Laurent Urban

Health Benefits of Vitamins and Secondary Metabolites of Fruits and Vegetables and Prospects To Increase Their Concentrations by Agronomic Approaches

Fruits and vegetables (FAVs) are an important part of the human diet and a major source of biologically active substances such as vitamins and secondary metabolites. The consumption of FAVs remains globally insufficient, so it should be encouraged, and it may be useful to propose to consumers FAVs with enhanced concentrations in vitamins and secondary metabolites. There are basically two ways to reach this target: the genetic approach or the environmental approach. This paper provides a comprehensive review of the results that have been obtained so far through purely agronomic approaches and brings them into perspective by comparing them with the achievements of genetic approaches. Although agronomic approaches offer very good perspectives, the existence of variability of responses suggests that the current understanding of the way regulatory and metabolic pathways are controlled needs to be increased. For this purpose, more in-depth study of the interactions existing between factors (light and temperature, for instance, genetic factors × environmental factors), between processes (primary metabolism and ontogeny, for example), and between organs (as there is some evidence that photooxidative stress in leaves affects antioxidant metabolism in fruits) is proposed.

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.

Carotenoid responses to environmental stimuli: integrating redox and carbon controls into a fruit model.

Carotenoids play an important role in plant adaptation to fluctuating environments as well as in the human diet by contributing to the prevention of chronic diseases. Insights have been gained recently into the way individual factors, genetic, environmental or developmental, control the carotenoid biosynthetic pathway at the molecular level. The identification of the rate-limiting steps of carotenogenesis has paved the way for programmes of breeding, and metabolic engineering, aimed at increasing the concentration of carotenoids in different crop species. However, the complexity that arises from the interactions between the different factors as well as from the coordination between organs remains poorly understood. This review focuses on recent advances in carotenoid responses to environmental stimuli and discusses how the interactions between the modulation factors and between organs affect carotenoid build-up. We develop the idea that reactive oxygen species/redox status and sugars/carbon status can be considered as integrated factors that account for most effects of the major environmental factors influencing carotenoid biosynthesis. The discussion highlights the concept of carotenoids or carotenoid-derivatives as stress signals that may be involved in feedback controls. We propose a conceptual model of the effects of environmental and developmental factors on carotenoid build-up in fruits.

Chlorophyll fluorescence, a nondestructive method to assess maturity of mango fruits (Cv. 'Cogshall') without growth conditions bias.

The quality of ripe mango fruits depends on maturity stage at harvest, which is usually assessed by visible criteria or from estimates of the age of fruit. The present study deals with the potential of chlorophyll fluorescence as a nondestructive method to assess the degree of fruit maturity regardless of fruit growing conditions. Chlorophyll fluorescence parameters were measured along with respiration rates of fruits still attached to the tree. At the same harvest stage, based on the fruit age or the thermal time sum (degree-days) method, physical and biochemical measurements related to fruit maturity and quality were made. Shaded fruits had a significantly greener flesh color, as well as a lower fruit density and flesh dry matter content, than well-exposed fruits, showing that fruits at the top of the canopy were more mature than fruits within the canopy, which were still in a growth phase. Additionally, chlorophyll fluorescence parameters, F(o), F(m), and F(v), were significantly lower for fruits taken from the top of the canopy than for those from within the canopy. The unique relationship observed between chlorophyll fluorescence parameters and fruit maturity, estimated by internal carbon dioxide content, on fruit still attached to trees is independent of growing conditions, such as the position of the fruit in the canopy and carbohydrate supply. The chlorophyll fluorescence method evaluates maturity much more accurately than the degree-day method and, moreover, nondestructively provides values for individual fruits before harvest.

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.

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.

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.

Effect of leaf-to-fruit ratio on leaf nitrogen content and net photosynthesis in girdled branches of Mangifera indica L.

Variations in leaf nitrogen concentration (Nm), mass-to-area ratio (Ma), amount of leaf nitrogen per unit leaf area (Na), and non-structural carbohydrates were measured in well-lit girdled branches of 11-year-old mango trees that were experiencing similar conditions of irradiance and gap fraction. The influence of source–sink relationships was studied by testing three levels of leaf-to-fruit ratio: 40, 70 and 150, during the period of linear fruit growth. Na was negatively correlated to the leaf-to-fruit ratio. Differences in Na were reflected in differences in net photosynthetic assimilation, Anet, although they could not fully account for them. All differences in Na resulted exclusively from differences in Nm, not Ma. Starch and total non-structural carbohydrates accumulated in the leaves as the result of higher leaf-to-fruit ratio, which suggests that the leaf carbohydrate status may play a role in photosynthetic acclimation to fruit load in mango. These observations complement previous findings where photosynthetic acclimation to light was found to be driven by changes in Ma, while Nm remained almost constant over a large range of gap fractions. Observations about the effect of fruit load were also in contrast with previous observations, since no evidence was found that leaf carbohydrate status played any role in photosynthetic acclimation to light. This study demonstrates that acclimation to changing source–sink relationships does not follow the same pattern as acclimation to progressive shading; but these observations do suggest that there may be different mechanisms by which leaves acclimatise to changing conditions.

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.

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.