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High-tech greenhouses are characterised by high yields and high energy consumption. Current trends towards expanded use of supplemental lighting further increase the intensity of crop production and energy use, and it is expected that the availability of light-emitting diodes (LEDs) will accelerate this trend. At the same time, an increase in greenhouse lighting reduces the heating energy needed from the heating system. This study presents a novel concept for greenhouses, where both lighting and heating are derived exclusively from lamps. Such greenhouses can be highly efficient, as light is used both for crop growth and for heating. If the electric grid is based on renewable sources, such greenhouses can also be carbon-neutral and fossil-free. By using model simulations for a tomato greenhouse in the Netherlands, it was found that such greenhouses could be realised by employing a heat storage system with a heat storage capacity of 2 MJ m−2 and LEDs with a power capacity of 150 W m−2 and a photosynthetic photon flux density (PPFD) of 450 μmol m−2 s−1. The greenhouse heated by light was predicted to have 44% higher yields and 60% higher energy inputs than a reference greenhouse, equipped with a boiler and LEDs with a PPFD of 200 μmol m−2 s−1. This result was part of a general trade-off that was found between yield and energy efficiency. This exploration helped identify avenues for further improvement of the energy efficiency of greenhouses heated by lamps, highlighting their promise as a potential new direction in greenhouse intensification.

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.

The relations between physical and chemical characteristics (e.g., color, firmness, volatile and non-volatile metabolites) of red ripe strawberry fruit and the natural spoilage caused by Botrytis cinerea were investigated. The spoilage rates differed between genotypes, and this was highly correlated over two successive years. Among seventeen genotypes, a more intense red coloration of the fruit skin was associated with a lower spoilage rate (r = –0.63). Additionally, weakly negative correlations were found between the levels of anthocyanins, ascorbic acid, malic acid and spoilage rates. No clear correlations were found between spoilage rates and soluble sugars, most volatiles, firmness and dry weight percentage. High levels of two volatile compounds, ethyl butanoate (r = 0.55) and 1-hexanol (r = 0.61), were correlated to high spoilage rates. These characteristics may assist strawberry breeders in selecting for genotypes with reduced susceptibility to B. cinerea.

Fruit set is a crucial plant developmental process, determining yield in many crops. Pepper (Capsicum annuum L.) is a crop with poor fruit set as typically about two-thirds of all flowers abort. A higher light integral improves fruit set. However, the role of light spectrum has hardly been investigated. Opportunities for detailed investigation of light spectrum effects on fruit set have strongly increased with the introduction of narrow-band LED lighting. To investigate whether additional far-red light (FR) influences the fruit set of sweet pepper, a climate chamber experiment was conducted. Four light treatments were applied to pepper plants grown under 130 µmol m−2 s−1 of red/white LED light. Treatments consisted of different intensities of FR (0, 50, 100 µmol m−2 s−1) applied throughout the day or applied at the end of day (EOD, 30 min, 30 µmol m−2 s−1). Treatments resulted in phytochrome photostationary state (PSS) values of 0.88, 0.77. 0.70 and EOD 0.16, respectively. Fruit set was determined 3 weeks after the last anthesis of the studied flowers. Additional FR light reduced fruit set in sweet pepper, regardless of whether FR light was provided during the whole day or only at the end of the day. Meanwhile, FR led to more stem elongation, more upright branches, and more dry mass partitioning to stems. Additional FR during the daytime increased total shoot dry weight but not when FR was applied at the end of day. Possible reasons for stimulated flower and fruit abortion by additional FR are discussed.

In purple-fruited Capsicum genotypes, fruit anthocyanin content first increases, and later decreases upon ripening. In this study, the response of this transient anthocyanin accumulation as a function of light intensity and light spectrum was investigated. Harvested fruit at the mature purple stage were illuminated by white-red LED lights at a photosynthetic photon flux density of 0 (darkness), 80, 160 and 320 μmol m-2 s-1 or illuminated by 24 (white light), 57%, 72% and 99% blue light (400–500 nm) at a photosynthetic photon flux density of 80 μmol m-2 s-1 for 28 d. Total anthocyanin, chlorophyll, capsanthin and soluble sugar contents, and their related gene expression, were measured. Anthocyanin content was hardly affected by white-red light intensity. Increasing the blue light fraction increased anthocyanin levels, via enhancing anthocyanin biosynthesis. This is supported by kinetic modelling and higher expression levels of the anthocyanin biosynthetic genes CaMYB, CaCHS, CaDFR, CaANS and CaUFGT. A higher blue light fraction delayed fruit ripening. This is supported by delayed sugar accumulation as indicated by kinetic modelling and transcriptome analysis. Transcriptome analysis indicated that 91% of the differentially expressed genes down-regulated during ripening had a higher expression in the 72% compared to 24% blue light treatment. In addition, PCA analysis of transcriptomic data indicated fruit ripening and senescence related genes NCED1, NCED2, NOR and RIN were expressed less in fruit illuminated by 72% blue light. Pathways upstream of ethylene production were downregulated in fruits illuminated with higher blue light fractions that might result in suppressed ethylene production. Higher fractions of blue light during postharvest storage might be applied to improve fruit anthocyanin levels but are not suited to create ripe anthocyanin rich pepper fruit.

When flower-bearing shoots in cut-rose (Rosa ×hybrida) are harvested (removed), a varying number of repressed axillary buds on the shoot remainder start to grow into new shoots (budbreak). Besides removing within-shoot correlative inhibition, it is hypothesized that shoot removal leads to 1) increased light intensity lower in the crop canopy; 2) changes in the light spectrum (particularly red:far-red ratio); and 3) changed source:sink ratio (i.e., the ratio between supply and demand of assimilates). As a fourth hypothesis it is proposed that the degree of budbreak on a shoot remainder is also influenced by the correlative inhibition exerted by other shoots on the plant. It is the goal of this work to determine which of these four hypotheses is most important for budbreak in a cut-rose crop. Four experiments were conducted, in which these factors were varied by leaf removal, removal of mature shoots, varying the number of young shoots, shading of the crop, and application of direct light on the buds. Increase in source:sink ratio was not consistently associated with higher budbreak. If source:sink ratio was decreased by removal of leaves or a mature shoot, budbreak showed even a tendency to increase. Budbreak was subject to correlative inhibition exerted by other shoots on the plant. Treatments where more light reached the bud (as a result of less shoots, no shading of the crop, application of local light) increased budbreak. Increased red:far-red ratio had the same result as more light reaching the bud but was often interrelated with light intensity. It was concluded that after removal of the flower-bearing shoot, among the factors tested, light intensity on the buds was an important and consistent factor explaining budbreak on the shoot remainder, whereas the effect of light spectrum should be further investigated.