Quantum Yield Of CO2 Assimilation Research Articles

Roles for leakiness and O2 evolution in explaining lower-than-theoretical quantum yields of photosynthesis in the PEP-CK subtype of C4 plants.

Theoretically, the PEP-CK C4 subtype has a higher quantum yield of CO2 assimilation ( ) than NADP-ME or NAD-ME subtypes because ATP required for operating the CO2-concentrating mechanism is believed to mostly come from the mitochondrial electron transport chain (mETC). However, reported is not higher in PEP-CK than in the other subtypes. We hypothesise, more photorespiration, associated with higher leakiness and O2 evolution in bundle-sheath (BS) cells, cancels out energetic advantages in PEP-CK species. Nine species (two to four species per subtype) were evaluated by gas exchange, chlorophyll fluorescence, and two-photon microscopy to estimate the BS conductance (gbs) and leakiness using a biochemical model. Average gbs estimates were 2.9, 4.8, and 5.0 mmol m-2 s-1 bar-1, and leakiness values were 0.129, 0.179, and 0.180, in NADP-ME, NAD-ME, and PEP-CK species, respectively. The BS CO2 level was somewhat higher, O2 level was marginally lower, and thus, photorespiratory loss was slightly lower, in NADP-ME than in NAD-ME and PEP-CK species. Differences in these parameters existed among species within a subtype, and gbs was co-determined by biochemical decarboxylating sites and anatomical characteristics. Our hypothesis and results partially explain variations in observed , but suggest that PEP-CK species probably use less ATP from mETC than classically defined PEP-CK mechanisms.

Grassland productivity responds unexpectedly to dynamic light and soil water environments induced by photovoltaic arrays

AbstractAgrivoltaic (AV) systems are designed to coproduce photovoltaic (PV) energy on lands simultaneously supporting agriculture (food/forage production). PV infrastructure in agroecosystems alters resources critical for plant growth, and water‐limited agroecosystems such as grasslands are likely to be particularly sensitive to the unique spatial and temporal patterns of incident sunlight and soil water inherent within AV systems. However, the impact of resource alteration on forage production, the primary ecosystem service from managed grasslands, is poorly resolved. Here, we evaluated seasonal patterns of soil moisture (SM) and diurnal variation in incident sunlight (photosynthetic photon flux density [PPFD]) in a single‐axis‐tracking AV system established in a formerly managed semiarid C3 grassland in Colorado. Our goals were to (1) quantify dynamic patterns of PPFD and SM within a 1.2 MW PV array in a perennial grassland, and (2) determine how aboveground net primary production (ANPP) and photosynthetic parameters responded to the resource patterns created by the PV array. We hypothesized that spatial variability in ANPP would be strongly related to SM patterns, typical of most grasslands. We measured significant reductions in ANPP directly beneath PV panels, where SM and PPFD were both low. However, in locations with significantly increased SM from the shedding and redistribution of precipitation by PV panels, ANPP was not increased. Instead, ANPP was greatest in locations where plants were shaded in the afternoon but received high levels of PPFD in the morning hours, when air temperatures and vapor pressure deficits were relatively low. Thus, contrary to expectations, we found relatively weak relationships between SM and ANPP despite significant spatial variability in both. Further, there was little evidence that light‐saturated photosynthesis (Asat) and quantum yield of CO2 assimilation (ϕCO2) differed for plants growing directly beneath (lowest PPFD) versus between (highest PPFD) PV panels. Overall, the AV system established in this semiarid managed grassland did not alter patterns of ANPP in ways predictable from past studies of controls of ANPP in open grasslands. However, our results suggest that the diurnal timing of low versus high periods of PPFD incident on plants is an important determinant of productivity patterns in grasslands.

Star Polymers with Designed Reactive Oxygen Species Scavenging and Agent Delivery Functionality Promote Plant Stress Tolerance.

Plant abiotic stress induces reactive oxygen species (ROS) accumulation in leaves that can decrease photosynthetic performance and crop yield. Materials that scavenge ROS and simultaneously provide nutrients in vivo are needed to manage this stress. Here, we incorporated both ROS scavenging and ROS triggered agent release functionality into an ∼20 nm ROS responsive star polymer (RSP) poly(acrylic acid)-block-poly((2-(methylsulfinyl)ethyl acrylate)-co-(2-(methylthio)ethyl acrylate)) (PAA-b-P(MSEA-co-MTEA)) that alleviated plant stress by simultaneous ROS scavenging and nutrient agent release. Hyperspectral imaging indicates that all of the RSP penetrates through the tomato leaf epidermis, and 32.7% of the applied RSP associates with chloroplasts in mesophyll. RSP scavenged up to 10 μmol mg-1 ROS in vitro and suppressed ROS in vivo in stressed tomato (Solanum lycopersicum) leaves. Reaction of the RSP with H2O2in vitro enhanced the release of nutrient agent (Mg2+) from star polymers. Foliar applied RSP increased photosynthesis in plants under heat and light stress compared to untreated controls, enhancing the carbon assimilation, quantum yield of CO2 assimilation, Rubisco carboxylation rate, and photosystem II quantum yield. Mg loaded RSP improved photosynthesis in Mg deficient plants, mainly by promoting Rubisco activity. These results indicate the potential of ROS scavenging nanocarriers like RSP to alleviate abiotic stress in crop plants, allowing crop plants to be more resilient to heat stress, and potentially other climate change induced abiotic stressors.

Impact of the facultative parasitic weed Rhamphicarpa fistulosa (Hochst.) Benth. on photosynthesis of its host Oryza sativa L.

Rhamphicarpa fistulosa is a facultative root parasitic annual forb, of the family Orobanchaceae that is native to sub-Saharan Africa. Parasitism results in yield reductions by the host plants but it is not known how exactly R. fistulosa affects its host or how the host responds physiologically. In three pot experiments, we investigated whether and when the parasite affects photosynthesis of rice, whether the level of impact was parasite density dependent and explored mechanisms underlying the response of rice photosynthesis to parasitism. Photosynthesis and related parameters were measured at a range of light use intensities. Host photosynthesis was negatively affected while light use efficiency was negatively affected only later on in the growth process. Except for dark respiration rates, which were never affected by parasite infection, suppression of host photosynthesis at light saturation, the initial light-use efficiency, chlorophyll content, specific leaf area and shoot weight were parasite density dependent with a stronger effect for higher parasite densities. Only at 56 days after sowing, the slope of the linear relationship between light adapted quantum efficiency of PSII electron transport (ΦPSII) and the quantum yield of CO2 assimilation (ΦCO2) of infected plants was less than those of un-infected plants. There was a considerable time lag between the parasite’s acquisition of benefits from the association, in terms of growth (previously observed around 42 DAS), and the reduction of host photosynthesis (around 56 DAS). Expression of relative reductions in host growth rates started at the same time as the relative suppression of host photosynthesis. This indicated that R. fistulosa affects host growth by first extracting assimilates and making considerable gains in growth, before impacting host photosynthesis and growth.

Can improved canopy light transmission ameliorate loss of photosynthetic efficiency in the shade? An investigation of natural variation in Sorghum bicolor.

Previous studies have found that maximum quantum yield of CO2 assimilation (Φ CO2,max,app) declines in lower canopies of maize and miscanthus, a maladaptive response to self-shading. These observations were limited to single genotypes, leaving it unclear whether the maladaptive shade response is a general property of this C4 grass tribe, the Andropogoneae. We explored the generality of this maladaptation by testing the hypothesis that erect leaf forms (erectophiles), which allow more light into the lower canopy, suffer less of a decline in photosynthetic efficiency than drooping leaf (planophile) forms. On average, Φ CO2,max,app declined 27% in lower canopy leaves across 35 accessions, but the decline was over twice as great in planophiles than in erectophiles. The loss of photosynthetic efficiency involved a decoupling between electron transport and assimilation. This was not associated with increased bundle sheath leakage, based on 13C measurements. In both planophiles and erectophiles, shaded leaves had greater leaf absorptivity and lower activities of key C4 enzymes than sun leaves. The erectophile form is considered more productive because it allows a more effective distribution of light through the canopy to support photosynthesis. We show that in sorghum, it provides a second benefit, maintenance of higher Φ CO2,max,app to support efficient use of that light resource.

Photosynthetic Physiology of Blue, Green, and Red Light: Light Intensity Effects and Underlying Mechanisms.

Red and blue light are traditionally believed to have a higher quantum yield of CO2 assimilation (QY, moles of CO2 assimilated per mole of photons) than green light, because green light is absorbed less efficiently. However, because of its lower absorptance, green light can penetrate deeper and excite chlorophyll deeper in leaves. We hypothesized that, at high photosynthetic photon flux density (PPFD), green light may achieve higher QY and net CO2 assimilation rate (An) than red or blue light, because of its more uniform absorption throughtout leaves. To test the interactive effects of PPFD and light spectrum on photosynthesis, we measured leaf An of “Green Tower” lettuce (Lactuca sativa) under red, blue, and green light, and combinations of those at PPFDs from 30 to 1,300 μmol⋅m–2⋅s–1. The electron transport rates (J) and the maximum Rubisco carboxylation rate (Vc,max) at low (200 μmol⋅m–2⋅s–1) and high PPFD (1,000 μmol⋅m–2⋅s–1) were estimated from photosynthetic CO2 response curves. Both QYm,inc (maximum QY on incident PPFD basis) and J at low PPFD were higher under red light than under blue and green light. Factoring in light absorption, QYm,abs (the maximum QY on absorbed PPFD basis) under green and red light were both higher than under blue light, indicating that the low QYm,inc under green light was due to lower absorptance, while absorbed blue photons were used inherently least efficiently. At high PPFD, the QYinc [gross CO2 assimilation (Ag)/incident PPFD] and J under red and green light were similar, and higher than under blue light, confirming our hypothesis. Vc,max may not limit photosynthesis at a PPFD of 200 μmol m–2 s–1 and was largely unaffected by light spectrum at 1,000 μmol⋅m–2⋅s–1. Ag and J under different spectra were positively correlated, suggesting that the interactive effect between light spectrum and PPFD on photosynthesis was due to effects on J. No interaction between the three colors of light was detected. In summary, at low PPFD, green light had the lowest photosynthetic efficiency because of its low absorptance. Contrary, at high PPFD, QYinc under green light was among the highest, likely resulting from more uniform distribution of green light in leaves.

BRIEF COMMUNICATIONPhotosynthetic light-response curves of light-demanding and shade-tolerant seedlings of neotropical tree species

Light-response curves of seedlings of five light-demanding (LD) and four shade-tolerant (ST) neotropical tree species, native to the Brazilian Atlantic biome, were analyzed, aiming to verify differences between these functional groups regarding the light use. We hypothesized that variations in the light requirement of seedlings would occur not only between LD and ST species, but also within these functional groups. The apparent quantum yield of CO2 assimilation, dark respiration rate (RD), photorespiration rate (RP), light-compensation point (Icomp), light-saturation point (Imax), and light-saturated photosynthetic rate (PNmax) were estimated from fitted light-responses curves and using equations available in the literature. The results revealed great differences between the functional groups. RD, RP, Icomp, Imax, and PNmax were effectively higher in the LD than those in the ST species. Furthermore, a continuum of strategies regarding the light use was observed, matching with some ecological characteristics of the species studied here.

Picea purpurea has a physiological advantage over its progenitors in alpine ecosystems due to transgressive segregation

ABSTRACTPicea purpurea originated from the homoploid hybrid speciation, which occupy novel alpine habitats in the north-east region of the Qinghai-Tibet Plateau (QTP) where its progenitors, P. wilsonii and P. likiangensis are excluded. Alpine habitats are usually characterized by low atmospheric CO2 concentrations and strong solar radiation with a large fraction of shortwave radiation (i.e. blue light). To test whether P. purpurea has the transgressive segregation in photosynthetic capacity and stomatal characteristics contributed to occupying such extreme habitats, we investigated the leaf gas exchange properties at two level of CO2 concentration (Ca: 200 and 400 μmol mol−1), light responsive curves of photosynthesis under three light qualities (red, blue and a mixture of both red and blue light at the ratio of 94:6), stomatal and other relative physiological traits for all three species. Our results show evidence of transgressive segregation in hybrid species; for instance, under blue and red light P. purpurea was demonstrated has the higher quantum yield of CO2 assimilation (φ) and maximum CO2 assimilation rate (Amax) than its progenitors. Particularly φ and Amax in P. purpurea were 2-fold higher than these in P. likiangensis. P. purpurea also exhibited higher stomatal conductance at low CO2 concentrations and instantaneous water use efficiency under all light qualities, have greater leaf pigments content, longer guard cell length and lower linear stomatal density relative to its progenitors. These traits may have enhanced the physiological fitness of P. purpurea and facilitated its colonization to alpine habitats.

Loss of photosynthetic efficiency in the shade. An Achilles heel for the dense modern stands of our most productive C4crops?

The wild progenitors of major C4 crops grew as individuals subjected to little shading. Today they are grown in dense stands where most leaves are shaded. Do they maintain photosynthetic efficiency in these low light conditions produced by modern cultivation? The apparent maximum quantum yield of CO2 assimilation (ΦCO2max,app), a key determinant of light-limited photosynthesis, has not been systematically studied in field stands of C4 crops. ΦCO2max,app was derived from the initial slope of the response of leaf CO2 uptake (A) to photon flux (Q). Leaf fractional light absorptance (α) was measured to determine the absolute maximum quantum yield of CO2 assimilation on an absorbed light basis (ΦCO2max,abs). Light response curves were determined on sun and shade leaves of 49 field plants of Miscanthus × giganteus and Zea mays following canopy closure. ΦCO2max,app and ΦCO2max,abs declined significantly by 15-27% (P<0.05) with canopy depth. Experimentally, leaf age was shown unlikely to cause this loss. Modeling canopy CO2 assimilation over diurnal courses suggested that the observed decline in ΦCO2max,app with canopy depth costs 10% of potential carbon gain. Overcoming this limitation could substantially increase the productivity of major C4 crops.

Chlorophyll fluorescence emission of tomato plants as a response to pulsed light based LEDs

The effects of pulsed light based-LEDs at eleven frequencies (0.1, 1, 10, 50, 100, 500 Hz, 1, 5, 10, 50 and 100 kHz) programmed at 50 % duty cycle were analyzed, obtaining important parameters of the fluorescence emission of chlorophyll such as: maximum fluorescence (Fm′), minimum fluorescence, the fluorescence emission in steady state, maximum efficiency of PSII (Fv′/Fm′), the fraction of PSII centers that are open, photochemical quenching, nonphotochemical quenching (NPQ), quantum efficiency of photosystem II (ΦPSII), electron transport rate (ETR) and quantum yield of CO2 assimilation (ϕCO2). For the study and validation of the results obtained in the experiments, the analysis of variance (ANOVA) was applied 0for each parameter with confidence intervals of 95 %. The results show that the frequencies of pulsed light had positive and negative effects on the fluorescence parameters with respect to the control treatment (continuous light). The frequencies that generated the best performance of Fv′/Fm′, NPQ, ΦPSII, ETR, ϕCO2 in tomato plants were 0.1, 1, 100 Hz, and 1 kHz. The increase obtained in these parameters can represent an optimal growth and productivity conditions for optimal energy consumption.

Leaf area, light environment, and gas exchange in Montepulciano grapevines trained to Tendone trellising system

Canopy light interception (CPFDInt), spectral irradiance, leaf water potential, gas- exchange and optical properties were measured in an irrigated vineyard (Vitis vinifera L. cv Montepulciano) trained to the so-called tendone system in which leaf area index (LAI) was varied by means of 50% (T50) or 75% (T75) cluster removal. The 20.5 t ha−1 yield in the unthinned treatment (UT) decreased by only 36% in T50 and by 52% in T75. LAI and CPFDInt similarly increased until summer pruning when LAI was 1.75 m2 m−2 in UT, and 25.6% or 62.2% higher in T50 and T75, respectively. The two thinned treatments had only 12.4% higher CPFDInt than in UT (1167.1 μmol m−2 s−1) due to the increased leaf self-shading. The red-to-far red ratio (R: FR) was as low as 0.10 below the canopy. Light-saturated CO2 assimilation (A max) in June averaged 14.4 μmol m−2 s−1 in sun-exposed leaves, and 7.6 μmol m−2 s−1 in shade leaves. By contrast, the apparent quantum yield of CO2 assimilation (φe) was not significantly affected by leaf position, averaging 0.029 and 0.070 mol mol−1 in June and October, respectively. Middle and low canopy leaves had only 27 or 6%, respectively, of the top canopy leaves actual CO2 assimilation rate.

Photosynthesis and photoinhibition in two differently coloured varieties of Oxalis triangularis - the effect of anthocyanin content

The purpose of this study was to clarify effects of anthocyanins on photosynthesis and photoinhibition in green and red leaves of Oxalis triangularis. Gas analysis indicated that green plants had the highest apparent quantum yield for CO2 assimilation [0.051 vs. 0.031 &mu;mol(CO2) &mu;mol-1(photon)] and the highest maximum photosynthesis [10.07 vs. 7.24 &mu;mol(CO2) m-2 s-1], while fluorescence measurements indicated that red plants had the highest PSII quantum yield [0.200 vs. 0.143 &mu;mol(e-) &mu;mol-1(photon)] and ETRmax [66.27 vs. 44.34 &mu;mol(e-) m-2 s-1]. Red plants had high contents of anthocyanins [20.11 mg g-1(DM)], while green plants had low and undetectable levels of anthocyanin. Red plants also had statistically significantly (0.05&gt;p&gt;0.01) lower contents of xanthophyll cycle components [0.63 vs. 0.76 mg g-1(DM)] and higher activities of the reactive oxygen scavenging enzyme ascorbate peroxidase [41.2 vs. 10.0 nkat g-1(DM)]. Anthocyanins act as a sunscreen, protecting the chloroplasts from high light intensities. This shading effect causes a lower photosynthetic CO2 assimilation in red plants compared to green plants, but a higher quantum efficiency of photosystem II (PSII). Anthocyanins contribute to photoprotection, compensating for lower xanthophyll content in red plants, and red plants are less photoinhibited than green plants, as illustrated by the Fv/Fm ratio.

Orange Rust Effects on Leaf Photosynthesis and Related Characters of Sugarcane.

Orange rust of sugarcane (Saccharum spp. hybrids), caused by Puccinia kuehnii, is a relatively new disease in the Western Hemisphere that substantially reduces yields in susceptible sugarcane genotypes. The objective of this study was to determine the physiological mechanisms of orange rust-induced reductions in sugarcane growth and yield by quantifying effects of the disease on leaf SPAD index (an indication of leaf chlorophyll content), net photosynthetic rate, dark respiration, maximum quantum yield of CO2 assimilation, carbon fixation efficiency, and the relationships between these leaf photosynthetic components and rust disease ratings. Plants growing in pots were inoculated with the orange rust pathogen using a leaf whorl inoculation method. A disease rating was assigned using a scale from 0 to 4 with intervals of 0.5. At disease ratings ≥2, the rust-infected leaf portion of inoculated plants showed significant reductions in SPAD index, maximum quantum yield, carbon fixation efficiency, stomatal conductance, leaf transpiration rate, and net photosynthetic rate; but the rusted portion of the infected leaves had increased intercellular CO2 concentration and leaf dark respiration rate. Although leaf SPAD index, photosynthetic rate, stomatal conductance, and transpiration rate at the rust-infected portion decreased linearly with increased rust rating, the effect of orange rust on photosynthetic rate was much greater than that on stomatal conductance and transpiration. Unlike earlier reports on other crops, reduction in leaf photosynthesis by orange rust under low light was greater than that under high light conditions. These results help improve the understanding of orange rust etiology and physiological bases of sugarcane yield loss caused by orange rust.

Relationships Between Quantum Yield for CO2 Assimilation, Activity of Key Enzymes and CO2 Leakiness in Amaranthus cruentus, a C4 Dicot, Grown in High or Low Light

In C(4) photosynthesis, a part of CO(2) fixed by phosphoenolpyruvate carboxylase (PEPC) leaks from the bundle-sheath cells. Because the CO(2) leak wastes ATP consumed in the C(4) cycle, the leak may decrease the efficiency of CO(2) assimilation. To examine this possibility, we studied the light dependence of CO(2) leakiness (phi), estimated by the concurrent measurements of gas exchange and carbon isotope discrimination, initial activities of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and pyruvate, orthophosphate dikinase (PPDK), the phosphorylation state of PEPC and the CO(2) assimilation rate using leaves of Amaranthus cruentus (NAD-malic enzyme subtype, dicot) plants grown in high light (HL) and low light (LL). phi was constant at photon flux densities (PFDs) >200 micromol m(-2) s(-1) and was around 0.3. At PFDs <150 micromol m(-2) s(-1), phi increased markedly as PFD decreased. At 40 micromol m(-2) s(-1), phi was 0.76 in HL and 0.55 in LL leaves, indicating that the efficiency of CO(2) assimilation at low PFD was greater in LL leaves. The activities of Rubisco and PPDK, and the phosphorylated state of PEPC all decreased as PFD decreased. Theoretical calculations with a mathematical model clearly showed that the increase in phi with decreasing PFD contributed to the decrease in the CO(2) assimilation rate. It was also shown that the 'conventional' quantum yield of photosynthesis obtained by fitting the straight line to the light response curve of the CO(2) assimilation rate at the low PFD region is seriously overestimated. Ecological implications of the increase in phi in LL are discussed.

Photosynthetic regulation of C4 desert plant Haloxylon ammodendron under drought stress

About 20-year-old desert plants of C4 species, Haloxylon ammodendron, growing at the southern edge of the Badain Jaran Desert in China, were selected to study the photosynthetic characteristics and changes in chlorophyll fluorescence when plants were subject to a normal arid environment (AE), moist atmospheric conditions during post-rain (PR), and the artificial supplement of soil water (SW). Results showed that under high radiation, in the AE, the species down-regulated its net assimilation rate (A) and maximum photochemical efficiency of PS II (Fv/Fm), indicating photoinhibition. However, under the PR and SW environments, A was up-regulated, with a unimodal diurnal course of A and a small diurnal change in Fv/Fm, suggesting no photoinhibition. When the air humidity or SW content was increased, the light compensation points were reduced; light saturation points were enhanced; while light saturated rate of CO2 assimilation (A max) and apparent quantum yield of CO2 assimilation (ΦC) increased. ΦC was higher while the A max was reduced under PR relative to the SW treatment. It was concluded that under high-radiation conditions drought stress causes photoinhibition of H. ammodendron. Increasing air humidity or soil moisture content can reduce photoinhibition and increase the efficiency of solar energy use.

Low temperature stress modifies the photochemical efficiency of a tropical tree species Hevea brasiliensis: effects of varying concentration of CO&lt;sub&gt;2&lt;/sub&gt; and photon flux density

Two clones of Hevea brasiliensis (RRII 105 and PB 235) were grown for one year in two distinct agroclimatic locations (warmer and colder, W and C) in peninsular India. We simultaneously measured gas exchange and chlorophyll (Chl) fluorescence on fully mature intact leaves at different photosynthetic photon flux densities (PPFDs) and ambient CO2 concentrations (Ca) and at constant ambient O2 concentration (21 %). Net photosynthetic rate (PN), apparent quantum yield for CO2 assimilation (Φc), in vivo carboxylation efficiency (CE), and photosystem 2 quantum yield (ΦPS2) were low in plants grown in C climate and these reductions were more predominant in RRII 105 than in PB 235 which was also reflected in their growth. We estimated in these clones the partitioning of photosynthetic electrons between CO2 reduction (JA) and processes other than CO2 reduction (J*) at low and high PPFDs and Ca. At high Ca (700 µmol mol-1) most of the photosynthetic electrons were used for CO2 assimilation and negligible amount went for other processes when PPFD was low (200-300 µmol m-2 s-1) both in the C and W climates. But at high PPFD (900-1 100 µmol m-2 s-1), J* was appreciably high even at a high Ca. Hence at normal ambient Ca and high irradiance, electrons can be generated in the photosynthetic apparatus far in excess of what can be safely utilised for photosynthetic CO2 reduction. However, at high Ca there was increased diversion of electrons to photosynthetic CO2 reduction which resulted in improved photosynthetic parameters even in plants grown in C climate.

Different Relationship Between Electron Transport and CO2 Assimilation in two Zea mays Cultivars as Influenced by Increasing Irradiance

Gas exchange and fluorescence parameters were measured simultaneously in two Zea mays L. cultivars (Liri and 121C D8) to assess the relationship between the quantum yield of electron transport (ΦPS2) and the quantum yield of CO2 assimilation (ΦCO2) in response to photosynthetic photon flux density (PPFD). The cv. Liri was grown under controlled environmental conditions in a climate chamber (CC) while cv. 121C D8 was grown in CC as well as outdoors (OT). By exposing the two maize cultivars grown in CC to an increasing PPFD, higher photosynthetic and photochemical rates were evidenced in cv. Liri than in cv. 121C D8. In Liri plants the ΦPS2/ΦCO2 ratio increased progressively up to 27 with increasing PPFD. This suggests that the reductive power was more utilised in non-assimilatory processes than in CO2 assimilation at high PPFD. On the contrary, by exposing 121C D8 plants to increasing PPFD, ΦPS2/ΦCO2 was fairly constant (around 11–13), indicating that the electron transport rate was tightly down regulated by CO2 assimilation. Although no significant differences were found between ΦPS2/ΦCO2 of the 121C D8 maize grown under CC and OT by exposing them to high PPFD, the photosynthetic rate and photochemical rates were higher in OT maize plants.

The relationship between photosystem II efficiency and quantum yield for CO(2) assimilation is not affected by nitrogen content in apple leaves.

Bench-grafted Fuji/M.26 apple (Malus domestica Borkh.) trees were fertigated with different concentrations of nitrogen by using a modified Hoagland's solution for 45 d. CO(2) assimilation and photosystem II (PSII) quantum efficiency in response to incident photon flux density (PFD) were measured simultaneously in recent fully expanded leaves under low O(2) (2%) and saturated CO(2) (1300 micromol mol(-1)) conditions. A single curvilinear relationship was found between true quantum yield for CO(2) assimilation and PSII quantum efficiency for leaves with a wide range of leaf N content. The relationship was linear up to a quantum yield of approximately 0.05 mol CO(2) mol(-1) quanta. It then became curvilinear with a further rise in quantum yield in response to decreasing PFD. This relationship was subsequently used as a calibration curve to assess the rate of non-cyclic electron transport associated with Rubisco and the partitioning of electron flow between CO(2) assimilation and photorespiration in different N leaves in response to intercellular CO(2) concentration (C(i)) under normal O(2) conditions. Both the rate of non-cyclic electron flow and the rate of electron flow to CO(2) or O(2) increased with increasing leaf N at any given C(i). The percentage of non-cyclic electron flow to CO(2) assimilation, however, remained the same regardless of leaf N content. As C(i) increased, the percentage of non-cyclic electron flow to CO(2) assimilation increased. In conclusion, the relationship between PSII quantum efficiency and quantum yield for CO(2) assimilation and the partitioning of electron flow between CO(2) assimilation and photorespiration are not affected by N content in apple leaves.

Effects of growth under UVA radiation on CO2 assimilation, carbon partitioning, PSII photochemistry and resistance to UVB radiation in Brassica napus cv. Topas

The effects of growth of Brassica napus L. cv. Topas under PAR or PAR+UVA radiation was assessed with respect to sensitivity to subsequent exposure to UVB radiation. Despite the fact that growth under PAR+UVA induced minimal effects in photosystem II (PSII) photochemistry, growth under PAR+UVA inhibited the accumulation of the photosynthetic end products, sucrose and starch. This was associated with a decreased capacity for ribulose 1,5-bisphosphate (RuBP) regeneration, a decreased capacity for light- and CO 2 -saturated rates of CO2 assimilation, a decrease in the apparent quantum yield for CO2 assimilation, an over-reduction of chloroplast stroma, an increased susceptibility to the feedback effects on photosynthesis and a stimulation of glycolysis compared to controls grown under PAR. Subsequent exposure to UVB decreased the maximum Rubisco activity in leaves of both PAR- and PAR+UVA-grown plants. However, the decrease in the capacity for CO2 assimilation in PAR-grown plants exposed to UVB did not appear to be associated with limitations at the level of PSII linear electron transport, but rather with a decreased capacity for sucrose biosynthesis, limited triose-P utilization and a decreased capacity for RuBP regeneration. In contrast, the decreased capacity for CO 2 assimilation in PAR+UVA-grown plants exposed to UVB was associated with an inhibition of PSII photochemistry and a decreased supply of ATP. Thus, growth under UVA radiation appears to induce feedback-limited photosynthesis and does not enhance resistance of Brassica napus to UVB radiation.

159 The Relationship between Actual Photosystem II Efficiency and Quantum Yield for CO2 Assimilation is Not Affected by Nitrogen Content in Apple Leaves

Bench-grafted Fuji/M26 apple (Malus domestica Borkh) trees were fertigated with different concentrations of nitrogen by using a modified Hoagland's solution for 45 days. CO2 assimilation and actual photosystem II (PSII) efficiency in response to incident photon flux density (PFD) were measured simultaneously in recent fully expanded leaves under low O2 (2%) and saturated CO2 (1300 ppm) conditions. A single curvilinear relationship was found between true quantum yield for CO2 assimilation and actual PSII efficiency for leaves with a wide range of leaf N content. The relationship was linear up to a quantum yield of approximately 0.05 mol CO2/mol quanta, then became curvilinear with a further rise in quantum yield in response to decreasing PFD. This relationship was subsequently used as a calibration curve to assess the rate of linear electron transport associated with rubisco and partitioning of electron flow between CO2 assimilation and photorespiration in different N leaves in response to intercellular CO2 concentration (Ci) under normal O2 conditions. Both the rate of linear electron flow, and the rate to CO2 or O2 increased with increasing leaf N at any given Ci, but the percentage of linear electron flow to CO2 assimilation remained the same regardless of leaf N content. As Ci increased, the percentage of linear electron flow to CO2 assimilation increased. In conclusion, the relationship between actual PSII efficiency and quantum yield for CO2 assimilation and the partitioning of electron flow between CO2 assimilation and photorespiration are not affected by N content in apple leaves.