Super-rice hybrids are two-line hybrid rice cultivars with 15 to 20 % higher yields than the raditional three-line hybrid rice cultivars. Response of photosynthetic functions to midday photoinhibition was compared between seedlings of the traditional hybrid rice (Oryza sativa L.) Shanyou63 and two super-rice hybrids, Hua-an3 and Liangyoupeijiu. Under strong midday sunlight, in comparison with Shanyou63, the two super-rice hybrids were less photoinhibited, as indicated by the lower loss of the net photosynthetic rate (PN), the quantum yield of photosystem 2 (ΦPS2), and the maximum and effective quantum yield of PS2 photochemistry (Fv/Fm and Fv'/Fm'). They also had a much higher transpiration rate. Hence the super-rice hybrids could protect themselves against midday photoinhibition at the cost of water. The photoprotective de-epoxidized xanthophyll cycle components, antheraxanthin (A) and zeaxanthin (Z), were accumulated more in Hua-an3 and Liangyoupeijiu than in Shanyou63, but the size of xanthophyll cycle pool of the seedlings was not affected by midday photoinhibition. Compared to Shanyou63, the super-rice hybrids were better photoprotected under natural high irradiance stress and the accumulation of Z and A, not the size of the xanthophyll pool protected the rice hybrids against photoinhibition. and Q. A. Wang, C. M. Lu, Q. D. Zhang.
The review deals with thermal dissipation of absorbed excitation energy within pigment-protein complexes of thylakoid membranes in higher plants. We focus on the de-excitation regulatory processes within photosystem 2 (PS2) that can be monitored as non-photochemical quenching of chlorophyll (Chl) a fluorescence consisting of three components known as energy-dependent quenching (qE), state-transition quenching (qT), and photoinhibitory quenching (qI). We summarize the role of thylakoid lumen pH, xanthophylls, and PS2 proteins in qE mechanism. Further, both the similarity between qE and qI and specific features of qI are described. The other routes of thermal energy dissipation are also mentioned, that is dissipation within photosystem 1 and dissipation through the triplet Chl pathway. The significance of the individual de-excitation processes in protection against photo-oxidative damage to the photosynthetic apparatus under excess photon supply is stretched. and M. Štroch, V. Špunda, I. Kurasová.
We investigated the effects of long-term acclimation of Eucalyptus nitens seedlings to ultraviolet-A (UV-A) irradiation (320-400 nm) on phenolic compounds (gallotannins, stilbenes, and flavonols), photochemical efficiency, and chlorophyll and carotenoid contents. Seedlings were raised under four nutrient regimes, ranging from low to high application rates, in an environment that included or excluded UV-A irradiance. Our aims were: to classify phenolic compounds that absorb in the UV-A and their relative contribution to total UV-A absorption; to identify how phenolic compounds respond to UV-A exposure and exclusion, and to determine how plant nutrient status affects acclimation of photo-and pigment-chemistry to UV-A exposure and exclusion. Gallotannins contributed to only a minor fraction of total absorption within the lower range (320-360 nm) of the UV-A spectrum. Stilbene and flavonol compounds dominated absorption within the 320-360 and 360-400 nm ranges, respectively. Contents of gallotannin were generally high in UV-A-exposed seedlings. Although there was a significant effect of UV-A on contents of stilbenes, a general response (across nutrient treatment comparisons) was not evident. Contents of flavonols were not affected by UV-A exposure. Contents of gallotannin, stilbene, and flavonols decreased from low to high nutrient-application treatments. There were no effects of UV-A on photochemical efficiency or pigment-chemistry. and D. C. Close ... [et al.].
The low chlorophyll b mutant of high yield rice had a lower light-harvesting complex 2 content than the wild type. The stability of oxygen evolution side of photosystem 2 was only slightly lower. A lower photon absorption rate and a stronger xanthophyll cycle capacity of this mutant led to a higher endurance to strong irradiance and a lower photoinhibition as compared with the wild type rice. and Xinbin Dai ... [et al.].
The concentrations of photosynthetic pigments decreased in both chilling stressed species but the ratios of chlorophyll (Chl) a/b and total carotenoids (Car)/Chls were depressed only in faba bean. The contents of α+β carotene and lutein+lutein-5,6-epoxide remained unaffected in both species, but the de-epoxidation state involving the components of xanthophyll cycle increased in pea. Under chilling stress the photosynthetic electron transport associated with photosystem 2, PS2 (with and without the water oxidising complex) decreased in both plant species, the inhibition being higher in faba bean. The intrachloroplast quinone pool also decreased in both stressed species, yet an opposite trend was found for cytochrome b559LP. Under stress an increasing peroxidation of thylakoid acyl lipids was detected in pea, but higher protein/Chl ratio was detected in faba bean. Thus the acceptor side of PS2 is mostly affected in both chilling stressed species, but faba bean is more sensitive. and F. C. Lidon ... [et al.].
At chilling stress, the contents of photosynthetic pigments decreased significantly in maize, but in wheat the contents of chlorophyll (Chl) remained unchanged whereas the contents of total carotenoids (Car) increased. In both species the contents of α+β carotene and lutein + lutein-5,6-epoxide remained unaffected, but the de-epoxidation state involving the components of the xanthophyll cycle increased. Under chilling stress the photosynthetic electron transport also displayed a general failure in maize but in wheat only photosystem (PS) 2 coupled to the water oxidation complex was inhibited. Moreover, in stressed maize the quinone pool decreased, while the low and high potential forms of cytochrome b559 increased. In wheat only the contents of cytochrome b559LP decreased. Peroxidation of acyl lipids in the chloroplast lamellae became more distinct in chilling stressed maize but could also be detected in wheat. Thus in chilling stressed maize prevails an impairment of the acceptor site of PS2 while in wheat photodamage is restricted to the electron donation pathway from water to P680 or to the oxygen evolving complex. and F. C. Lidon ... [et al.].
Exposure of plants to irradiation, in excess to saturate photosynthesis, leads to reduction in photosynthetic capacity without any change in bulk pigment content. This effect is known as photoinhibition. Photoinhibition is followed by destruction of carotenoids (Cars), bleaching of chlorophylls (Chls), and increased lipid peroxidation due to formation of reactive oxygen species if the excess irradiance exposure continues. Photoinhibition of photosystem 2 (PS2) in vivo is often a photoprotective strategy rather than a damaging process. For sustainable maintenance of chloroplast function under high irradiance, the plants develop various photoprotective strategies. Cars perform essential photoprotective roles in chloroplasts by quenching the triplet Chl and scavenging singlet oxygen and other reactive oxygen species. Recently photoprotective role of xanthophylls (zeaxanthin) for dissipation of excess excitation energy under irradiance stress has been emphasised. The inter-conversion of violaxanthin (Vx) into zeaxanthin (Zx) in the light-harvesting complexes (LHC) serves to regulate photon harvesting and subsequent energy dissipation. De-epoxidation of Vx to Zx leads to changes in structure and properties of these xanthophylls which brings about significant structural changes in the LHC complex. This ultimately results in (1) direct quenching of Chl fluorescence by singlet-singlet energy transfer from Chl to Zx, (2) trans-thylakoid membrane mediated, ΔpH-dependent indirect quenching of Chl fluorescence. Apart from these, other processes such as early light-inducible proteins, D1 turnover, and several enzymatic defence mechanisms, operate in the chloroplasts, either for tolerance or to neutralise the harmful effect of high irradiance. and N. K. Choudhury, R. K. Behera.
Chlorophyll (Chl) fluorescence decay measurements were performed on higher plant leaves to investigate the photoprotective mechanisms under in vivo conditions. Measurements on leaves with different amounts of zeaxanthin pointed out that zeaxanthin is necessary for most of the observed nonphotochemical energy quenching, that has to be activated by a transthylakoid ΔpH. An additional sustained energy quenching component was clearly resolved in leaves with high amounts of zeaxanthin. The changes of the Chl fluorescence decay parameters did not correlate with a photoprotective energy dissipation in the reaction centre of photosystem 2 (P680), nor with a ΔpH-mediated, zeaxanthin-independent aggregation of the antenna complexes; no indications for a state 1/state 2 transition of the main light-harvesting complex LHC2 were found.
The content of cytokinins (CKs), the plant inhibitors of the final phase of plant development, senescence, is effectively controlled by irreversible degradation catalysed by cytokinin oxidase/dehydrogenase (CKX). In transgenic tobacco, denoted as AtCKX, with over-expressed CKX causing lowered CK content, we investigated changes in the time courses of chlorophyll (Chl) and xanthophyll (violaxanthin, antheraxanthin, zeaxanthin, neoxanthin, and lutein) contents. We also determined parameters of slow Chl fluorescence kinetics such as minimum Chl fluorescence yield in the darkadapted state F0, maximum quantum yield of PS2 photochemistry (Fv/Fm), maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in photosystem 2 (PS2), Fv/F0, non-photochemical quenching (NPQ), and effective quantum yield of photochemical energy conversion in PS2 (Φ2). We used three different developmental leaf stages, old, mature, and young, and compared this with time courses of these characteristics in leaves with natural CK levels. The parameters Fv/Fm, Fv/F0, and Φ2 were unchanged during ageing in AtCKX plants in contrast to control ones where a significant decrease in old leaves was found. In control plants F0 increased during ageing, but in the oldest leaf a considerable decrease was observed. This could indicate progressive damage to PS2 reaction centres and then detachment and rapid degradation of Chl. This is in agreement with time course of Chl content. NPQ decreased with age and was similar in both plant types. We observed a decline of xanthophyll contents in the oldest leaves in both plant types, but the contents were enhanced in AtCKX compared to control plants, especially of neoxanthin. The higher xanthophyll contents in the transgenic plants contribute to a better photoprotection and the fluorescence parameters indicated that photosynthetic apparatus was in better condition compared to control and it consequently postponed the onset of leaf senescence. and Z. Mýtinová, D. Haisel, N. Wilhelmová.
When cells get metals in small excess, mechanisms of avoidance occur, such as exclusion, sequestration, or compartmentation. When the excess reaches sub-lethal concentrations, the oxidative stress, that toxic metals trigger, leads to persistent active oxygen species. Biomolecules are then destroyed and metabolism is highly disturbed. At the chloroplast level, changes in pigment content and lipid peroxidation are observed. The disorganized thylakoids impair the photosynthetic efficiency. The Calvin cycle is also less efficient and the photosynthetic organism grows slowly. When an essential metal is given together with a harmful one, the damages are less severe than with the toxic element alone. Combined metals and phytochelatins may act against metal toxicity. and M. Bertrand, I. Poirier.