Summary The results of a single publication stating that terrestrial plants emit methane has sparked a discussion in several scientific journals, but an independent test has not yet been performed. Here it is shown, with the use of the stable isotope 13C and a laser‐based measuring technique, that there is no evidence for substantial aerobic methane emission by terrestrial plants, maximally 0.3% (0.4 ng g−1 h−1) of the previously published values. Data presented here indicate that the contribution of terrestrial plants to global methane emission is very small at best. Therefore, a revision of carbon sequestration accounting practices based on the earlier reported contribution of methane from terrestrial vegetation is redundant.

Diffuse light enhanced crop photosynthesis. A more uniform horizontal PPFD distribution played the most important role in this enhancement, and a more uniform vertical PPFD distribution and higher leaf photosynthetic capacity contributed more to the enhancement of crop photosynthesis than did higher values of LAI.

Light is not evenly distributed in Dutch glass greenhouses, but this can be improved with diffuse light. Modern greenhouse coverings are able to transform most of the light entering the greenhouse into diffuse light. Wageningen UR Greenhouse Horticulture has studied the effect of diffuse light on crops for several years. Modelling and experimental studies showed that crops such as fruit vegetables with a high plant canopy as well as ornamentals with a small plant canopy can utilize diffuse light better than direct light. Diffuse light penetrates the middle layers of a high-grown crop and results in a better horizontal light distribution in the greenhouse. Diffuse light is absorbed to a better degree by the middle leaf layers of cucumber, resulting in a higher photosynthesis. The actual photosynthesis of four pot plant species was found to be increased and crop temperatures were lower during high irradiation. The yield of cucumbers was increased, and the growth rate of several potted plants was increased. These investigations have resulted in a quantitative foundation for the potentials of diffuse light in Dutch horticultural greenhouses and the selection and verification of technological methods to convert direct sunlight into diffuse light.

Reliable and quick assessment of energy conservation measures in greenhouse cultivation supports growers in their operations. Such an overview should quantify the consequences of changes in energy flows for total energy consumption, amount and quality of production, and farm economy.Using tomato as an example crop, comprehensive energy balances were developed for a reference situation in The Netherlands. Solar radiation, primary and secondary heating circuits and CO 2 from the flue gasses of the heating system were quantified as energy sources. Energy use for air and leaf temperature increase, crop photosynthesis, crop transpiration, as well as energy losses through the roof, walls and ground surface were quantified. Subsequently, the effects of 11 energy conservation measures were computed. Consequences for gas consumption and production were simulated with a greenhouse and a crop growth model, respectively, consequences for quality were assessed on the basis of expert knowledge, and economic consequences were simulated with a cost-benefit model.For tomato, most energy was saved by increased insulation of the greenhouse cover (23% saving) and lowered temperature set point (16%), followed by increased set point for air relative humidity, screen gap control in steps, and temperature integration (all about 5%). Fresh tomato production fell in most cases, except in case of increased light transmission by the greenhouse cover. Energy use efficiency was defined as the amount of energy required to produce a certain quantity of fresh harvestable product. Energy-conservation aims to decrease the energy use efficiency. Greatest gains were reached through insulation (-20%), lowered temperature set point (-12%) and improved light transmission (-8%). Improved light transmission resulted in the strongest increase of the balance of yield and costs (€2.6, or 10%), followed by increase of RH set point, crop-based RH control, crop-based use of the energy screen, increased size of the thermal storage tank and reduction of crop transpiration (all less than €0.5).Although energy conservation reduces fuel costs, its implementation depends on the effects on production an overall economic profitability of the farm. Improved roof insulation, reduced temperature set point, screen gap control in steps, increase of the RH set point, temperature integration, and crop-based RH control are first candidates for (further) implementation. Other measures require prior technological advancements or fine-tuning. INTRODUCTIONGiven the high costs of energy and obligations imposed on national governments by the Kyoto protocol (UNFCCC, 1997), energy conservation in horticulture has become increasingly important. Reliable and quick assessment of measures to conserve energy supports growers in their operations, and policy makers in directing research funds. Energy conservation assessments require an overview of the most important energy flows and their consequences. Changes in energy flows may have consequences for the

Next to its intensity, the spectral composition of light is one of the most important factors affecting plant growth and morphology. The introduction of light emitting diodes (LEDs) offers perspectives to design optimal light spectra for plant production systems. However, knowledge on the effects of light quality on physiological plant processes is still limited. The aim of this study is to determine the effects of six light qualities on growth and plant architecture of young tomato plants, and to upscale these effects to the crop level using a multispectral, functional-structural plant model. Young tomato plants were grown under 210 μmol m -2 s -1 blue, green, amber, red, white or red/blue (92%/8%) LED light with a low intensity of sunlight as background. Plants grown under blue light were shorter and developed smaller leaves which were obliquely oriented upward. Leaves grown under blue light contained the highest levels of light harvesting pigments, but when exposed to blue light only, they had the lowest rate of leaf photosynthesis. However, when exposed to white light these leaves had the highest rate of photosynthesis. Under green light, tomato plants were taller and leaves were nearly horizontally oriented, with a high specific leaf area. The open plant structure combined with a high light transmission and reflection at the leaf level allowed green light to penetrate deeper into the canopy. Plants grown under red, amber and white light were comparable with respect to height, leaf area and biomass production. The 3D model simulations indicated that the observed changes in plant architecture had a significant impact on light absorbance at the leaf and crop level. The combination of plant architecture and spectrum dependent photosynthesis was found to result in the highest rate of crop photosynthesis under red light in plants initially grown under green light. These results suggest that dynamic light spectra may offer perspectives to increase growth and production in high value production systems such as greenhouse horticulture and vertical farming.

At high irradiation levels, diffuse greenhouse coverings result in better light distribution, lower crop temperature, decreased transpiration, and increased photosynthesis and growth. Various greenhouse coverings (plastic films, glass panes and temporal coatings) can be used to transform direct light into diffuse light. However, light diffusing properties of materials are hardly known. Optical properties of a wide range of materials are being currently investigated, including direct light transmission under different angles of incidence, transmission for hemispherical light and haze. The potential of diffusing greenhouse covering materials is estimated by analyzing the global radiation data for different climatic regions: marine winter climate (The Netherlands), Mediterranean mild winter climate (Italy) and semi-arid climate (Arizona). The required optical properties differ for the various climates. With modern material technology, the optical properties (haze and light transmission) can be altered to meet the requirements for different climatic regions in order to optimize crop performance in the future.

Wageningen UR investigated the potential of several NIR-filtering methods to be applied in horticulture. In this paper the analysis of the optical properties of available NIR-filtering materials is given including a calculation method to quantify the energy reduction under these materials and to estimate the contribution for greenhouse cooling. It can be concluded that the optimum NIR-filtering material is still not found but there is good potential for further developments. NIR-filtering multilayer coatings applied to plastic film or glass filter out NIR most effectively. NIR-filtering is not desirable during winter-time in most climatic regions. Therefore, NIR-filtering coverings should not be used in unheated greenhouses since they cause an undesirable temperature drop. NIR-filtering white washes still reduce PAR too much. NIR-filtering (moveable) screens could be an alternative in the future. Future studies should also investigate whether it is possible to use the reflected NIR energy for other processes or cost-effectively storage.

The response to salt spray and soil salinity of two sand dune strandline species (Cakile maritima Scop. and Salsola kali L.) and two salt marsh strand‐line species (Atriplex hastata L. and A. littoralis L.) was compared in sand‐compost cultures. The growth of the salt‐marsh species remained unaffected, while the growth of the sand dune species Cakile maritima was strongly reduced by NaCl (150 and 300 mM) absorbed via the root system. All four species were resistant to airborne salinity, and under conditions of low soil fertility, salt spray increased the dry matter production, especially of the sand dune species. Mineral analysis revealed foliar uptake of Na, K, Cl, Ca and Mg. Na and Cl ions absorbed from seawater droplets induced succulence. Both salt spray and soil salt increased the methylated quaternary ammonium compound content in the shoot tissue. Under non‐saline conditions a considerable amount of these osmotic solutes was still present, while turgor pressure potential in these plants was rather low. The relation between salt, compatible osmotic solutes, turgor pressure potential and growth is discussed. Next to the major constituents of seawater, Na and Cl, especially magnesium and to a lesser extent, calcium, accumulated in the shoot tissue. Based on the positive growth response of the sand dune species to airborne salt, they should be termed ‘aerohalophytes’, whereas ‘soil halophytes’ should be used when referring to the Atriplex species, which are more specifically adapted to the increased salinity of salt marsh soils.