Flashes of UV-C Light Stimulate Defenses of Vitis vinifera L. 'Chardonnay' Against Erysiphe necator in Greenhouse and Vineyard Conditions.

Using detached leaves, UV-C light in the form of 1-s flashes has recently been shown to stimulate defenses of several plants against different pathogens better than 1-min exposures under greenhouse conditions. In the present work, the pathological tests were conducted using undetached leaves under greenhouse and vineyard conditions. In a first trial, two flashes of UV-C light were applied to plants of Vitis vinifera L. 'Chardonnay' grown under greenhouse conditions, at an interval of 10 days. Plants were inoculated with Erysiphe necator 2 days after the last light treatment. After 18 days of inoculation, the symptom severity on leaves was reduced by 60% when compared with the untreated control. In a second trial, flashes of UV-C light were applied to grapevine Chardonnay plants under field conditions in the southeast of France every 10 days from 18 April until 10 July 2019. The symptom severity resulting from natural contaminations by E. necator was reduced by 42% in leaves on 4 July 2019 and by 65% in clusters on 25 July 2019. In a third trial, we observed that UV-C light did not have any effect on net photosynthesis, maximal net photosynthesis, dark respiration, maximal quantum efficiency of photosystem II, the performance index of Strasser, and, generally, any parameter derived from induction curves of maximal chlorophyll fluorescence. It was concluded that flashes of UV-C light have true potential for stimulating plant defenses against E. necator under vineyard conditions and, therefore, help in reducing fungicide use.

The influence of pulsed light exposure mode on quality and bioactive compounds of fresh-cut mangoes.

This study investigated the effect of pulsed light (PL) on the respiratory rate, quality (firmness, color and soluble solid content), bioactive compounds (ascorbate and carotenoid) and total antioxidant activity of fresh-cut "Tommy Atkins" mangoes. Fresh-cut mangoes were subjected to PL treatments: control (0P), 1 pulse (1P; 0.7 J cm-2), 4 successive pulses (4P; 2.80 J cm-2) and 1 pulse per day for 4 days (1P4D; 2.80 J cm-2) before storage for 7 days at 6 °C. The 1P and the 4P treatments reduced fresh mass loss during storage, while 4P-treated samples also showed a slower decline of yellow color, as shown by parameter b and overall better visual appearance. After 7 days of storage, total ascorbate content was 40% higher in the 1P4D treatment than in control, whereas total carotenoid content (0.894 mg g-1 FM) and total antioxidant activity (144 µmol trolox 100 g-1 FM) were the highest in the 4P-treated samples. Results suggest that PL mode of application is more important than the fluence or final dose received by fresh-cut mangoes; moreover, 4P is an effective method to preserve, or even improve quality of fresh-cut mangoes.

Water shortage and quality of fleshy fruits--making the most of the unavoidable.

Extreme climatic events, including drought, are predicted to increase in intensity, frequency, and geographic extent as a consequence of global climate change. In general, to grow crops successfully in the future, growers will need to adapt to less available water and to take better advantage of the positive effects of drought. Fortunately, there are positive effects associated with drought. Drought stimulates the secondary metabolism, thereby potentially increasing plant defences and the concentrations of compounds involved in plant quality, particularly taste and health benefits. The role of drought on the production of secondary metabolites is of paramount importance for fruit crops. However, to manage crops effectively under conditions of limited water supply, for example by applying deficit irrigation, growers must consider not only the impact of drought on productivity but also on how plants manage the primary and secondary metabolisms. This question is obviously complex because during water deficit, trade-offs among productivity, defence, and quality depend upon the intensity, duration, and repetition of events of water deficit. The stage of plant development during the period of water deficit is also crucial, as are the effects of other stressors. In addition, growers must rely on relevant indicators of water status, i.e. parameters involved in the relevant metabolic processes, including those affecting quality. Although many reports on the effects of drought on plant function and crop productivity have been published, these issues have not been reviewed thus far. Here, we provide an up-to-date review of current knowledge of the effects of different forms of drought on fruit quality relative to the primary and secondary metabolisms and their interactions. We also review conventional and less conventional indicators of water status that could be used for monitoring purposes, such as volatile compounds. We focus on fruit crops owing to the importance of secondary metabolism in fruit quality and the importance of fruits in the human diet. The issue of defence is also briefly discussed.

Carbohydrate control over carotenoid build-up is conditional on fruit ontogeny in clementine fruits.

The final contents of primary and secondary metabolites of the ripe fruit depend on metabolic processes that are tightly regulated during fruit ontogeny. Carbohydrate supply during fruit development is known to influence these processes but, with respect to secondary metabolites, we do not really know whether this influence is direct or indirect. Here, we hypothesized that the sensitivity of clementine fruit metabolism to carbohydrate supply was conditional on fruit developmental stage. We applied treatments increasing fruit load reversibly or irreversibly at three key stages of clementine (Citrus clementina Hort. ex Tan.) fruit development: early after cell division, at the onset of fruit coloration (color break) and near maturity. The highest fruit load obtained by early defoliation (irreversible) had the highest impact on fruit growth, maturity and metabolism, followed by the highest fruit load obtained by early shading (reversible). Final fruit size decreased by 21 and 18% in these early irreversible and reversible treatments, respectively. Soluble sugars decreased by 18% in the early irreversible treatment, whereas organic acids increased by 46 and 29% in these early irreversible and reversible treatments, respectively. Interestingly, total carotenoids increased by 50 and 18%, respectively. Changes in leaf starch content and photosynthesis supported that these early treatments triggered a carbon starvation in the young fruits, with irreversible effects. Furthermore, our observations on the early treatments challenge the common view that carbohydrate supply influences positively carotenoid accumulation in fruits. We propose that early carbon starvation irreversibly promotes carotenoid accumulation.

Metabolism in orange fruits is driven by photooxidative stress in the leaves.

In plants, stress signals propagate to trigger distant responses and thus stress acclimation in non-exposed organs. We tested here the hypothesis that leaves submitted to photooxidative stress may influence the metabolism of nearby fruits and thus quality criteria. Leaves of orange trees (Citrus sinensis (L.) Osbeck cv. 'Navelate') were acclimated to shade for 1 week and then submitted to full (FL) and medium light (ML) conditions. As expected, photoinhibition was detected in leaves of both FL and ML treatments as revealed by stress indicators (Fv /Fm , Performance Index) for at least 99 h after treatments. In the fruits near the stressed leaves, we then determined the activities of enzymes related to oxidative stress, superoxide dismutase, catalase and the enzymes of the ascorbate (AA)/glutathione cycle, as well as the contents in sugars, organic acids and carotenoids. Ascorbate peroxidase and monodehydroascorbate reductase activities in the pulp of fruits were dramatically higher in both treatments when compared to the control. AA and total sugars were not affected by the photooxidative stress. However, the FL treatment resulted in a 16% increase in total organic acids, with succinic acid being the major contributor, a shift towards less glucose + fructose and more sucrose, and a 15% increase in total carotenoids, with cis-violaxanthin being the major contributor. Our observations strongly suggest the existence of a signal generated in leaves in consequence of photooxidative stress, transmitted to nearby fruits. Exploiting such a signal by agronomic means promises exciting perspectives in managing quality criteria in fruits accumulating carotenoids.

Effect of high salinity and of priming of non-germinated seeds by UV-C light on photosynthesis of lettuce plants grown in a controlled soilless system.

High salinity results in a decrease in plant photosynthesis and crop productivity. The aim of the present study was to evaluate the effect of UV-C priming treatments of lettuce seeds on photosynthesis of plants grown at high salinity. Non-primed and primed seeds were grown in an hydroponic system, with a standard nutrient solution, either supplemented with 100 mM NaCl (high salinity), or not (control). Considering that leaf and root K+ concentrations remained constant and that chlorophyll fluorescence parameters and root growth were not affected negatively in the high salinity treatment, we conclude that the latter was at the origin of a moderate stress only. A substantial decrease in leaf net photosynthetic assimilation (Anet) was however observed as a consequence of stomatal and non-stomatal limitations in the high salinity treatment. This decrease in Anet translated into a decrease in growth parameters; it may be attributed partially to the high salinity-associated increase in leaf concentration in abscisic acid and decrease in stomatal conductance. Priming by UV-C light resulted in an increase in total photosynthetic electron transport rate and Anet in the leaves of plants grown at high salinity. The increase of the latter translated into a moderate increase in growth parameters. It is hypothesized that the positive effect of UV-C priming on Anet and growth of the aerial part of lettuce plants grown at high salinity, is mainly due to its stimulating effect on leaf concentration in salicylic acid. Even though leaf cytokinins' concentration was higher in plants from primed seeds, maintenance of the cytokinins-to-abscisic acid ratio also supports the idea that UV-C priming resulted in protection of plants exposed to high salinity.

Effect of fruit load on maturity and carotenoid content of clementine (Citrus clementina Hort. ex Tan.) fruits.

BACKGROUND: Citrus fruits contain many secondary metabolites displaying valuable health properties. There is a lot of interest in enhancing citrus quality traits, especially carotenoid contents, by agronomic approaches. In this study the influence of carbohydrate availability on maturity and quality criteria was investigated in clementine fruits during ripening. Fruiting branches were girdled and defoliated after fruit set to obtain three levels of fruit load: high (five leaves per fruit), medium (15) and low (30). RESULTS: Considering the soluble solid content/titratable acidity (SSC/TA) ratio, it was found that fruits of the high and medium fruit load treatments reached maturity 1.5 months later than fruits of the control. At the time of maturity the SSC/TA ratio of fruits of all treatments was about 13.6. At harvest, fruits were 23% smaller and total sugar concentration of the endocarp was 12.6% lower in the high fruit load treatment than in the control. In contrast, the concentrations of organic acids and total carotenoids were 55.4 and 93.0% higher respectively. Total carotenoids were not positively correlated with either soluble sugars or total carbohydrates. CONCLUSION: Taken together, the results do not support the common view that carbohydrate availability directly determines carotenoid synthesis by influencing precursor availability.

Temperature and storage time increase provitamin A carotenoid concentrations and bioaccessibility in post-harvest carrots.

The aim was to enhance provitamin A carotenoid (proVA CAR) concentrations and bioaccessibility in carrots by manipulating post-harvest factors. To that end, we assessed the effects of Ultraviolet-C light, pulsed light, storage temperature, and storage duration. We also measured CAR bioaccessibility by using an in vitro model. Pulsed light, but not Ultraviolet-C, treatment increased proVA CAR concentrations in the cortex tissue (p < 0.05). Longer storage times and higher temperatures also increased concentrations (p < 0.05). The maximal increase induced by pulsed light was obtained after treatment with 20 kJ/m2 and 3-days of storage at 20 °C. However, the positive effect induced by pulsed light decreased considerably over the next seven days. ProVA CAR in carrots with the highest concentrations also proved to be more bioaccessible (p < 0.05). Thus, proVA CAR concentrations in stored carrots can be increased significantly through storage times and temperatures. Pulsed light can also significantly increase proVA CAR concentrations, but only temporarily.

An overview of models of stomatal conductance at the leaf level.

Stomata play a key role in plant adaptation to changing environmental conditions as they control both water losses and CO(2) uptake. Particularly, in the context of global change, simulations of the consequences of drought on crop plants are needed to design more efficient and water-saving cropping systems. However, most of the models of stomatal conductance (g(s)) developed at the leaf level link g(s) to environmental factors or net photosynthesis (A(net)), but do not include satisfactorily the effects of drought, impairing our capacity to simulate plant functioning in conditions of limited water supply. The objective of this review was to draw an up-to-date picture of the g(s) models, from the empirical to the process-based ones, along with their mechanistic or deterministic bases. It focuses on models capable to account for multiple environmental influences with emphasis on drought conditions. We examine how models that have been proposed for well-watered conditions can be combined with those specifically designed to deal with drought conditions. Ideas for future improvements of g(s) models are discussed: the issue of co-regulation of g(s) and A(net); the roles of CO(2), absissic acid and H(2)O(2); and finally, how to better address the new challenges arising from the issue of global change.

Flashes of UV-C light: An innovative method for stimulating plant defences.

Leaves of lettuce, pepper, tomato and grapevine plants grown in greenhouse conditions were exposed to UV-C light for either 60 s or 1 s, using a specific LEDs-based device, and wavelengths and energy were the same among different light treatments. Doses of UV-C light that both effectively stimulated plant defences and were innocuous were determined beforehand. Tomato plants and lettuce plants were inoculated with Botrytis cinerea, pepper plants with Phytophthora capsici, and grapevine with Plasmopara viticola. In some experiments we investigated the effect of a repetition of treatments over periods of several days. All plants were inoculated 48 h after exposure to the last UV-C treatment. Lesions on surfaces were measured up to 12 days after inoculation, depending on the experiment and the pathogen. The results confirmed that UV-C light stimulates plant resistance; they show that irradiation for one second is more effective than irradiation for 60 s, and that repetition of treatments is more effective than single light treatments. Moreover a systemic effect was observed in unexposed leaves that were close to exposed leaves. The mechanisms of perception and of the signalling and metabolic pathways triggered by flashes of UV-C light vs. 60 s irradiation exposures are briefly discussed, as well as the prospects for field use of UV-C flashes in viticulture and horticulture.

Long-term drought results in a reversible decline in photosynthetic capacity in mango leaves, not just a decrease in stomatal conductance.

The negative effects of drought on plant growth, development of natural plant communities and crop productivity are well established, but some of the responses remain poorly characterized, particularly the effect of long-term drought on photosynthetic capacity. We hypothesized that long-term drought results in a decline in leaf photosynthetic capacity, and not just a decrease in diffusive conductance. To test this hypothesis, we studied the effect of drought, slowly developed over 3.5 months, in leaves of eight potted mango (Mangifera indica L.) trees. We found that photosynthesis was not only limited by stomatal closure, but was also downregulated as a consequence of a strong decrease in photosynthetic capacity assessed by the measurements of maximal net photosynthesis (A(max)) and the light-saturated rate of photosynthetic electron transport (J(max)). The rapid recovery of A(max) and J(max), after only 1 week of rewatering, the maintenance of a stable pool of leaf nitrogen throughout the trial, and the decrease in quantum efficiency of open centers of photosystem II, indicate that the photosynthetic machinery escaped photodamage in the drought-treated trees and was simply downregulated during drought. The hexose-to-sucrose ratio was higher in leaves from drought-treated trees than in control leaves, suggesting that photosynthetic capacity decreased as a consequence of sink limitation.

Xenon lamps used for fruit surface sterilization can increase the content of total flavonols in leaves of Lactuca sativa L. without any negative effect on net photosynthesis.

One (1P), two (2P), three (3P) or four (4P) pulses of light supplied by a xenon lamp, were applied to young lettuce plants grown in pots. The lamp used in the trial was similar to those used for fruit surface sterilization. Total flavonols were measured in leaves using the Dualex method. In a first trial conducted in greenhouse conditions, 6 days after the pulsed light (PL) treatment, flavonols were increased by 312% and 525% in the 3P and 4P treatments, respectively, in comparison to the those in the untreated control. Changes in the chlorophyll fluorescence parameters suggest that the PL treatment may induce limited and transient damage to the photosynthetic machinery and that the damage increases with the increasing number of pulses. The performance parameters were not significantly affected by PL and recovered fully by 6 days after the treatments. The 1P and the 2P treatments 6 days after the treatment showed a 28.6% and a 32.5% increase, respectively, in net photosynthetic assimilation, when compared to that of the control. However, 8 days after the treatment, there was no longer a difference between the treatments and the control in net photosynthetic assimilation. Eight days after the light treatment, the 3P treatment showed a 38.4% increase in maximal net photosynthetic assimilation over that of the control, which is an indication of positive long-term adaptation of photosynthetic capacity. As a whole, our observations suggest that PL could be used on field or greenhouse crops to increase their phytochemical content. No long-lasting or strong negative effects on photosynthesis were associated with PL within the range of doses we tested; some observations even suggest that certain treatments could result in an additional positive effect. This conclusion is supported by a second trial conducted in phytotrons. More studies are required to better understand the roles of the different wavelengths supplied by PL and their interactions.

Interpreting the decrease in leaf photosynthesis during flowering in mango.

Little is known about the effect of flowering on leaf photosynthesis. To understand why net photosynthesis (A(net)) is lower in Mangifera indica L. leaves close to inflorescences than in leaves on vegetative shoots, we measured nitrogen and carbohydrate concentrations, chlorophyll a fluorescence and gas exchange in recently matured leaves on vegetative terminals and on floral terminals of 4-year-old trees. We used models to estimate photosynthetic electron fluxes and mesophyll conductance (g(m)). Lower A(net) in leaves close to developing inflorescences was attributable to substantial decreases in stomatal conductance and g(m), and also in photosynthetic capacity as indicated by the decrease in the light-saturated rate of photosynthetic electron transport (J(max)). The decrease in J(max) was the result of decreases in the amount of foliar nitrogen per unit leaf area, and may have been triggered by a decrease in sink activity as indicated by the increase in the hexose:sucrose ratio. Parameters measured on leaves close to panicles bearing set fruits were generally intermediate between those measured on leaves on vegetative shoots and on leaves close to inflorescences, suggesting that the changes in A(net) associated with flowering are reversible.

Assessing the Effects of Water Deficit on Photosynthesis Using Parameters Derived from Measurements of Leaf Gas Exchange and of Chlorophyll a Fluorescence.

Water deficit (WD) is expected to increase in intensity, frequency and duration in many parts of the world as a consequence of global change, with potential negative effects on plant gas exchange and growth. We review here the parameters that can be derived from measurements made on leaves, in the field, and that can be used to assess the effects of WD on the components of plant photosynthetic rate, including stomatal conductance, mesophyll conductance, photosynthetic capacity, light absorbance, and efficiency of absorbed light conversion into photosynthetic electron transport. We also review some of the parameters related to dissipation of excess energy and to rerouting of electron fluxes. Our focus is mainly on the techniques of gas exchange measurements and of measurements of chlorophyll a fluorescence (ChlF), either alone or combined. But we put also emphasis on some of the parameters derived from analysis of the induction phase of maximal ChlF, notably because they could be used to assess damage to photosystem II. Eventually we briefly present the non-destructive methods based on the ChlF excitation ratio method which can be used to evaluate non-destructively leaf contents in anthocyanins and flavonols.

Influence of Environmental Conditions and Genetic Background of Arabica Coffee (C. arabica L) on Leaf Rust (Hemileia vastatrix) Pathogenesis.

Global warming is a major threat to agriculture worldwide. Between 2008 and 2013, some coffee producing countries in South and Central America suffered from severe epidemics of coffee leaf rust (CLR), resulting in high economic losses with social implications for coffee growers. The climatic events not only favored the development of the pathogen but also affected the physiological status of the coffee plant. The main objectives of the study were to evaluate how the physiological status of the coffee plant modified by different environmental conditions impact on the pathogenesis of CLR and to identify indicators of the physiological status able to predict rust incidence. Three rust susceptible genotypes (one inbred line and two hybrids) were grown in controlled conditions with a combination of thermal regime (TR), nitrogen and light intensity close to the field situation before being inoculated with the rust fungus Hemileia vastatrix. It has been demonstrated that a TR of 27-22°C resulted in 2000 times higher sporulation than with a TR of 23-18°C. It has been also shown that high light intensity combined with low nitrogen fertilization modified the CLR pathogenesis resulting in huge sporulation. CLR sporulation was significantly lower in the F1 hybrids than in the inbred line. The hybrid vigor may have reduced disease incidence. Among the many parameters studied, parameters related to photosystem II and photosynthetic electron transport chain components appeared as indicators of the physiological status of the coffee plant able to predict rust sporulation intensity. Taken together, these results show that CLR sporulation not only depends on the TR but also on the physiological status of the coffee plant, which itself depends on agronomic conditions. Our work suggests that vigorous varieties combined with a shaded system and appropriate nitrogen fertilization should be part of an agro-ecological approach to disease control.

Juvenile Coffee Leaves Acclimated to Low Light Are Unable to Cope with a Moderate Light Increase.

The understorey origin of coffee trees and the strong plasticity of Coffea arabica leaves in relation to contrasting light environments have been largely shown. The adaptability of coffee leaves to changes in light was tested under controlled conditions by increasing the illumination rate on C. arabica var. Naryelis seedlings acclimated to low light conditions and observing leaf responses at three different developmental stages (juvenile, growing and mature). Only mature leaves proved capable of adapting to new light conditions. In these leaves, different major mechanisms were found to contribute to maintaining a good photosynthetic level. With increased illumination, a high photosynthetic response was conserved thanks to fast nitrogen remobilization, as indicated by SPAD values and the photorespiration rate. Efficient photoprotection was accompanied by a great ability to export sucrose, which prevented excessive inhibition of the Calvin cycle by hexose accumulation. In contrast, in younger leaves, increased illumination caused photodamage, observable even after 9 days of treatment. One major finding was that young coffee leaves rely on the accumulation of chlorogenic acids, powerful antioxidant phenolic compounds, to deal with the accumulation of reactive oxygen species rather than on antioxidant enzymes. Due to a lack of efficient photoprotection, a poor ability to export sucrose and inadequate antioxidant protection, younger leaves seemed to be unable to cope with increased illumination. In these leaves, an absence of induced antioxidant enzyme activity was accompanied, in growing leaves, by an absence of antioxidant synthesis or, in juvenile leaves, inefficient synthesis of flavonoids because located in some epidermis cells. These observations showed that coffee leaves, at the beginning of their development, are not equipped to withstand quick switches to higher light levels. Our results confirm that coffee trees, even selected for full sunlight conditions, remain shade plants possessing leaves able to adapt to higher light levels only when mature.

Season effects on leaf nitrogen partitioning and photosynthetic water use efficiency in mango.

The key parameters of photosynthetic capacity (maximum carboxylation rate (V(cmax)), electron transport capacity (J(max)) and dark respiration rate (R(d))) and the slope (m) of the stomatal conductance model of Ball et al. [Progress in photosynthetic research, Martinus Nijhoff, Dordrecht, 1987] were measured for a whole growing season in fully expanded leaves of 12-year-old mango trees cv. Cogshall in La Réunion island. Leaf nitrogen partitioning into carboxylation (P(c)) and bioenergetic (P(b)) pools were computed according to the model of Niinemets and Tenhunen [Plant Cell Environ 1997;20: 845-66]. V(cmax), J(max), R(d), P(c) and P(b) remained relatively stable over the whole study period, with the exception of the period of linear fruit growth when J(max), R(d) and P(b) were slightly lower, and leaf non-structural carbohydrate content higher. During the pre-floral and floral periods, m decreased by more than 50%, indicating an increase in photosynthetic water use efficiency and m increased again during the period of linear fruit growth. Our results show that, in tropical orchard conditions characterized by mild seasonal climatic changes and non-limiting water supply, leaf nitrogen partitioning is rather stable. Our results also advocate for more studies on the effect of phenology on m and photosynthetic water use efficiency, which is of paramount importance for building coupled biochemical models of photosynthetic carbon assimilation.

Water deficit effects on tomato quality depend on fruit developmental stage and genotype.

Many studies have advocated that water deficit (WD) may exert beneficial effects on fruit quality. However, the fruit response to WD at specific developmental stages was seldom investigated, although different mechanisms could be involved at each stage and lead to different effects on final fruit quality. In the present study, a moderate WD (-60% of water supply compared to control) was applied during each of the three major phases of fruit development, namely cell division (CD), cell expansion (CE) and maturation (MT). Two cocktail tomato (Solanum lycopersicum L.) genotypes were studied, one producing poor quality fruits (LA1420), and the other one producing tasty fruits (PlovdivXXIVa named Plovdiv). Contrasted responses were observed between the two genotypes. For both of them, fruit fresh mass and size were not significantly reduced by WD, whatever the developmental phase affected. Osmotic regulations were likely involved in the CD treatment for LA1420 fruits, which accumulated more sugars (both on a dry and fresh matter basis) and less acids (on a dry matter basis). In the CE treatment, other adaptive strategies involving sugar metabolism and sub-cellular compartmentation were suggested. In contrast, the composition of Plovdiv fruits changed only under the MT treatment, with less sugars, acids and carotenoids compared to control fruits (both on a dry and fresh matter basis). Total ascorbic acid (AsA) was not significantly influenced by treatments in both genotypes. On their whole, results suggest that, depending on genotypes, fruits are sweeter and less acidic under WD, but that the nutritive value related to vitamin and carotenoid contents may be lessened. The sensitivity of each developmental phase highly depends on the genotype. All phases were sensitive to WD for LA1420, but only the ripening phase for Plovdiv. Interestingly, major changes in fruit composition were observed in LA1420 which presents poor fruit quality under control conditions. This suggests the onset of fast adaptive response to WD at the fruit level in this genotype.

Understanding the physiological effects of UV-C light and exploiting its agronomic potential before and after harvest.

There is an abundant literature about the biological and physiological effects of UV-B light and the signaling and metabolic pathways it triggers and influences. Much less is known about UV-C light even though it seems to have a lot of potential for being effective in less time than UV-B light. UV-C light is known since long to exert direct and indirect inhibitory and damaging effects on living cells and is therefore commonly used for disinfection purposes. More recent observations suggest that UV-C light can also be exploited to stimulate the production of health-promoting phytochemicals, to extent shelf life of fruits and vegetables and to stimulate mechanisms of adaptation to biotic and abiotic stresses. Clearly some of these effects may be related to the stimulating effect of UV-C light on the production of reactive oxygen species (ROS) and to the stimulation of antioxidant molecules and mechanisms, although UV-C light could also trigger and regulate signaling pathways independently from its effect on the production of ROS. Our review clearly underlines the high potential of UV-C light in agriculture and therefore advocates for more work to be done to improve its efficiency and also to increase our understanding of the way UV-C light is perceived and influences the physiology of plants.

A User's View of the Parameters Derived from the Induction Curves of Maximal Chlorophyll a Fluorescence: Perspectives for Analyzing Stress.

Analysis of the fast kinetics of the induction curve of maximal fluorescence represents a relatively recent development for chlorophyll a fluorescence measurements. The parameters of the so-called JIP-test are exploited by an increasingly large community of users to assess plant stress and its consequences. We provide here evidence that these parameters are capable to distinguish between stresses of different natures or intensities, and between stressed plants of different genetic background or at different developmental stages at the time of stress. It is, however, important to keep in mind that the JIP-test is inherently limited in scope, that it is based on assumptions which are not fully validated and that precautions must be taken to ensure that measurements are meaningful. Recent advances suggest that some improvements could be implemented to increase the reliability of measurements and the pertinence of the parameters calculated. We moreover advocate for using the JIP-test in combination with other techniques to build comprehensive pictures of plant responses to stress.