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.

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.

Taken together, the results do not support the common view that carbohydrate availability directly determines carotenoid synthesis by influencing precursor availability.

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.