Chlorophyll fluorescence has been routinely used for many years to monitor the photosynthetic performance of plants non-invasively. The relationships between chlorophyll fluorescence parameters and leaf photosynthetic performance are reviewed in the context of applications of fluorescence measurements to screening programmes which seek to identify improved plant performance. The potential role of chlorophyll fluorescence imaging in increasing both the sensitivity and throughput of plant screening programmes is examined. Finally, consideration is given to possible specific applications of chlorophyll fluorescence for screening of plants for tolerance to environmental stresses and for improvements in glasshouse production and post-harvest handling of crops.

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.

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.