Abiotic stresses due to environmental factors could adversely affect the growth and development of crops. Among the abiotic stresses, drought and heat stress are two critical threats to crop growth and sustainable agriculture worldwide. Considering global climate change, incidence of combined drought and heat stress is likely to increase. The aim of this study was to shed light on plant growth performance and leaf physiology of three tomatoes cultivars (‘Arvento’, ‘LA1994’ and ‘LA2093’) under control, drought, heat and combined stress. Shoot fresh and dry weight, leaf area and relative water content of all cultivars significantly decreased under drought and combined stress as compared to control. The net photosynthesis and starch content were significantly lower under drought and combined stress than control in the three cultivars. Stomata and pore length of the three cultivars significantly decreased under drought and combined stress as compared to control. The tomato ‘Arvento’ was more affected by heat stress than ‘LA1994’ and ‘LA2093’ due to significant decreases in shoot dry weight, chlorophyll a and carotenoid content, starch content and NPQ (non-photochemical quenching) only in ‘Arvento’ under heat treatment. By comparison, the two heat-tolerant tomatoes were more affected by drought stress compared to ‘Arvento’ as shown by small stomatal and pore area, decreased sucrose content, ΦPSII (quantum yield of photosystem II), ETR (electron transport rate) and qL (fraction of open PSII centers) in ‘LA1994’ and ‘LA2093’. The three cultivars showed similar response when subjected to the combination of drought and heat stress as shown by most physiological parameters, even though only ‘LA1994’ and ‘LA2093’ showed decreased Fv/Fm (maximum potential quantum efficiency of photosystem II), ΦPSII, ETR and qL under combined stress. The cultivars differing in heat sensitivity did not show difference in the combined stress sensitivity, indicating that selection for tomatoes with combined stress tolerance might not be correlated with the single stress tolerance. In this study, drought stress had a predominant effect on tomato over heat stress, which explained why simultaneous application of heat and drought revealed similar physiological responses to the drought stress. These results will uncover the difference and linkage between the physiological response of tomatoes to drought, heat and combined stress and be important for the selection and breeding of tolerant tomato cultivars under single and combine stress.

With the expeditious development of optoelectronics, the light-emitting diode (LED) technology as supplementary light has shown great advancement in protected cultivation. One of the greatest challenges for the LED as alternative light source for greenhouses and closed environments is the diversity of the way experiments are conducted that often makes results difficult to compare. In this review, we aim to give an overview of the impacts of light spectra on plant physiology and on secondary metabolism in relation to greenhouse production. We indicate the possibility of a targeted use of LEDs to shape plants morphologically, increase the amount of protective metabolites to enhance food quality and taste, and potentially trigger defense mechanisms of plants. The outcome shows a direct transfer of knowledge obtained in controlled environments to greenhouses to be difficult, as the natural light will reduce the effects of specific spectra with species or cultivar-specific differences. To use the existing high-efficiency LED units in greenhouses might be both energy saving and beneficial to plants as they contain higher blue light portion than traditional high-pressure sodium (HPS) lamps, but the design of light modules for closed environment might need to be developed in terms of dynamic light level and spectral composition during the day to secure plants with desired quality with respect to growth, postharvest performance, and specific metabolites.

Background and Aims Leaf gas exchange is influenced by stomatal size, density, distribution between the leaf adaxial and abaxial sides, as well as by pore dimensions. This study aims to quantify which of these traits mainly underlie genetic differences in operating stomatal conductance (g_s) and addresses possible links between anatomical traits and regulation of pore width. Methods Stomatal responsiveness to desiccation, g_s-related anatomical traits of each leaf side and estimated g_s (based on these traits) were determined for 54 introgression lines (ILs) generated by introgressing segments of Solanum pennelli into the S. lycopersicum ‘M82’. A quantitative trait locus (QTL) analysis for stomatal traits was also performed. Key Results A wide genetic variation in stomatal responsiveness to desiccation was observed, a large part of which was explained by stomatal length. Operating g_s ranged over a factor of five between ILs. The pore area per stomatal area varied 8-fold among ILs (2–16 %), and was the main determinant of differences in operating g_s between ILs. Operating g_s was primarily positioned on the abaxial surface (60–83 %), due to higher abaxial stomatal density and, secondarily, to larger abaxial pore area. An analysis revealed 64 QTLs for stomatal traits in the ILs, most of which were in the direction of S. pennellii. Conclusions The data indicate that operating and maximum g_s of non-stressed leaves maintained under stable conditions deviate considerably (by 45–91 %), because stomatal size inadequately reflects operating pore area (R² = 0·46). Furthermore, it was found that variation between ILs in both stomatal sensitivity to desiccation and operating g_s is associated with features of individual stoma. In contrast, genotypic variation in g_s partitioning depends on the distribution of stomata between the leaf adaxial and abaxial epidermis.

To evaluate the effect of blue light intensity and timing, two cultivars of lettuce [Lactuca sativa cv. ’Batavia’ (green) and cv. ‘Lollo Rossa’ (red)] were grown in a greenhouse compartment in late winter under natural light and supplemental high pressure sodium (SON-T) lamps yielding 90 (±10) µmol m-2 s-1 for up to 20 hr, but never between 17:00 and 21:00. The temperature in the greenhouse compartments was 22/11°C day/night, respectively. The five light-emitting diode (LED) light treatments were Control (no blue addition), 1B 06-08 (Blue light at 45 µmol m-2 s-1 from 06:00 to 08:00), 1B 21-08 (Blue light at 45 µmol m-2 s-1 from 21:00 to 08:00), 2B 17-19 (Blue at 80 µmol m-2 s-1 from 17:00 to 19:00), and (1B 17-19) Blue at 45 µmol m-2 s-1from 17:00 to 19:00. Total fresh and dry weight was not affected with additional blue light; however, plants treated with additional blue light were more compact. The stomatal conductance in the green lettuce cultivar was higher for all treatments with blue light compared to the Control. Photosynthetic yields measured with chlorophyll fluorescence showed different response between the cultivars; in red lettuce, the quantum yield of PSII decreased and the yield of non-photochemical quenching increased with increasing blue light, whereas in green lettuce no difference was observed. Quantification of secondary metabolites showed that all four treatments with additional blue light had higher amount of pigments, phenolic acids, and flavonoids compared to the Control. The effect was more prominent in red lettuce, highlighting that the results vary among treatments and compounds. Our results indicate that not only high light level triggers photoprotective heat dissipation in the plant, but also the specific spectral composition of the light itself at low intensities. However, these plant responses to light are cultivar dependent.