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The objective of this research was to investigate the effects of gradually increasing or decreasing photosynthetic photon flux density (PPFD) during cultivation compared to a constant PPFD on biomass production. Lettuce plants (Lactuca sativa L. ‘Expertise’) were grown in climate rooms in which every three days the PPFD was increased by 16 µmol m−2 s−1 (from 140 to 300 µmol m−2 s−1 from day 0 to 30), decreased (from 300 to 140 µmol m−2 s−1), or kept constant (221 µmol m−2 s−1), while the total light integral at the end of the cultivation period (30 d) was the same for all three treatments. Gradually increasing PPFD resulted in a 16 or 13% increase in total plant dry weight compared to treatments with decreasing or constant PPFD, respectively. This increase was explained by a higher light interception mainly because, in this treatment, most of the light was provided at the end of the cultivation period when the leaf area index was high. Consequently, the light use efficiency based on incident PPFD was highest when PPFD gradually increased, even though the light use efficiency based on intercepted PPFD was highest when PPFD gradually decreased during cultivation. Despite the higher shoot dry weight when PPFD gradually increased, shoot fresh weight was not significantly affected by the light treatments. This difference in response between fresh and dry weight resulted from a higher shoot dry matter content when PPFD gradually increased. Our results show that gradually increasing PPFD had a positive effect on dry weight accumulation and increased dry matter content, but did not affect the shelf life. So, although vertical farms enable growers to keep all conditions constant, some dynamic variation of conditions might be needed for optimizing the light use efficiency.

As water quality and availability decreases in many parts of the world, salinity is becoming a major challenge that reduces crop yield, even in soilless cultivation systems. Therefore, novel strategies are needed to promote plant salt tolerance in these systems. We hypothesized that the non-essential element silicon (Si) and plant-growth promoting Bacillus spp. can alleviate salt stress of cucumber (Cucumis sativus L.) grown in hydroponics. We tested this hypothesis by growing cucumber seedlings with and without salt stress (75 mM NaCl) and with and without 1.5 mM Si and an inoculum of six rhizosphere Bacillus species in a full-factorial design. Seedlings were grown in a climate room for two weeks in independent deep-water culture containers. The applied salt stress strongly reduced plant biomass, whereas Si application under salt stress resulted in a substantial increase in cucumber shoot and root biomass. This beneficial impact of Si was also observed in increased plant height, leaf area, specific leaf area, root length, specific root length, root surface area and root volume. The Bacillus species increased root dry weight, specific leaf area as well as specific root length. In seedlings grown under salt stress, Si application increased shoot and root Si concentration, whereas Cl− concentration was reduced in the plant shoots. A reduction in Cl− concentration of the shoots was also apparent in the Bacillus treatment. Under non-stress conditions, neither Si nor Bacillus species affected plant growth parameters. However, shoot mineral content was affected as Si application reduced shoot Cl−and Ca2+ concentrations, and inoculation with Bacillus species decreased K concentration. We conclude that Si does promote salt stress alleviation during the early growth stage of cucumber grown in deep water culture and this has implications for soilless crop production. Seed inoculation with Bacillus species showed a beneficial trend for some plant growth characteristics and nutrient status under high salinity, although not as pronounced as for Si.

Effects of preharvest sprays of calcium with and without traditional and novel growth regulators (gibberellic acid, abscisic acid and salicylic acid, glycine betaine and a biostimulant (Ascophyllum nodosum)) on quality and sensory attributes of ‘Skeena’ and ‘Sweetheart’ sweet cherries were investigated at harvest. Experiments were carried out during 2015 and 2016 in a Portuguese orchard. Spray treatment effects were affected by cultivar and year, with spray treatments eliciting more responses in 2016 and for ‘Sweetheart’ cherries. Abscisic acid spray treatment showed small effects on quality attributes at harvest. Gibberellic and salicylic acid increased fruit weight, while glycine betaine with calcium increased fruit weight, appearance and colour development. The spray treatments did not affect the soluble sugars, while salicylic acid and Ascophyllum nodosum with calcium increased total organic acid levels. Gibberellic acid and glycine betaine sprays are candidates to increase quality attributes at harvest for two important commercial cherry cultivars.

Sweet cherry (Prunus avium L.) is highly perishable and is greatly affected by orchard management and environmental conditions, such as excess rainfall before harvest. Rain-induced cracking is the major cause of crop loss in sweet cherry in most production areas of the world. Advances in understanding the physiological and molecular mechanisms involved in cracking physiology in combination with orchard management strategies to limit cherry cracking are discussed. The current opinions to explain fruit cracking is that the process initiates with water uptake by the fruit surface that results in localised bursting of cells that release malic acid into the apoplast. This results in swelling of the epidermis and weakening of the epidermal and hypodermal cells until macroscopic fruit cracking. This review focusses on management strategies such as rain cover protection, mineral sprays, anti-transpirants and growth regulators. Tree responses to growth regulators and biostimulants vary with cultivar, application frequency, concentration and type, making it hard to generalize their effects. New approaches to limit cracking are presented, including the development of tolerant cultivars, candidate mineral sprays, biostimulants and technologies for rainwater removal such as orchard air-blast sprayers or creating downwash by helicopters.

Red and blue light play pivotal roles on mediating plant developmental processes, while evidence is accumulating that responses to changes in light spectrum are species dependent. This study aims to evaluate responses in cucumber and tomato which are major horticultural species and model plants for light spectrum studies, to red and blue light at different integrational levels, and at distinguishing to what extent these two species differ from each other. Plants were grown under a range of blue (B) light percentage: 0B, 25B, 50B, 75B, and 100B, the remaining percentage was red (R) light, with identical photosynthetic photon flux density (100 μmol⋅m−2⋅s-1). Peak intensities of B and R were at wavelengths of 454 nm and 663 nm, respectively. We showed that an increase in the blue light percentage resulted in a decreased plant height in both species, except under 100B where cucumber exhibited the greatest plant height, while in tomato the tallest plants were under 0B. Cucumber grown under 100B had the greatest shoot dry weight and largest leaf area, whereas in tomato 25B had the greatest plant growth. In both species, the function of leaf photosynthetic apparatus was strongly repressed by 0B, compared with the other treatments, while the magnitude of this repression was greater in cucumber than in tomato. Leaf total nitrogen, total carbon and chlorophyll contents were lowest under 0B in both species. HY5 expression exhibited limited response to 0B, but it was significantly increased by blue light (i.e. 50B and 100B) except with cucumber that was non-sensitive to 100B. We conclude that blue light is needed for maintaining normal plant growth and developmental processes, and cucumber is more sensitive to red and blue light than tomato, indicating that plant responses to red and blue light are species specific. Our research stresses the significance to consider the species or even cultivar difference when drawing conclusions from light quality study.

Although it is well known that, as a short-term response, stomata close at low relative humidity (RH) (high Vapour Pressure Deficit) and open at high RH (low Vapour Pressure Deficit), effects of long-term exposure to different Vapour Pressure Deficits (VPD’s) have only been studied in a few economically important horticultural crops, especially cut roses. To save energy costs, often low VPD conditions are present in greenhouses, due to low ventilation. We studied stomatal and desiccation responses of leaves of cut chrysanthemums after the plants had been exposed to different VPD conditions. While lower transpiration rate (E) was recorded for plants growing at low VPD (L-plants) in comparison with growth at moderate VPD (M-plants), higher stomatal conductance (g s ) was found in L-plants compared to M-plants. Bigger size of stomata and higher stomatal density were observed in L-plants. Besides differences in g s , E and stomatal characteristics during growth of the plants at different VPDs, stomatal closure response to desiccation (under the same VPD condition) was different when plants had been grown before at different VPD’s. Following desiccation, leaves of L-plants showed higher E compared with M-plants. Also E of leaves grown at moderate VPD but exposed to low VPD for only 5 days (M → L-plants) decreased less as response to desiccation than E of M-plants. After rapid desiccation, leaves of L and M → L plants had larger stomatal openings compared with leaves that were grown continuous at M condition, as indicated by their PSII efficiencies under non-photorespiratory conditions (Φ PSII ). In conclusion, control of VPD during growth will be important to prevent subsequent wilting of leaves of chrysanthemum cut flowers.

Indoor plant cultivation systems are gaining increasing popularity because of their ability to meet the needs of producing food in unfavourable climatic contexts and in urban environments, allowing high yield, high quality, and great efficiency in the use of resources such as water and nutrients. While light is one of the most important environmental factors affecting plant development and morphology, electricity costs can limit the widespread adoption of indoor plant cultivation systems at a commercial scale. LED lighting technologies for plant cultivation are also rapidly evolving, and lamps for indoor cultivation are often designed to optimize their light emissions in the photosynthetically active spectrum (i.e. red and blue), in order to reduce energetic requirements for satisfactory yield. Under these light regimens, however, little information is available in literature about minimum photosynthetic photon flux density (PPFD) for indoor production of leafy vegetables and herbs, while existing literature often adopts light intensities from 100 to 300 μmol m-2 s-1. This study aims at defining the optimal PPFD for indoor cultivation of basil (Ocimum basilicum L.) and lettuce (Lactuca sativa L.), by linking resource use efficiency to physiological responses and biomass production under different light intensities. Basil and lettuce plants were cultivated at 24 °C and 450 μmol mol-1 CO2 under red and blue light (with red:blue ratio of 3) and a photoperiod of 16 h d-1 of light in growth chambers using five PPFD (100, 150, 200, 250 and 300 μmol m-2 s-1, resulting in daily light integrals, DLI, of 5.8, 8.6, 11.5, 14.4 and 17.3 mol m-2 d-1, respectively). A progressive increase of biomass production for both lettuce and basil up to a PPFD of 250 μmol m-2 s-1 was observed, whereas no further yield increases were associated with higher PPFD (300 μmol m-2 s-1). Despite the highest stomatal conductance associated to a PPFD of 250 μmol m-2 s-1 in lettuce and to a PPFD ≥ 200 μmol m-2 s-1 in basil, water use efficiency was maximized under a PPFD ≥ 200 μmol m-2 s-1 in lettuce and PPFD ≥ 250 μmol m-2 s-1 in basil. Energy and light use efficiencies were increased under a PPFD of 200 and 250 μmol m-2 s-1 in lettuce and under a PPFD of 250 μmol m-2 s-1 in basil. Furthermore, in lettuce grown under 250 μmol m-2 s-1 antioxidant capacity, phenolics and flavonoids were higher as compared with plants supplied with PPFD ≤ 150 μmol m-2 s-1. Accordingly, a PPFD of 250 μmol m-2 s-1 seems suitable for optimizing yield and resource use efficiency in red and blue LED lighting for indoor cultivation of lettuce and basil under the prevailing conditions of the used indoor farming set-up.

Effects of repeated sprayings expected to affect phenolic, anthocyanin, carotenoid and ascorbic acid content in ‘Skeena’ and ‘Sweetheart’ cherries were observed during two years (without addition of calcium (Ca) in 2015, and with Ca in 2016). A shift in phytonutrients, with higher phenolic and carotenoid- and lower ascorbic acid content was observed when comparing Ca and the control (water) treatments in 2016 compared to 2015. Higher radiation, higher temperatures and less precipitation in 2015 compared to 2016 likely contributed to this shift. Gibberellic acid (GA3), abscisic acid (ABA), salicylic acid (SA) and glycine betaine (GB) sprays increased anthocyanin content in 2015 and for ‘Skeena’ cherries in 2016. GA3 and GB induced lower carotenoid content for ‘Skeena’- in 2015 and for ‘Sweetheart’ cherries in 2016 and lowered ascorbic acid content for ‘Sweetheart’ cherries. GA3 sprays induced the largest changes, increasing anthocyanin- (42 %), lowering carotenoid (19 %) and ascorbic acid content (53 %) compared to control. Ascophyllum nodosum, one of the novel spray treatments next to GB, appears to induce an effect opposite to GB, increasing carotenoid and ascorbic acid, but lowering phenolic content. Whether these phytonutrient shifts, due to climate conditions or to spray treatments, are beneficial to consumer health is unclear.

Size is an important property of all fruit as it determines, together with crop load, fruit yield. The aim of this study is to model the increase in fruit size during cell expansion based on diameter or length and to predict fruit size and volume, including the variation in size and volume, at harvest. The size of individual fruit during growth of fruit (four apple, two tomato and six pepper cultivars) was measured from early fruit set until harvest at several seasons. Size data were evaluated as diameter and length using the von Bertalanffy growth model, adapted to include the variation in size between individuals and the effect of growing temperature. The adapted von Bertalanffy model describes the increase in diameter and length and their variation. For any fruit type, size increased along the same generic growth model. A single stochastic variable, the biological shift factor, was sufficient to describe the variation in development time, initial size and maximum size. For all batches the percentage variation explained for were well over 93% with growth rate constants similar per species and only a minor effect of growing temperature. Prediction of the maximum size distribution based on measurements half way the cell expansion period is explored and discussed. Size and its distribution can be predicted based on a single measuring point in time but with two or three points in time the prediction system becomes more robust.