Interplay of Stress Responses in Pear Tree Revealed by Chlorophyll Fluorescence Under Combined Erwinia amylovora Infection and Temperature Stress

Trees in urban areas provide important ecosystem services and are an essential element of urban green space. The constant increase in artificial light from anthropogenic activities around the world creates photopollution that affects the phenology and physiology of plants. Here we conducted a field study to investigate the anthropogenic impacts on six urban trees (Saraca asoca, Terminalia catappa, Bauhinia variegata, Holoptelea integrifolia, Ficus benjamina and Thevetia peruviana) using chlorophyll fluorescence analysis. OJIP curve, maximum quantum yield of primary photochemistry (ΦPo), quantum yield of electron transport (ΦEo), probability that an absorbed photon will be dissipated (ΦDo), photosynthetic performance index (PIcsm) and reaction center photochemistry were assessed. According to the results, various parameters of chlorophyll fluorescence showed significant and important effects on different tree species. T. peruviana and F. benjamina were found to be tolerant to street lighting, while on the other hand, S. asoca, T. catappa, B. variegata and H. integrifolia were found to be sensitive to artificial light induced by street lamps. This study clearly indicates that chlorophyll fluorescence analysis is a potent method for screening the tolerance of tree species to photopollution induced by artificial lights.

Chlorophyll fluorescence analysis as a measure of photosynthesis is increasingly used in eco-physiological studies. It is particularly useful in investigations on the photosynthetic performance of plants in stressful environments. Chlorophyll fluorescence, which can be measured rapidly and conveniently, provides insights into a plant's ability to tolerate environmental stresses such as those of a drought. Damage to the photosynthetic apparatus due to moisture or heat stress, which would reduce photosynthetic assimilation and eventually the yield, can be detected at a very early stage of the drought. This technique is widely used to identify stress tolerant plants and crop varieties. However, this technique has not been utilized to identify putative drought tolerant coconut varieties. Hence, the objective of this study was to set out the basic principles of the technique, discuss its applications in eco-physiological studies, and evaluate its potential to screen coconut varieties for drought tolerance. San Ramon, a tall form of coconut introduced to Sri Lanka few decades ago and a few other local tall accessions in the germplasm conservation blocks in the Coconut Research Institute of Sri Lanka appeared to possess a photosynthetic apparatus which was clearly more tolerant to moisture stress conditions than the other accessions. SRR, SRG, SA, CL PI & PW were the most drought tolerant of the 40 genotypes tested. The results generated by using Fv/Fm, were consistent with results of other important parameters such as the rate of photosynthesis and water use efficiency. Even the basic parameters of chlorophyll fluorescence, for instance the maximum quantum yield of photosystem II (Fv/Fm) measured on dark-adapted coconut leaves showed promise as a selection criterion in future screening programmes due to its quick, non-invasive and reliable nature. Similarities of fluorescence parameters observed between seedlings and adult palms enables fairly confident predictions on the performance of adult palms, based on studies with seedlings. In general, chlorophyll fluorescence appears as a promising tool for screening coconut palms for their drought tolerance. Key words: Coconut, drought tolerance, chlorophyll fluorescence, photosynthesis, Fv/Fm doi: 10.4038/cocos.v18i0.991 COCOS (2007), 18

Photosystems of higher plants alleviate heat-induced damage in the presence of light under moderate stressed conditions; however, in the absence of light (i.e., in the dark), the same plants are damaged more easily. (Yamauchi and Kimura, 2011) We demonstrate that regulating photochemical energy transfer in heat-treated wheat at 40 °C with light contributed to heat tolerance of the photosystem. Chlorophyll fluorescence analysis using heat-stressed wheat seedlings in light showed increased non-photochemical quenching (NPQ) of chlorophyll fluorescence, which was due to thermal dissipation that was increased by state 1 to state 2 transition. Transmission electron microscopy revealed structural changes in thylakoid membranes, including unstacking of grana regions under heat stress in light. It was accompanied by the phosphorylation of thylakoid proteins such as D1 and D2 proteins and the light harvesting complex II proteins Lhcb1 and Lhcb2. These results suggest that heat stress at 40 °C in light induces state 1 to state 2 transition for the preferential excitation of photosystem I (PSI) by phosphorylating thylakoid proteins more strongly. Structural changes of thylakoid membrane also assist the remodeling of photosystems and regulation of energy distribution by transition toward state 2 probably contributes to plastoquione oxidation; thus, light-driven electrons flowing through PSI play a protective role against PSII damage under heat stress.

A comprehensive understanding of the quantitative relationship among phenotypic and photosynthetic traits at key growth stages in wheat can guide trait selection and management strategies that can contribute to increased productivity. In this study, we quantified the physiological basis of morphological and phenological attributes of wheat using chlorophyll fluorescence analysis. Chlorophyll fluorescence parameters were measured using a handheld MultispeQ V2.0 device, connected to a PhotosynQ platform, for 72 genotypes grown in a randomized complete block design (RCBD). Days to heading (DH), percentage digital ground cover (DGC%), and flag leaf length and width were recorded at specific individual growth stages, and chlorophyll fluorescence parameters were measured at 95, 103, and 108days after sowing to the grain filling stage. Variance analysis revealed a significant difference among the three measurement time points in the fluorescence parameters. A notable increase in relative chlorophyll content was observed in the flag leaf, which plays a central role in photosynthesis; higher chlorophyll levels enhance light capture capacity and thereby support greater photosynthetic activity. The DGC% played a vital role in radiation absorption and photosynthesis. Significant positive correlations were observed among most fluorescence and physiological traits, although some traits showed negative correlations. Path analysis revealed that leaf temperature and ambient temperature indirectly affected morphophysiological traits via DH. Such relationships may assist breeders in selecting elite genotypes.

The potential use of chlorophyll fluorescence (CF) induction assay as a tool for screening and characterizing the tolerance of ozone (O3), contrasting cultivars of snapbeans (Phaseolus vulgaris L.), was investigated. A range of CF parameters was examined for snapbeans treated with O3. Chlorophyll fluorescence parameters such as Fo, Fmax, and Fv, Fv/Fmax were compared in O3 tolerant and susceptible snapbeans cultivars grown under O3 stress conditions. O3-stressed leaves showed significantly higher constant-yield (Fo) but greatly reduced variable fluorescence (Fv) and decreased Fv/Fmax ratios. In the O3-sensitive cultivar snapbean cv BBL-290, O3 stress resulted in a strong inhibition of the fast and slow fluorescence-induction transients and altered the form of the kinetic curves of CF in leaves. In particular, the fluorescence quenching rate and Fv/Fmax ratios were markedly decreased in O3-stressed leaves. In contrast, leaves of the O3-resistant cultivar cv Astro showed only minor changes in CF. The values of the Fv/Fmax ratio decreased in the O3-sensitive cultivar much more drastically than the O3-resistant cultivar. Based on CF measurements, it appears that O3-induced stress blocked photosynthetic electron transport between photosystem (PS) II and PS I. The close agreement between changes in fluorescence and visual symptoms of O3-induced injury suggest that the CF patterns, the rate of fluorescence-induction transients, and the Fv/Fmax ratio can provide valuable tools to investigate the photosynthetic and metabolic mechanisms affects by O3-induced stress. Chlorophyll fluorescence analysis could also be a useful technique which could be used by plant breeders to screen large numbers of plant reapidly for air pollution sensitivity.

The measurements of chlorophyll fluorescence play an important role in studies of lichen physiology. Usually, for foliose lichens fluorescence kinetics is recorded from the upper thalline side often exhibiting green color reflecting the presence of photosynthetic pigments. The lower side of such lichens is grey, dark-brown or black. At the first time, we evaluated photosynthetic activity distribution by chlorophyll fluorescence analysis on both lower and upper thallus sides for the foliose lichen Nephroma arcticum. We have demonstrated that photosynthesis proceeds not only on the green-colored upper side, but also on the gray lower side of the curled growing edges of the thallus lobes. These sides were differed in terms of PSII photochemical quantum yield, activity of non-regulatory dissipation and non-photochemical quenching of excited chlorophyll states (NPQ). Upper side was characterized by higher maximal PSII efficiency, whereas the lower one of the curled edges was characterized by higher actual photochemical quantum yield during actinic light acclimation. NPQ was higher on the upper surface, whereas, on the lower side (of the curled edges) non-regulatory dissipation was predominant. In terms of photosynthetic activity measurements, these results show, that actinic and measuring light reached the layer of phycobiont despite its shielding by mycobiont hyphae. On the melanized lower side in the basal thalline zone attached to the substratum photosynthesis was not detected. Lower side demonstrated higher level of light scattering in the reflectance spectra. We believe that different photoprotective mechanisms against high light are crucial on the upper and lower sides: NPQ on the upper surface, and light scattering and shielding by mycobiont on the lower side. Possible biological role of photosynthesis on the lower side is discussed.

Chlorophyll fluorescence analysis is one of the most powerful and widely used techniques to study the effect of stresses on the photosynthetic process. From the first utilization, the Fv/Fm ratio has been largely used as a sensitive indicator of plant photosynthetic performance. Decreases of this index are indicative of the reduction of photosystem II (PSII) efficiency, namely photoinhibition. In the last 20 years, application of chlorophyll fluorescence has been largely improved, and many other informative parameters have been established to detect PSII photochemical efficiency and the partitioning of light energy to alternative dissipative mechanisms (qE, energy-dependent quenching; qZ, zeaxanthin-dependent quenching and qI, photoinhibitory quenching; qH, sustained photoprotective antenna quenching; qM, quenching dependent to chloroplast movement; qT, light harvesting complexes II–I state-transition) such as the recently developed “photoprotective power” of non-photochemical quenching (pNPQ). This review reports a brief description of the main chlorophyll fluorescence parameters and a wide analysis of the current bibliography on the use of different parameters which are useful to detect events of PSII photoinhibition. In addition, in view of the inherent differences in morpho-anatomical, physiological and biochemical features between C3 and C4 metabolism, possible differences in terms of photoinhibition between C3 and C4 plant species under stress conditions are proposed. The attempt is to highlight the limits of their comparison in terms of susceptibility to photoinhibition and to propose direction of future research which, assisted by chlorophyll fluorescence, should improve the knowledge of the different sensitivity of C3 and C4 to abiotic stressors.

The determination of chlorophyll fluorescence emission is a powerful tool for assessing the status of PSII and the allocation of absorbed light to photosynthesis v. photoprotective energy dissipation. The development of field-portable fluorometers has enabled growing numbers of scientists to measure fluorescence emission from plants in diverse field settings. However, the ease of operation of contemporary fluorometers masks the many challenges associated with collecting meaningful and interpretable fluorescence signals from leaves exposed to relevant environmental conditions. Here, we offer methodological advice aimed at, but not limited to, the non-specialist for the proper measurement of fluorescence parameters, with an emphasis on avoiding common errors in the use of fluorescence under field conditions. Chief among our suggestions is (1) to delay use of automatically calculated fluorescence parameters, presented by the instrument software, until raw data 'traces' have been carefully inspected to ensure the integrity of findings, and (2) to combine chlorophyll fluorescence analysis, as a rapid, preliminary method of assessing plant responses to stress, with additional methods of characterising the system of interest (e.g. gas exchange, foliar pigment composition, thylakoid protein composition).

The effects of a) glucose, b) mannose and 2-deoxyglucose (glucose analogues capable of triggering hexokinase-mediated photosynthesis repression) and c) sodium citrate on chlorophyll fluorescence parameters of the blue-green alga Spirulina platensis (Nordst.) Geitl. were investigated. The addition of glucose evoked, at first, the lowering of all photochemical fluorescence quenching parameters (FV/FM, FV'/FM', qP and ΦPS II). The decrease correlated with glucose concentration. The inhibition of photochemical activity relaxed after the third day of the experiment, perhaps, as the consequence of glucose utilization. If the concentration of glucose in the cultural medium was high enough (50 mM), on the seventh day all the indices of photochemical quenching began to diminish again, at which time qP reduction was more significant than that for FV'/FM'. The relief of glucose repressive effects was light-dependent. Glucose analogues caused the decrease in only one photochemical quenching parameter, qP. Unlike mannose, the effect of 2-deoxyglucose was more pronounced and did not weaken till the end of experiments. Nonphotochemical quenching fluorescence parameters (qN and NPQ) were also reduced under monosaccharide treatment. The influence of sodium citrate on chlorophyll fluorescence parameters was negligible. Therefore, the multiplicity of glucose effects in the course of inhibition of S. platensis photosystem II activity was demonstrated, while hexokinase-dependent repression mechanism was playing only a partial role and resulting in the blocking of electron flow from photosystem II reaction centers down the electron transport chain.

With a portable PAM-2000 fluorometer it was observed that responses of initial chlorophyll fluorescence Fo level to strong light were different in various plant species examined. When the photochemical efficiency of Photosystem II, Fv/Fm, declined, Fo increased significantly in leaves of some plants such as soybean and cotton, while Fo decreased remarkably in other plants such as wheat and barley. In order to explore the mechanism of the increase in Fo in soybean leaves, the change in D1 protein amount and effects of lincomycin and far-red light on these fluorescence parameters were observed by SDS–PAGE combined with gel scanning and chlorophyll fluorescence analysis. The following results were obtained. (1) The amount of inactive PS II reaction centers increased under strong light and decreased during subsequent dark recovery [Hong and Xu (1997) Chinese Sci Bull 42(8): 684–689]. (2) No net loss of D1 protein occurred after strong light treatment. (3) Lincomycin taken up through petioles following strong light treatment had no significant effect on D1 protein level and the decay of Fo in the dark. (4) Far-red light applied after strong light treatment could largely attenuate the increase in Fo and accelerate Fo decay in the dark. Based on these results, it is deduced that the increase in Fo under strong light is mainly due to reversible inactivation of part of PS II reaction centers, rather than the net loss of D1 protein and that reversible inactivation of PS II is prevalent in some plants.

Chlorophyll fluorescence analysis is a method that has been widely disseminated among researchers for ensuring fast, accurate and non-destructive results. Thus, this study aimed to assess the chlorophyll a fluorescence of pepper seedlings grown from seeds that were unprimed, hydroprimed and primed with 24-EpiBL at 10-8 M after induction of salt stress at three concentrations (0.0, -0.2 and -0.4 MPa). To carry out the experiment, the Seed Reporter Camera Spectral & Color Imaging System (PhenoVation Life Sciences, Wageningen, Netherlands) was used. Three lots of seeds of the pepper cultivar ‘Biquinho’ were used. Chlorophyll fluorescence in the seedlings was evaluated on the seventh day after the seeds were subjected to different levels of salt stress. The chlorophyll fluorescence images obtained were analyzed by the CFTI - Analysis software (version 4.5). Through the data obtained it was possible to verify that, in general, there was an increase in chlorophyll a fluorescence in seedlings grown from seeds primed with 24-EpiBL. In short, the chlorophyll fluorescence technique proved to be viable and efficient, in addition to being a practical tool that can be used to support the analysis of the quality of pepper seedlings, whether or not subjected to saline stress conditions.

Chlorophyll a fluorescence analysis is widely used to measure photosynthetic behaviors in intact plants, and has resulted in the development of many parameters that efficiently measure photosynthesis. Leaf reflectance analysis provides several vegetation indices in ecology and agriculture, including the photochemical reflectance index (PRI), which can be used as an indicator of thermal energy dissipation during photosynthesis because it correlates with non-photochemical quenching (NPQ). However, since NPQ is a composite parameter, its validation is required to understand the nature of the PRI parameter. To obtain physiological evidence for evaluation of the PRI parameter, we simultaneously measured chlorophyll fluorescence and leaf reflectance in xanthophyll cycle defective mutant (npq1) and wild-type Arabidopsis plants. Additionally, the qZ parameter, which likely reflects the xanthophyll cycle, was extracted from the results of chlorophyll fluorescence analysis by monitoring relaxation kinetics of NPQ after switching the light off. These simultaneous measurements were carried out using a pulse-amplitude modulation (PAM) chlorophyll fluorometer and a spectral radiometer. The fiber probes from both instruments were positioned close to each other to detect signals from the same leaf position. An external light source was used to activate photosynthesis, and the measuring lights and saturated light were provided from the PAM instrument. This experimental system enabled us to monitor light-dependent PRI in the intact plant and revealed that light-dependent changes in PRI differ significantly between the wild type and npq1 mutant. Furthermore, PRI was strongly correlated with qZ, meaning that qZ reflects the xanthophyll cycle. Together, these measurements demonstrated that simultaneous measurement of leaf reflectance and chlorophyll fluorescence is a valid approach for parameter evaluation.

Combined impact of heat stress and phosphate deficiency on growth and photochemical activity of sheepgrass (Leymus chinensis)

Acute toxicity of excess mercury on the photosynthetic performance of cyanobacterium, S. platensis – assessment by chlorophyll fluorescence analysis

The frequency of abiotic stress impairing wheat root growth and yield production has been increasing with global warming. Diverse root traits have been widely targeted to improve wheat adaptivity to different abiotic stress, but most research has been conducted under controlled environments with a single stress factor, hindering transferability to fields conditions with multiple stresses. It is essential to distinguish the valuable root traits for both individual and combined abiotic stresses, and identify agronomic practices that can mitigate the detrimental effects on wheat production. This review summarizes morphological, physiological and anatomical root traits of wheat under stresses of drought, soil compaction, nitrogen (N) and phosphorus (P) deficiency, and waterlogging. Variations of root traits are further discussed under the co-occurrence of these abiotic stresses. In general, thick wide-angle seminal roots, deep sparse roots, and lengthy laterals roots are superior root traits under the stress combinations of drought combined with either soil compaction, N deficiency, or P deficiency. Dense adventitious, thin and sparse roots, and lengthy laterals with aerenchyma formation are superior root traits under the stress combinations of waterlogging combined with either soil compaction, N deficiency, or P deficiency. Wheat production loss from multiple stress conditions can be relieved by optimal crop and soil management strategies, including fertigation and subsurface drainage. Future development of wheat production should focus on taking advantage of adaptative root traits and agronomic optimization.