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M. Carmen González-Mas, M. José Llosa, Antonio Quijano and M. Angeles Forner-Giner

efficiency of PSII photochemistry ( A ), q P , photochemical quenching ( B ), and q N , nonphotochemical quenching ( C ) of ‘Navelina’ trees grown in calcareous soil. Data are mean values and sd for n = 8. For all of measurements, sd of Φ PSII , q P , and

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Shuyang Zhen and Marc W. van Iersel

( Baker, 2008 ; Maxwell and Johnson, 2000 ). Nonphotochemical quenching, which provides an index of the amount of absorbed light that is dissipated as heat, was calculated as NPQ = ( F m − F m ′)/ F m ′ ( Maxwell and Johnson, 2000 ). The PPF inside

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Marc W. van Iersel, Geoffrey Weaver, Michael T. Martin, Rhuanito S. Ferrarezi, Erico Mattos and Mark Haidekker

target ETR of 100 µmol·m −2 ·s −1 , Φ PSII gradually declined, whereas ( C ) nonphotochemical quenching increased. As a result, ( D ) PPFD had to be gradually increased throughout the 16-h light period to maintain a target ETR of 100 µmol·m −2 ·s −1

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Lailiang Cheng, Leslie H. Fuchigami and Patrick J. Breen

Bench-grafted `Fuji' apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] trees on Malling 26 (M.26) rootstocks were fertigated for 6 weeks with N concentrations ranging from 0 to 20 mm. These treatments produced levels of leaf N ranging from 0.9 to 4.3 g·m-2. Over this range, leaf absorptance increased curvilinearly from 74.8% to 92.5%. The light saturation point for CO2 assimilation expressed on the basis of absorbed light increased linearly at first with increasing leaf N, then reached a plateau at a leaf N content of ≈3 g·m-2. Under high light conditions (photosynthetic photon flux of 1500 μmol·m-2·s-1), the amount of absorbed light in excess of that required to saturate CO2 assimilation decreased with increasing leaf N. Chlorophyll fluorescence measurements revealed that the maximum photosystem II (PSII) efficiency of dark-adapted leaves was relatively constant over the leaf N range, except for a slight decrease at the lower end. As leaf N increased, nonphotochemical quenching declined under high light, and there was an increase in the efficiency with which the absorbed photons were delivered to open PSII centers. The photochemical quenching coefficient remained high except for a decrease at the lower end of the leaf N range. Actual PSII efficiency increased curvilinearly with increasing leaf N, and was highly correlated with light-saturated CO2 assimilation. The fraction of absorbed light potentially going into singlet oxygen formation was estimated to be ≈10%, regardless of leaf N status. It was concluded that there was more excess absorbed light in low N leaves than in high N leaves under high light conditions. Nonphotochemical quenching was enhanced with decreasing leaf N to reduce both the PSII efficiency and the probability of damage from photooxidation by excess absorbed light.

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Madhulika Sagaram and Jacqueline K. Burns

more chlorophyll fluorescence, photochemical, and/or nonphotochemical quenching components in leaves produce a signature that identifies asymptomatic HLB-infected citrus trees. Because leaf starch content is expected to vary in HLB-affected leaves

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Qin Shi, Yunlong Yin, Zhiquan Wang, Wencai Fan and Jianfeng Hua

chlorophyll contents (Chl t ), nonphotochemical quenching (NPQ), superoxide dismutase (SOD), and proline. Discussion The influence of water deficit on plants performance described in several species may be restrictive and even devastating ( Álvarez et al

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Jieshan Cheng, Peige Fan, Zhenchang Liang, Yanqiu Wang, Ning Niu, Weidong Li and Shaohua Li

′/F m ′). The following calculations were made: (1) photochemical quenching (qP) = (F m ′ − F s )/(F m ′ – F 0 ′), and nonphotochemical quenching (NPQ) = (F m /F m ′) – 1; (2) ΦPSII = (F m ′ − F s )/F m ′ ( Genty et al., 1989 ). Carbohydrate

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Lailiang Cheng, Leslie H. Fuchigami and Patrick J. Breen

Photosystem II (PSII) efficiency and CO2 assimilation in response to photon flux density (PFD) and intercellular CO2 concentration (Ci) were monitored simultaneously in leaves of apple, pear, apricot, and cherry with a combined system for measuring chlorophyll fluorescence and gas exchange. When photorespiration was minimized by low O2 (2%) and saturated CO2 (1300 ppm), a linear relationship was found between PSII efficiency and the quantum yield for CO2 assimilation with altering PFD, indicating CO2 assimilation in this case is closely linked to PSII activity. As PFD increased from 80 to 1900 μmol·m–2·s–1 under ambient CO2 (350 ppm) and O2 (21%) conditions, PSII efficiency decreased by increased nonphotochemical quenching and decreased concentration of open PSII reaction centers. The rate of linear electron transport showed a similar response to PFD as CO2 assimilation. As Ci increased from 50 to 1000 ppm under saturating PFD (1000 μmol·m–2·s–1) and ambient O2, PSII efficiency was increased initially by decreased nonphotochemical quenching and increased concentration of open PSII reaction centers and then leveled off with further a rise in Ci. CO2 assimilation reached a plateau at a higher Ci than PSII efficiency because increasing Ci diverted electron flow from O2 reduction to CO2 assimilation by depressing photorespiration. It is concluded that PSII efficiency is regulated by both nonphotochemical quenching and concentration of open PSII reaction centers in response to light and CO2 to meet the requirement for photosynthetic electron transport.

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Susan Lurie, Reuven Ronen and Shimon Meier

Storing `Maor' green bell peppers (Capsicum annuum L.) for 3 weeks at 2C resulted in the development of chilling injury (CI) evidenced as surface pitting. Fruit held at 8C did not develop any CI symptoms, but, after 3 weeks of storage, the fruit began to change color from green to red. PAM fluorometry was used to measure changes in photosynthetic competency in whole green bell peppers. Three photosynthetic characteristics could be measured by this method: quantum yield (Fm/Fe), photochemical quenching (Qp), and nonphotochemical quenching (Qnp). Fm/Fo decreased 90% during the first week of storage at 2C and remained low thereafter, while Qnp decreased after 2 weeks at 2C, just before the peppers began to develop CL Qp was similar at both storage temperatures. Potassium leakage as a CI measurement also increased in excised pepper discs after 2 weeks at 2C. The results indicate that PAM fluorometry can measure CI nondestructively before tissue damage is visible in green peppers.

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Hector Valenzuela, Stacy Riede and Harry Yamamoto

Portable chlorophyll fluorometers have made it possible to evaluate the photosynthetic efficiency of photosystem 11 for vegetable crops under ambient conditions. A sampling protocol was first established to eliminate variability due to positioning of the fiber optics in relation to the leaf, leaf selection, and natural environmental variability. Fluorescence parameters of the quantum yield of noncyclic electron transport (DF/Fm') and electron transport rate (ETR) were taken from several economically important vegetables under ambient conditions between 11 and 14 h. The objective of the second part of the study was to conduct in situ chlorophyll fluorescence and biomass determinations as affected by salt stress and N deficiency. DF/Fm' and ETR were studied in rhizobium inoculated, noninoculated and inorganic N-fed soybean and differences in fluorescence were related to yield. The influence that salt stress, and several N rates have on fluorescence photochemical quenching (qP) and nonphotochemical quenching (qN), NPQ ([Fm-Fm']/Fm'), DF/Fm' and ETR for hydroponically grown lettuce will also be presented.