ethylene in regulating growth of these sunflower tissues was investigated. The results showed that gradual decrease in PAR level increases hypocotyl elongation and decreases ethylene evolution. However, gradual decrease in PAR level decreases cotyledon and leaf growth and increases ethylene evolution. Thus it seems possible that PAR regulation of shoot growth is mediated by changes in ethylene evolution in tissue specific manner. This hypothesis was supported by experiments with the ethylene releasing factor, ethephon, and the ethylene biosynthesis inhibitor, AVG, as well as by transfer experiments where sunflower seedlings were transferred from one PAR regime to another with subsequent growth and ethylene measurements.     

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 (P_Nmax) increased from 2 mol CO₂ m^-2 s^-1 under 5% daylight to 4.4 mol 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 (), 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, P_Ng, and P_Nmax).     

Photoinhibitory responses of the epiphytic bromeliad Guzmania monostachia during the dry season in Trinidad maintain photochemical integrity under adverse conditions

The C₃–CAM epiphytic bromeliad Guzmania monostachia var. monostachia may be exposed to high incident photosynthetically active radiation (PAR) during the dry season in Trinidad, and resultant variations in photochemical efficiency have been investigated for ‘exposed’ (receiving ～100% incident PAR), ‘semi-exposed’ (～60% PAR) and shaded populations under natural conditions. The more succulent leaves of the plants growing fully exposed within the canopy had higher overall CAM activity (measured as ΔH⁺, the dawn-dusk titratable acidity), a smaller proportion of chlorenchyma and lower total chlorophyll content. There was a gradation of morphological and physiological characteristics between these and shaded leaves. Diurnal time-courses of photosynthetic light responses (as O₂ evolution) showed marked variations in apparent quantum yield (AQY) and light-saturated rates for both exposed and semi-exposed populations, dependent on incident PAR during the day. Similar measurements of photosystem II fluorescence characteristics showed that F_v/F_m declined from 0·70 to 0·42 at midday for exposed plants (on a day when total incident PAR was 44 mol photon m^?2), indicating non-photochemical quenching (qNP) of photosynthesis. However, in contrast to AQY determinations, F_v/F_m recovered during the afternoon. The decrease in F_v/F_m was reduced from 0·72 to 0·64 under 24 mol photon m^?2 d^?1. The long–term recovery of photo-synthetic efficiency was assessed for exposed plants placed under three shading regimes (60, 30 and 3% of incident PAR) over a 17-d period. During this time, total chlorophyll content increased from 228 to 515 and 585 μg g^?1 fresh weight (for 3 and 30%, respectively) and chlorophyll a:b declined. While AQY recovery was much longer under the lowest PAR (17d), under 30% PAR both AQY and F_v/F_m had recovered after 2d shading. The differences between timing of recovery for F_v/F_m during diurnal time courses and in the long term suggest that, while quenching associated with PSII recovers rapidly, enzyme activation and/or protein synthesis of other photosynthetic components may be limiting under low PAR. However, it is suggested that the occurrence of qNP on a daily basis may preclude long-term photoinhibitory damage under natural conditions during the dry season.     

Physiologic and Yield Responses of Shaded Cotton to the Plant Growth Regulator PGR-IV

The plant growth regulator PGR-IV has been reported to improve the growth, boll retention, and yield of cotton (Gossypium hirsutum L.) under optimum growing conditions. However, little is known about the response of cotton to PGR-IV under low light stress. A 3-year field study was conducted to determine if applying PGR-IV before an 8-day period of shade (63% light reduction) benefitted the growth and yield of shaded cotton. Shading during early squaring did not affect yield. Shading after the first flower stage significantly increased leaf chlorophyll concentration and fruit abscission and decreased the leaf photosynthetic rate, nonstructural carbohydrate concentrations, and lint yield. Foliar application of PGR-IV at 292 mL ha⁻¹ at early squaring and first flower did not improve the leaf photosynthetic rate of shaded cotton. However, shaded plants receiving PGR-IV had higher nonstructural carbohydrate concentrations in the floral buds and significantly lower fruit abscission than the shaded plants without PGR-IV. Applying PGR-IV to the foliage before shading resulted in a numeric increase (6–18%) in lint yield compared with shaded plants without PGR-IV. The decreased fruit abscission from the application of PGR-IV was associated with improved assimilate translocation. The yield enhancement from foliar application of PGR-IV was attributed to increased fruit retention. However, the average boll weight of shaded plants with PGR-IV tended to be lower than that of shaded plants without PGR-IV. Lint percentage was not affected by PGR-IV. Foliar application of PGR-IV appears beneficial for increasing the fruit retention of shaded cotton. Received June 12, 1997; accepted January 19, 1998     

Photosynthetic acclimation of plants to growth irradiance: the relative importance of specific leaf area and nitrogen partitioning in maximizing carbon gain

Changes in specific leaf area (SLA, projected leaf area per unit leaf dry mass) and nitrogen partitioning between proteins within leaves occur during the acclimation of plants to their growth irradiance. In this paper, the relative importance of both of these changes in maximizing carbon gain is quantified. Photosynthesis, SLA and nitrogen partitioning within leaves was determined from 10 dicotyledonous C₃ species grown in photon irradiances of 200 and 1000 µmol m^?2 s^?1. Photosynthetic rate per unit leaf area measured under the growth irradiance was, on average, three times higher for high‐light‐grown plants than for those grown under low light, and two times higher when measured near light saturation. However, light‐saturated photosynthetic rate per unit leaf dry mass was unaltered by growth irradiance because low‐light plants had double the SLA. Nitrogen concentrations per unit leaf mass were constant between the two light treatments, but plants grown in low light partitioned a larger fraction of leaf nitrogen into light harvesting. Leaf absorptance was curvilinearly related to chlorophyll content and independent of SLA. Daily photosynthesis per unit leaf dry mass under low‐light conditions was much more responsive to changes in SLA than to nitrogen partitioning. Under high light, sensitivity to nitrogen partitioning increased, but changes in SLA were still more important.     

PHYSIOLOGICAL AND MORPHOLOGICAL MODIFICATIONS OF PLANTAGO MAJOR (PLANTAGINACEAE) IN RESPONSE TO LIGHT CONDITIONS

Physiological and morphological differences between Plantago major L. (Plantaginaceae) growing in full sunlight and shaded conditions were examined. Photosynthesis of isolated leaves was saturated by irradiance around 300 μE m^−-2 sec^−-1 and 170 μE m^−-2 sec^−-1, respectively. In contrast to previous studies of sun/shade leaf responses, initial slopes of curves from shaded plants are significantly less than those taken from full-sun plants. Within the 400–500 nm and 600–700 nm ranges, leaves 5.0 cm or longer are essentially opaque, transmitting less than 1.25% of incident light. Chlorophyll content per unit leaf area is nearly equivalent for leaves from plants growing under the two extremes in light levels. Morphometric comparisons indicate shaded plants bear fewer leaves, have less leaf overlap, lower total leaf area, and longer petioles than full-sun plants. Leaf elongation rates are lower and the duration between the emergence of successive leaves is longer in shaded plants. Computer analyses of both types of rosette morphology reveal shaded plants have an equal or greater capacity to intercept light than full-sun plants, principally because of the minimization of leaf overlap and the large variation in the deflection angles of leaves in shaded rosette morphologies. Simulations, calculated on the basis of light interception, and taking into account the transition between photosynthate-importing and -exporting leaves, predict relative growth rates for full-sun and shaded rosette morphologies that are in reasonable agreement with empirically determined leaf growth rates. However, the data indicate that significant physiological and morphological differences exist among leaves from a single rosette, and among developmentally comparable leaves from rosettes growing under different ambient light environments. Differences among leaves on a single plant must be accommodated in computerized techniques attempting to simulate light interception and its consequences on potential growth rates.     

Improved growth and water use efficiency of cherry saplings under reduced light intensity

Cherry (Prunus avium L.) saplings were grown under natural sunlight (controls) or moderate shading (up to 30%, depending on the incident light intensity and the hour of the day). Reduced light intensity increased the dry mass of each of the plant components studied. Consequently, the total dry mass of shaded plants was significantly greater than that of controls at the end of the growing season. However, the diurnal trend in the level of photosynthesis (per unit of leaf area) of shaded plants was similar to the controls in August, but lower in September. As the growing season proceeded, reduced photosynthetic rates, thinner mesophyll and larger specific leaf area in the shaded plants indicated that leaf development had adapted to shaded conditions throughout the growing season. It is suggested that increased growth of shaded plants was caused by a higher initial relative growth rate and a greater whole-plant photosynthesis. Shading consistently reduced transpiration over the season, therefore improving water use efficiency of shaded leaves. Our results suggest that a moderate reduction in light intensity can be a useful method for improving growth and saving water in hot and dry environments.     

Growth and development of cucumber Cucumis sativus L. in the prereproductive period under long photoperiods

When plants are grown in a greenhouse or in controlled environment growth rooms, prolonging the photoperiod, including towards continuous light, is one of the ways to increase plant productivity and energy savings. However, exposing some plant species to long photoperiods causes leaf injuries and growth reductions. We studied the effect of the photoperiod (8, 12, 16, 20, and 24 h) and photosynthetic photon flux density (60, 120, and 160 μmol/m² s PAR) on cucumber plants Cucumis sativus L. in a prereproductive period. It was shown that the response of the cucumber plants to a photoperiod over 20 h, including continuous light, depending on the plant age and light intensity, may include leaf movement or paraheliotropism, non-photochemical energy dissipation, and/or reversible photoinhibition of a reaction center of photosystem II, development of reversible chlorosis, reduction of a light-harvesting complex, and increase in the content of carotenoids. Reaction of immature and virginile plants to long photoperiods was different, which high-lights the need for experimental separation of the prereproductive period in terms of age states and consideration of this when developing crop production plan.     

Characteristics of photosynthesis in maize leaves (С<Subscript>4</Subscript> plants) upon changes in the level of illuminance and nitrate nutrition

We studied assimilation of ¹⁴СО₂ and distribution of ¹⁴С among the products of 3-min-long photosynthesis of maize (Zea mays L.) leaves. The day before the experiment, half of the plants were fertilized with Ca(NO₃)₂ (1 g/L of water) at a rate of 6 L/m². Five days before the experiment, some plants were shaded for adaptation (illuminance was reduced by 50%). On the day of the experiment (before the application of ¹⁴СО₂), several shaded plants were exposed to direct sunlight for 3 min, and some plants grown at full light (light plants) were shaded for 3 min (illuminance of 50%). Unfertilized plants adapted for 5 days to shading showed photosynthesis of 75.9% of control level (full light). If light plants were transferred to shading for 3 min, their photosynthesis decreased to 42.1%. In plants shaded for 5 days and then transferred to full light, photosynthesis in 3 min was 96.3% of control level. At full light, fertilization with nitrate boosted photosynthesis to 132.6% as compared with control material, but photosynthesis decreased to 43.5 and 65.4% of control level in plants shaded for 5 days and those shaded for 3 min, respectively. At the same time, the plants shaded for 5 days and then exposed for 3 min to full light restored photosynthesis to almost control level (95.5%). Analysis of ¹⁴С distribution among the products of 3-min-long photosynthesis showed that, the same as in C₃ plants, a decrease in illuminance (especially a sudden one) in maize reduced the ratio between labeled sucrose and hexoses and elevates incorporation of ¹⁴С into malate, which indicated that its consumption in bundle sheath cells was suppressed. A decrease in the ratio between labeled sucrose and hexoses became more pronounced under the influence of nitrates with this effect also occurring in transport products of photosynthesis (20 cm below ¹⁴С-providing leaf area). In plants fertilized with nitrates, radioactivity of sucrose (% of radioactivity of soluble compounds) decreased in all the types of illumination. When illuminance was suddenly reduced for 3 min, incorporation of ¹⁴С into sucrose was 21.5 against 51.2% in light plants, and radioactivity of aspartate and malate sharply rose to 13.7 and 26.1% (against 2.1 and 8.9% in control material). Incorporation of ¹⁴С into compounds of glycolate pathway was low (less than 2.5%), but it was somewhat greater in nitrate plants. We concluded that the same mechanism of interaction between stomatal apparatus of leaf epidermis, invertase of mesophyll apoplast, and photosynthetic metabolism of carbon with electron flux via electron transport chain in chloroplasts of bundle sheath cells, which governs the rate of photosynthesis and assimilate export from the leaf but is triggered by the extent of consumption in the bundle sheath cells of C₄ acids produced in the mesophyll operates in C₄ plants (the same as in C₃ plants).     

Photosynthetic characteristics and productivity of spring rape plants as related to crop density

Optimal density of spring rape (Brassica napus L.) crop stand was determined by plant photosynthetic characteristics at the beginning of flowering. As crop density increased from 100 to 350 plants/m², leaf surface index (LSI) of the crop was found to increase by 18.2–80.2%, and LSI decreased by 38.8–67.3% as compared with the sparsest crop (50–100 plants/m²). LSI depended on the rate of incident PAR reaching 0.5 and 0.25 heights of the crop stand and to the soil surface. When crop density increased from 100 to 350 plants/m², the photosynthetic potential (PP) of the crop increased 1.8 times as compared with the sparsest crop. PP of the densest rape crop stand was 3 times lower than in the sparsest crop. When the crop density increased from 100 to 250 plants/m², the daily increment in biomass calculated per leaf surface unit increased by 27.0% as compared with the sparsest crop and depended on LSI. When leaf area decreased, the daily increment in biomass calculated per leaf surface unit declined; in the densest stand, this characteristic was by 58.3% lower than in the sparsest crop. Rape productivity at the flowering stage depended on the crop density, LSI of plants, rate of PAR reaching 0.5 and 0.25 heights of the crop stand and to the soil surface, PP, and the daily increment in biomass calculated per leaf surface unit. Crop productivity at the flowering stage and the rape seed yield were associated by a significant parabolic relationship. When crop density increased from 100 to 350 plants/m², seed yield per plant considerably decreased (by 33.1–78.5%) as compared with the sparsest crop. The greatest influence on seed yield per plant was exerted by LSI and the daily increment in biomass calculated per leaf surface unit. When crop density increased to 250–300 plants/m², the seed yield considerably rose (by 28.6–58.8%) as compared with the sparsest crop; when this index reached 300–350 plants/m², the seed yield decreased because plant growth was suppressed, with the productivity reduced. The results thus obtained suggest that the photometric characteristics of spring rape were at optimum at crop density of 100–250 plants/m². The agroclimatic conditions of Lithuania ensure potential for rapid accumulation of total biomass and high seed yield.     

Plasticity in relative growth rate and its components following a change in irradiance

A total of 244 plants from two species, Lythrum salicaria and Epilobium glandulosum, were grown individually in hydroponic sand culture from seed for 36 d. Until day 27 all plants experienced an irradiance of 550 μmol m^?2 s^?1 PFD and on day 27 half of the plants were subjected to a neutral shade treatment in which irradiance was reduced to 100 μmol m^?2 s^?1 photon fluy density (PFD). Measures of relative growth rate, net assimilation rate, specific leaf area, biomass partitioning to leaves, roots, structural tissues (i.e. stems, petioles and inflorescences) and tissue density were obtained from intensive harvests three or four times per day. The shade treatment caused an immediate decrease in relative growth rate and net assimilation rate. Within hours the specific leaf area of the shaded plants increased and leaf tissue density decreased, thus partially offsetting the decrease in relative growth rate. Biomass partitioning was not affected.     

Factors affecting photosynthesis and its seasonal variation in the seagrasses Cymodocea nodosa (Ucria) Aschers,and Posidonia oceanica (L.) Delile in the mediterranean

Measurements of photosynthesis, dark respiration, and leaf chlorophyll content were made in the laboratory on both shallow (1 to 5 m) and deep (25 to 33 m) leaves of Cymooceu nodosa (Ucria) Aschers, and Posidonia oceanica (L.) Delile in Malta in April and August. Light saturated photosynthetic rates in Cymodocea were similar in spring (18 μg C cm^?2h^?1) and summer (25μg Ccm^?2h^?1) if the 9 C increase in water temperature in summer is taken into account: however, photosynthetic rates in Posidonia were higher in spring than in summer, especially in shallow leaves which fixed ≈ 10 μg C cm^?2h^?1 in spring but less than half that in summer when rates of carbon accretion were close to compensation point. Levels of irradiance at which photosynthesis was light saturated (Iλ were ≈ 3 mW cm^?2 PAR for Cymodocea and 2 mW cm^?2 PAR for Posidonia: underwater irradiance at the lower depth limit for these plants (≈33 m) was ≈3 mW cm^?2 PAR. corresponding closely to the saturation irradiances. Compensation irradiance for both species was between 0.3 and 0.5 mW cm^?2 PAR.Photosynthesis in both species had a temperature optimum at about 30 C (slightly higher in Cymodocea in summer). Dark respiration rates were generally similar in spring and summer, in the region of 3 μg C cm^?2 h^?1 in Cymodocea and 1.5 to 2 μg C cm^?2 h^?1 in Posidonia. Increase in dark respiration rates with increased temperature was considerably greater in spring than in summer in both species. Photosynthesis was directly proportional to chlorophyll content in Posidonia in the range encountered (up to 58 μg Chl cm^?2) and the summer reduction in photosynthesis was closely correlated with reduction in chlorophyll content. It seems unlikely that environmental factors such as seasonal changes in light intensity, nutrient availability or water temperature were directly responsible for this loss of chlorophyll and it is suggested that this is a manifestation of general leaf senescence, probably induced by daylength changes but possibly enhanced by increased water temperature. Cymodocea showed a similar reduction in chlorophyll content in summer but this was not reflected in reduced photosynthesis. Thus, although Cymodocea may grow rapidly throughout the spring and summer with an overall productivity of 3.6 g C m^?2 day^?1 in shallow water, the luxuriant growths of Posidonia must develop in the first half of the year when a dense meadow may produce up to 2.1 g C m^?2 day^?1 in shallow water, declining to ?0.6 g C m^?2 day^?1 in summer.     

Photosynthesis Regulation by Glucohexaose Through Redox Changes in Cucumis sativus

To investigate the effects of glucohexaose (P6) on cucumber, leaf CO₂ assimilation, chlorophyll fluorescence parameters, chlorophyll content, and carbohydrate metabolism were examined in cucumber plants. The net photosynthetic rate (P _n ) of cucumber leaves was enhanced after being treated with 10 μg mL^?1 P6. The increase was correlated with increases in transpiration rate (E) and stomatal conductance (G _s), whereas the intercellular CO₂ concentration (C _i) was not different from the control plants. Chlorophyll content, absorption of light energy per unit area (ABS/CS), capture of light energy per unit area (TR_o/CS), quantum yield of electron transport per unit area (ET_o/CS), maximum photochemical efficiency of PSII (φP _o), quantum yield of photosynthetic institution electron transfer (φE _o), probability of other electron acceptors that captured exciton-transferred electrons to the electronic chain which exceeds QA (ψ _o), number of reaction centers per unit leaf area (RC/CS_o), and the performance index on absorption basis (PI_ABS) were improved, but heat dissipation per unit area (DI_o/CS) and maximum quantum yield of non-chemical quenching (φD _o) were reduced. In addition, increases in sucrose, soluble sugars, and starch contents were observed in P6-treated plants. However, H₂O₂ scavenger (DMTU) or NADPH oxidase inhibitor (DPI) pretreatment significantly abolished the effect of P6 on photosynthesis. The results demonstrated that ROS played a critical role in P6-induced photosynthesis. The increase in chlorophyll content together with efficient light absorption, transmission, and conversion in P6-treated plants is important for increasing photosynthesis.     

Spine influences on PAR interception, stem temperature, and nocturnal acid accumulation by cacti

Abstract. The influence of spines on nocturnal acid accumulation was examined for two Crassulacean acid metabolism plants, the barrel cactus Ferocactus acanthodes (Lemaire) Britton & Rose var. lecontei (Engelm.) Lindsay and the cylindropuntia Opuntia bigelovii Engelm. var. bigelovii. The ambient photo-synthetically active radiation (PAR) required for 90% saturation of nocturnal acid accumulation by O. bigelovii was 23 mol m^?2 d^?1 for stems that were 32% shaded by spines and 16 mol m^?2 d^?1 when the spines were removed. For F. acanthodes, 90% saturation occurred for an ambient PAR of 45 mol m^?2 d^?1 where the stem was 78% shaded by spines and 21 mol m^?2 d^?1 when the spines were removed. For comparison, the ambient PAR averaged 21 mol m^?2 d^?1 at various times of the year and stem positions for O. bigelovii fully exposed to solar irradiation. Although spines can reflect some PAR toward the stem surface, their presence decreases the PAR incident on the stem surface, thereby decreasing the nocturnal acid accumulation. For O. bigelovii, periodically removing the spines led to a 60% greater increase in stem volume over 2^1/2 years. The effect of variation in spine-shading on stem surface temperature was determined for F. acanthodes using computer simulations and an energy budget model. A substantial reduction in daytime stem temperatures and a smaller increase in night-time temperatures as the spine-shading increased had very little influence on nocturnal acid accumulation for both winter and summer days. Thus, the main effect of spines on the metabolism of cacti is apparently to reduce the PAR incident on the stem surface and thereby to reduce productivity.     

Response of plasmodesmata formation in leaves of C4 grasses to growth irradiance

Rapid metabolite diffusion across the mesophyll (M) and bundle sheath (BS) cell interface in C₄ leaves is a key requirement for C₄ photosynthesis and occurs via plasmodesmata (PD). Here, we investigated how growth irradiance affects PD density between M and BS cells and between M cells in two C₄ species using our PD quantification method, which combines three‐dimensional laser confocal fluorescence microscopy and scanning electron microscopy. The response of leaf anatomy and physiology of NADP‐ME species, Setaria viridis and Zea mays to growth under different irradiances, low light (100 μmol m^?2 s^?1), and high light (1,000 μmol m^?2 s^?1), was observed both at seedling and established growth stages. We found that the effect of growth irradiance on C₄ leaf PD density depended on plant age and species. The high light treatment resulted in two to four‐fold greater PD density per unit leaf area than at low light, due to greater area of PD clusters and greater PD size in high light plants. These results along with our finding that the effect of light on M‐BS PD density was not tightly linked to photosynthetic capacity suggest a complex mechanism underlying the dynamic response of C₄ leaf PD formation to growth irradiance.     

Photosynthesis in leaves and siliques of winter oilseed rape (Brassica napus L.)

The rate of photosynthesis and its relation to tissue nitrogen content was studied in leaves and siliques of winter oilseed rape (Brassica napus L.) growing under field conditions including three rates of nitrogen application (0, 100 or 200 kg N ha^-1) and two levels of irrigation (rainfed or irrigated at a deficit of 20 mm). The predominant effect of increasing N application under conditions without water deficiency was enhanced expansion of photosynthetically active leaf and silique surfaces, while the rate of photosynthesis per unit leaf or silique surface area was similar in the different N treatments. Thus, oilseed rape did not increase N investment in leaf area expansion before a decline in photosynthetic rate per unit leaf area due to N deficiency could be avoided. Much less photosynthetically active radiation penetrated into high-N canopies than into low-N canopies. The specific leaf area increased markedly in low light conditions, causing leaves in shade to be less dense than leaves exposed to ample light. In both leaves and siliques the photosynthetic rate per unit surface area responded linearly to increasing N content up to about 2 g m^-2, thus showing a constant rate of net CO₂ assimilation per unit increment in N (constant photosynthetic N use efficiency). At higher tissue N contents, photosynthetic rate responded less to changes in N status. Expressed per unit N, light saturated photosynthetic rate was three times higher in leaves than in silique valves, indicating a more efficient photosynthetic N utilization in leaves than in siliques. Nevertheless, from about two weeks after completion of flowering and onwards total net CO₂ fixation in silique valves exceeded that in leaves because siliques received much higher radiation intensities than leaves and because the leaf area declined rapidly during the reproductive phase of growth. Water deficiency in late vegetative and early reproductive growth stages reduced the photosynthetic rate in leaves and, in particular, siliques of medium- and high-N plants, but not of low-N plants.     

Inhibition of photosynthesis by solar radiation in Dunaliella salina: relative efficiencies of UV-B, UV-A and PAR

Inhibition of photosynthesis after exposure to solar radiation was investigated in the marine green alga Dunaliella salina by monitoring photosynthetic optimal quantum yield F_v/F_m and efficiency of oxygen production. Samples were exposed to solar radiation in Ancient Korinth, Greece (37°58′ N, 23°0′ E) in August 1994. Within 30 min, F_v/F_m and efficiency of oxygen production decreased with similar kinetics with increasing exposure time. The inhibition, however, diminished when ultraviolet radiation was progressively excluded by means of colour filter glasses. Samples exposed for 3 h showed complete or partial recovery of photosynthesis, with almost the same rate under all irradition conditions. The fit of the experimental data with an analytical model describing inhibition of photosynthesis as a function of a linear combination of the photon fluence in the UV-B, UV-A and PAR allows one to estimate the relative mean effectiveness for inhibition by the three spectral ranges [about 2 × 10^?4, 4 × 10^?6 and 2 × 10^?7 (μmol photons m^?2)^?1 for UV-B, UV-A and PAR, respectively].     

Growth and physiological responses of cotton (Gossypium hirsutum L.) to elevated carbon dioxide and ultraviolet-B radiation under controlled environmental conditions

Better understanding of crop responses to projected changes in climate is an important requirement. An experiment was conducted in sunlit, controlled environment chambers known as soil–plant–atmosphere–research units to determine the interactive effects of atmospheric carbon dioxide concentration [CO₂] and ultraviolet‐B (UV‐B) radiation on cotton (Gossypium hirsutum L.) growth, development and leaf photosynthetic characteristics. Six treatments were used, comprising two levels of [CO₂] (360 and 720 µmol mol^?1) and three levels of 0 (control), 7.7 and 15.1 kJ m^?2 d^?1 biologically effective UV‐B radiations within each CO₂ level. Treatments were imposed for 66 d from emergence until 3 weeks after the first flower stage. Plants grown in elevated [CO₂] had greater leaf area and higher leaf photosynthesis, non‐structural carbohydrates, and total biomass than plants in ambient [CO₂]. Neither dry matter partitioning among plant organs nor pigment concentrations was affected by elevated [CO₂]. On the other hand, high UV‐B (15.1 kJ m^?2 d^?1) radiation treatment altered growth resulting in shorter stem and branch lengths and smaller leaf area. Shorter plants at high UV‐B radiation were related to internode lengths rather than the number of mainstem nodes. Fruit dry matter accumulation was most sensitive to UV‐B radiation due to fruit abscission. Even under 7.7 kJ m^?2 d^?1 of UV‐B radiation, fruit dry weight was significantly lower than the control although total biomass and leaf photosynthesis did not differ from the control. The UV‐B radiation of 15.1 kJ m^?2 d^?1 reduced both total (43%) and fruit (88%) dry weights due to smaller leaf area and lower leaf net photosynthesis. Elevated [CO₂] did not ameliorate the adverse effects of UV‐B radiation on cotton growth and physiology, particularly the boll retention under UV‐B stress.