Sugars play an important role in grapevine flowering. This complex process from inflorescence initiation to fruit maturity takes two growing seasons. Currently, most of the available data concern the involvement of sugars as energy sources during the formation of reproductive structures from initiation of inflorescences during the summer of the first year, until flower opening during the following spring. Sugars devoted to the development of reproductive structures are supplied either by wood reserves or by photosynthesis in leaves or inflorescences, depending on the stage of development. Female meiosis appears to be a key point in the success of flower formation because (i) flowers are vulnerable at this stage and (ii) it corresponds in the whole plant to the transition between reserve mobilization from perennial organs (roots, trunk, and canes) towards efficient leaf photosynthesis. The perturbation of reserve replenishment during the previous year provokes perturbation in the development of inflorescences, whereas altering the photosynthetic sources affects the formation of flowers during the same year. In particular, a lack of sugar availability in flowers at female meiosis caused by various environmental or physiological fluctuations may lead to drastic flower abortion. Apart from energy, sugars also play roles as regulators of gene expression and as signal molecules that may be involved in stress responses. In the future, these two topics should be further investigated in the grapevine considering the sensitivity of flowers to environmental stresses at meiosis.

Fungicides are widely used to control pests in crop plants. However, it has been reported that these pesticides may have negative effects on crop physiology, especially on photosynthesis. An alteration in photosynthesis might lead to a reduction in photoassimilate production, resulting in a decrease in both growth and yield of crop plants. For example, a contact fungicide such as copper inhibits photosynthesis by destroying chloroplasts, affecting photosystem II activity and chlorophyll biosynthesis. Systemic fungicides such as benzimidazoles, anilides, and pyrimidine are also phytotoxic, whereas azoles stimulate photosynthesis. This article focuses on the available information about toxic effects of fungicides on photosynthesis in crop plants, highlighting the mechanisms of perturbation, interaction, and the target sites of different classes of fungicides.

In plants, flowering is a crucial process for reproductive success and continuity of the species through time. Fruit production requires the perfect development of reproductive structures. Abscission, a natural process, can occur to facilitate shedding of no longer needed, infected, or damaged organs. If stress occurs during flower development, abscission can intervene at flower level, leading to reduced yield. Flower abscission is a highly regulated developmental process simultaneously influenced and activated in response to exogenous (changing environmental conditions, interactions with microorganisms) and endogenous (physiological modifications) stimuli. During climate change, plant communities will be more susceptible to environmental stresses, leading to increased flower and fruit abscission, and consequently a decrease in fruit yield. Understanding the impacts of stress on the reproductive phase is therefore critical for managing future agricultural productivity. Here, current knowledge on flower/fruit abscission is summarized by focusing specifically on effects of environmental stresses leading to this process in woody plants. Many of these stresses impair hormonal balance and/or carbohydrate metabolism, but the exact mechanisms are far from completely known. Hormones are the abscission effectors and the auxin/ethylene balance is of particular importance. The carbohydrate pathway is the result of complex regulatory processes involving the balance between photosynthesis and mobilization of reserves. Hormones and carbohydrates together participate in complex signal transduction systems, especially in response to stress. The available data are discussed in relation to reproductive organ development and the process of abscission.

Plant infection by pathogens generates various forms of symptoms. Most of them have been described as soon as they become visible, whereas preceding, discrete signs during incubation are poorly or not understood. In Vitis vinifera, esca-related pathogenic fungi inhabit living trunk wood and induce the so-called apoplexy, a sudden wilting of leaves within a few days. To further understand the apoplexy expression, the period preceding symptom appearance was investigated by following physiological and molecular markers associated with photosynthetic mechanisms and stress responses. Within the week preceding symptoms, drastic physiological alterations of photosynthesis were registered in pre-apoplectic vines, as revealed by a decrease in gas exchange, changes in chlorophyll fluorescence, and repression of photosynthesis-related genes. In the meantime, expression of defense-related genes was induced and amplified during symptom expression. Water-stress-related genes were specifically investigated because water transport may be impeded by clogging xylem vessels due to esca-causing fungi. Neither of the tested water-stress-related genes was affected in pre-apoplectic grapevine leaves whereas these genes were downregulated in drying leaves. Our results suggest that, during incubation of esca apoplexy, grapevine perceives some signals (likely fungi-originated toxins) and reacts by reducing photosynthesis and triggering defense mechanisms.

The oomycete Plasmopara viticola is responsible for downy mildew, a severe grapevine disease. In infected grapevine leaves, we have observed an abnormal starch accumulation at the end of the dark period, suggesting modifications in starch metabolism. Therefore, several complementary approaches, including transcriptomic analyses, measurements of enzyme activities, and sugar quantification, were performed in order to investigate and to understand the effects of P. viticola infection on leaf starch and-to a larger extent-carbohydrate metabolism. Our results indicate that starch accumulation is associated with an increase in ADP-glucose pyrophosphorylase (AGPase) activity and modifications in the starch degradation pathway, especially an increased α-amylase activity. Together with these alterations in starch metabolism, we have observed an accumulation of hexoses, an increase in invertase activity, and a reduction of photosynthesis, indicating a source-to-sink transition in infected leaf tissue. Additionally, we have measured an accumulation of the disaccharide trehalose correlated to an increased trehalase gene expression and enzyme activity. Altogether, these results highlight a dramatic alteration of carbohydrate metabolism correlated with later stages of P. viticola development in leaves.