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Abstract: The capacity of a Quercus pubescens forest to resist recurrent drought was assessed on an in situ experimental platform through the measurement of a large set of traits (ecophysiological and metabolic) studied under natural drought (ND) and amplified drought (AD) induced by partial rain exclusion. This study was performed during the third and fourth years of AD, which correspond to conditions of moderate AD in 2014 and harsher AD in 2015, respectively. Although water potential (Ψ) and net photosynthesis (Pn) were noticeably reduced under AD in 2015 compared to ND, trees showed similar growth and no oxidative stress. The absence of oxidative damage could be due to a strong accumulation of α‐tocopherol, suggesting that this compound is a major component of the Q. pubescens antioxidant system. Other antioxidants were rather stable under AD in 2014, but slight changes started to be observed in 2015 (carotenoids and isoprene) due to harsher conditions. Our results indicate that Q. pubescens could be able to cope with AD, for at least 4 years, likely due to its antioxidant system. However, growth decrease was observed during the fifth year (2016) of AD, suggesting that this resistance could be threatened over longer periods of recurrent drought. [ABSTRACT FROM AUTHOR]

Charcoal canker of oak is a common disease in the western regions of Iran, which has widely spread due to climate change and increasing drought in recent years. Biscogniauxia mediterranea and Obolarina persica are known as two agents of oak charcoal canker in Zagros forests causing the dieback of oak trees. The effects of charcoal disease agent pathogens on the growth and physiological response of two-year-old seedlings of Quercus brantii were evaluated under drought stress in greenhouse over a period of nine months. Survival was 21.7% lower in seedlings inoculated with B. mediterranea subjected to drought stress compared with control treatment. The length of lesions on stems was 68% more under charcoal pathogens in drought stressed compared with the length of lesions caused by charcoal pathogens only. On the other hand, stem lesion length caused by B. mediterranea was 25% greater than those caused by O. persica. Some morphophysiological characteristics were affected by charcoal pathogens alone and/or in combination with drought. Diameter, height and biomass decreased the most in seedlings inoculated with charcoal pathogens under drought stress. Pathogenic agents alone and in combination with drought stress reduced photosynthetic rates, stomatal conductance, transpiration, the maximum Rubisco activity ( Vcmax), maximum photochemical efficiency of photosystem II ( Fv/ Fm) and xylem water potential. In general, drought increased the aggressiveness of fungus and intensification of their destructive effects in Quercus brantii. Moreover, there was no significant difference between the effect of B. mediterranea and O. persica on the morphophysiological parameters studied. [ABSTRACT FROM AUTHOR]

This study was undertaken to investigate how different cultivars of sweet basil ( Ocimum basilicum) responded to boron (B) excess. Two purple-leafed and eight green-leafed cultivars were hydroponically grown for 20 d with 0.2 or 20 mg L-1 B in the nutrient solution. Leaf B concentration, gas exchanges, chlorophyll a fluorescence, and oxidative stress were determined at the end of the treatment along with the severity of leaf necrosis. A range of tolerance to B toxicity was found: the green cultivars were more susceptible than the purple-leafed ones characterized by a higher constitutive anthocyanin concentration. In all the genotypes B excess resulted in oxidative stress as determined by accumulation of malondialdehyde by-products (MDA), reduced photosynthesis, and the occurrence of leaf burn. A close correlation was found between leaf B accumulation and oxidative stress, as well as between oxidative stress and the severity of leaf burn. Net photosynthesis ( Pn) was reduced due to both stomatal and nonstomatal limitations in the green cultivars whereas the reduction of Pn in the purple leaves was only attributable to stomatal factors. Chlorophyll a fluorescence revealed a decrease in the maximum quantum yield of PSII ( Fv/ Fm) and in the electron transport rate (ETR) in plants grown with B excess although less reduction was observed in the purple genotypes. The quantum yield of PSII (ΦPSII) decreased as a result of B toxicity only in the green cultivars. It is concluded that anthocyanins are involved in attenuation of the negative effects of B toxicity. [ABSTRACT FROM AUTHOR]

© New Phytologist (2004) doi: 10.1111/j.1469-8137.2004.01000.x [ABSTRACT FROM AUTHOR]

Summary• The combined effects of the two pollutants, cadmium and ozone, on sunflower (Helianthus annuus) metabolism are analysed here. • Photosysnthetic processes and ascorbate metabolism were studied in sunflower plants grown for 15 d in the presence of cadmium and exposed to acute O3 treatments. • CO2 assimilation rate was reduced in plants subjected to Cd(II) and/or O3 treatments, but no alterations in stomatal conductance and Fv : Fm ratio were observed. Rubisco activity was significantly reduced only in plants grown in the presence of cadmium indicating that the photosynthetic process is mainly altered by this factor. Photochemical quenching and the quantum efficiency of PSII in steady-state conditions were significantly depressed and nonphotochemical quenching increased in stressed plants. Cd(II) and O3 also strongly affected ascorbate metabolism. • The changes in ascorbate redox state and the increase in ascorbate-redox enzymes strongly supported an ascorbate over-utilization in Cd(II) and/or O3–treated plants. However, the increase in ascorbate-based detoxification mechanisms did not provide complete protection against the oxidative stress imposed by the two pollutants, since an increase in lipid peroxidation and protein oxidation accompanied a decrease in photosynthesis under pollutant exposure. [ABSTRACT FROM AUTHOR]

CO2 assimilation, xanthophyll cycle pigments and PSII efficiency were analyzed in two different ages of pumpkin leaves (Cucurbita pepo L. cv. Ambassador) exposed to 150 nmol mol-1of ozone (5 days, 5 h day-1). Gas-exchange measurements revealed a reduction in CO2 assimilation and stomatal conductance, accompanied by an increase in the intercellular CO2 concentration both in young and in mature leaves as compared to their respective controls. In both leaves, F0 remained unchanged, while Fm and the Fv/Fm ratio decreased after O3 fumigation, indicating that ozone may induce an alteration in the capability of photosystem H (PSII) to reduce the primary acceptor QA. In the mature leaves the photochemical quenching (qp) was significantly lowered by the pollutant, but this was not the case in the young leaves where qp did not change. In both mature and young ozonated pumpkin leaves, the development of non-photochemical quenching caused a decrease in the PSII photochemical rate, as shown by the correlation between Fv/Fm and the de-epoxidation state of dark-adapted leaves. Decreases in the Fv/Fm ratio are generally attributed to damage to the PSII reaction centre, apart from the down-regulation of the capacity of PSII electron transport. While in young ozonated leaves the decrease in the Fv/Fm ratio was not associated with damage to the D1 protein, in mature ozonated pumpkin leaves, the decrease in the Fv/Fm was accompanied by a significant decline in the D1 content. In conclusion, ozone exposure induces alterations in the light reactions of photosynthesis in both young and mature leaves. However, in young leaves the engagement of the xanthophyll cycle appears to counteract ozone effects against the photosynthetic apparatus as demonstrated by the absence of damage to the D1 protein. On the other hand, the loss of D1 protein in mature fumigated leaves suggests that the activation of the xanthophyll cycle is not sufficient to prevent photoinhibition, probably because a physiological state of senescence adds to the oxidative stress. [ABSTRACT FROM AUTHOR]

Ultrastructural and physiological characteristics of the C3-C4 intermediate Neurachne minor S. T. Blake (Poaceae) are compared with those of C3and C4 relatives, and C3-C4 Panicum milioides Nees ex Trin . . N. minor consistently exhibits very low CO2 compensation points (Γ : ≤ 1.0, usually 0.0.6 Pa) yet has C3-like δ13C values. CO2 assimilation rates (A) respond like those of C3 plants to a decrease in O2 partial pressure (2 × 104-1.9 × 103 Pa) at ambient CO2 levels, but this response is progressively attenuated until negligible at very low CO2. By contrast, other species of the Neurachneae are clearly either C4 (two spp.) or C3 (seven spp.). For plants grown and measured at different photon flux densities (PFDs), Γ for N. minor and P. milioides increases from 0.5 to 1.0, and from 1.0 to 2.1 Pa, respectively, as PFD is decreased from 1860 to 460 μmol m-2s-1. In N. minor, the O2response of Γ is either biphasic as in P. milioides, but much diminished and with a higher transition point, or is very C4-like. As in C4 relatives, inner sheath cells contain numerous chloroplasts. Their walls possess a suberized lamella, which may make them more CO2-tight than bundle sheath cells of P. milioides, contributing to the almost C4-like Γ characteristics of N. minor. The biochemical basis of C3-C4 intermediacy is considered. [ABSTRACT FROM AUTHOR]