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Jason J. Griffin, Thomas G. Ranney, and D. Mason Pharr

Tolerance to high solar irradiation is an important aspect of stress tolerance for landscape plants, particularly for species native to understory conditions. The objective of this study was to evaluate differential tolerance to high solar irradiation and underlying photosynthetic characteristics of diverse taxa of Illicium L. grown under full sun or 50% shade. Eleven commercially available taxa of Illicium were evaluated for light tolerance by measuring light-saturated photosynthetic capacity (Amax), dark-adapted quantum efficiency of photosystem II (Fv/Fm), and relative chlorophyll content using a SPAD chlorophyll meter. Comparisons of Amax indicated that three of the 11 taxa (I. anisatum L., I. parviflorum Michx. ex Vent., and I. parviflorum `Forest Green') maintained similar rates of light-saturated carbon assimilation when grown in either shade or full sun. All other taxa experienced a significant reduction in Amax when grown in full sun. Chlorophyll fluorescence analysis demonstrated that Fv/Fm was similar between sun and shade plants for the same three taxa that were able to maintain Amax. These taxa appeared to experience less photoinhibition than the others and maintained greater maximum photochemical efficiency of absorbed light. SPAD readings were not significantly reduced in these three taxa either, whereas most other taxa experienced a significant reduction. In fact, SPAD readings were significantly higher in I. parviflorum `Forest Green' when grown under full sun, which also maintained the highest Amax of all the taxa. These results suggest that there is considerable variation in light tolerance among these taxa, with I. parviflorum `Forest Green' demonstrating superior tolerance to high light among the plants compared. A more rigorous examination of I. parviflorum `Forest Green' (high light tolerance) and I. floridanum Ellis (low-light tolerance) demonstrated that I. parviflorum `Forest Green' had a considerably higher Amax, a higher light saturation point, greater potential photosynthetic capacity, reduced susceptibility to photoinhibition as indicated by superior PSII efficiency following light exposure, greater capacity for thermal de-excitation as indicated by a higher rate of nonphotochemical quenching (NPQ) under full sun, greater apparent electron transport rate (ETR) at mid-day, and higher concentrations of the free-radical scavenger myo-inositol. All of these factors contribute potentially to a greater capacity to use light energy for carbon fixation while minimizing photodamage.

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Brandon R. Smith and Lailiang Cheng

The objective of this study was to quantify how photoprotective mechanisms in the leaves of `Concord' grapevines (Vitis labruscana Bailey) respond to a range of iron (Fe) supply. Own-rooted, 1-year-old container-grown vines were fertigated twice weekly for 11 weeks with a complete nutrient solution containing 1, 10, 20, 50, or 100 μm Fe from ferric ethylenediamine di (o-hydroxyphenylacetic) acid (Fe-EDDHA). Leaf total Fe content did not increase in response to Fe supply; however, “active” Fe (extracted with 2,2′-dipyridyl) and chlorophyll (Chl) increased on a leaf area basis as applied Fe increased. At the lowest active Fe level, leaf absorptance and the efficiency of excitation transfer (Fv′/Fm′) was lower, and nonphotochemical quenching (NPQ) was significantly greater. Photosystem II (PSII) quantum efficiency decreased curvilinearly, and the proportion of PSII reaction centers in the open state (qP) decreased linearly as active Fe content decreased. On a Chl basis, the xanthophyll cycle pool size [violaxanthin (V) + antheraxanthin (A) + zeaxanthin (Z)], lutein, and β-carotene increased curvilinearly as active Fe decreased, and neoxanthin (Neo) increased at the lowest Fe level. On a leaf area basis, as active Fe decreased, V+A+Z and β-carotene decreased curvilinearly, and lutein and Neo decreased linearly. At noon, conversion of V to A and Z increased as active Fe decreased. On a Chl basis, activities of antioxidant enzymes superoxide dismutase (SOD), monodehydroascorbate reductase (MDAR), and dehydroascorbate reductase (DHAR) increased curvilinearly, and glutathione reductase (GR) activity increased linearly as active Fe levels declined. Ascorbate peroxidase (APX) and catalase (CAT), on a Chl basis, were relatively constant. On a leaf area basis, a decrease in active Fe increased SOD and MDAR activity, whereas APX, CAT, DHAR and GR activity decreased. Antioxidant metabolites ascorbate (AsA), dehydroascorbate (DAsA), reduced glutathione (GSH) and oxidized glutathione (GSSG) also increased in response to Fe limitation when expressed on a Chl basis, whereas on a leaf area basis AsA and DAsA decreased and GSH increased curvilinearly. The GSH:GSSG ratio increased as active Fe declined, whereas the AsA:DAsA ratio did not change. In conclusion, both photoprotective mechanisms, xanthophyll cycle-dependent thermal dissipation and the ascorbate-glutathione antioxidant system, are enhanced in response to Fe deficiency to cope with excess absorbed light. In a low soil pH tolerant species such as V. labruscana, the foliar antioxidant system was upregulated in response to excess absorbed light from Fe deficiency-induced chlorosis, and there was no evidence of an increase in oxidative stress from high rates of applied Fe-EDDHA.

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Jia Li, Liyun Liu, Huanqi Zhou, and Meng Li

-control software (PAMWin 3.0). The photochemical quenching (qP) = (F m – F s )/(F m ′ – F o ′), nonphotochemical quenching (NPQ) = (F m – F m ′)/F m ′, maximum photochemical efficiency of PSII (F v /F m , F v = F m – F o ), and actual photochemical efficiency

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Jinhong Yuan, Man Xu, Wei Duan, Peige Fan, and Shaohua Li

ways for leaves to dissipate excess light energy such as non-photochemical quenching, photorespiration, and Mehler peroxidase reactions ( Asada, 1999 ; Chen et al., 2004a ; Dai et al., 2004 ; Jiang et al., 2006 ; Noctor et al., 2002 ). Non-photochemical

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Mengyang Liu, Yin Lu, Shan Wang, Fang Wu, Jingrui Li, Yanhua Wang, Jianjun Zhao, and Shuxing Shen

s)/( F m′ – Fo ), non-photochemical quenching (NPQ) = ( F m − F m′)/ F m′, the effective quantum yields of PSII (ΦII) = ( F m′– F )/ F m′, quantum yield of regulatory energy dissipation [ Y (NPQ)] = 1 – ΦII – 1/[NPQ + 1 + qL( F m/ F o – 1

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Jiunn-Yan Hou, Tim L. Setter, and Yao-Chien Alex Chang

fluorescence (Fm), quantum yield, photochemical quenching (Qp), and non-photochemical quenching (Qn) were measured after a saturation pulse with a photosynthesis yield analyzer (MINIPAM; Heinz Walz, Effeltrich, Germany). The fluorescence ratio Fv/Fm, where Fv

Open access

Joshua K. Craver, Krishna S. Nemali, and Roberto G. Lopez

-term exposure to high radiation intensities can result in increased nonphotochemical quenching of “absorbed” radiation ( Müller et al., 2001 ). Absorbed radiation must be used in photosynthesis or converted to thermal energy through the process of

Open access

Zhenghai Zhang, Hai Sun, Cai Shao, Huixia Lei, Jiaqi Qian, Yinyin Ruan, and Yayu Zhang

protect itself against damage by excess illumination ( Bizarre et al., 2019 ; Klughammer and Schreiber, 2008 ). Y(NPQ) is linked to the xanthophyll cycle, which involves nonphotochemical quenching of excess light energy in PS II ( Jahns and Holzwarth

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Ritu Dhir, Richard L. Harkess, and Guihong Bi

cultivars with thick, broad leaves and higher stomatal frequency had higher transpirational cooling, gas exchange, and CO 2 fixation ( Natarajan and Kuehny, 2008 ). Non-photochemical quenching and antioxidant enzymes were the main mechanisms in seedlings of

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Young-Hwan Shin, Rui Yang, Yun-Long Shi, Xu-Min Li, Qiu-Yue Fu, Jian-Liang Lu, Jian-Hui Ye, Kai-Rong Wang, Shi-Cheng Ma, Xin-Qiang Zheng, and Yue-Rong Liang

the photosystems under high-light conditions ( Powles, 1984 ). Carotenoids function to dissipate excessive energy or excitation nondestructively as heat, a phenomenon being called nonphotochemical quenching (NPQ) of chlorophyll a fluorescence