Influence of Pulsed, Scanning and Constant (16- and 24-h) Modes of LED Irradiation on the Physiological, Biochemical and Morphometric Parameters of Lettuce Plants (Lactuca sativa L.) while Cultivated in Vertical Farms

Abstract

In city farming, when growing green crops, a significant part of the production cost is the cost of electricity for lighting. The physiology, biochemistry, morphology and productivity of plants can be affected by changing irradiation modes and these changes reduce electricity costs. However, the results of studies in the literature are contradictory. In this work, we investigated the effect of impulse (frequency 1000 Hz and duty cycle 67%), scanning (the principle of running lights) and constant 16 h and 24 h modes of operation of white light LED irradiators on the physiological, biochemical and morphometric parameters of lettuce with red and green leaves. The daytime integral of light in all variants remained unchanged ~15.6 mol m−2 day−1. Daily electricity consumption also did not differ significantly. Plants were grown on racks in a climatic chamber up to 35 days of age. For lettuce with red leaves, the most optimal for biomass accumulation and synthesis of anthocyanins was the impulse illumination mode, while for lettuce with green leaves, no statistically significant differences in biomass were observed under different irradiation modes. For red-leaved lettuce, it was found that the highest concentration of carotenoids in the leaf was observed under constant (24 h) and scanning irradiation, which is associated with a more active reaction of the photosynthetic system to prolonged irradiation and increased intensity during scanning irradiation. Also, increased photosynthetic activity was found in both varieties of lettuce at 16 h of operation of LED irradiators, which, however, did not affect their final productivity. The results may be useful for the development of LED illuminators for use in rack growing.

Effect of Duration of LED Lighting on Growth, Photosynthesis and Respiration in Lettuce

“…In turn, LEF suppression can result from the decreasing of the fraction of the actinic light absorbed by the leaf and fraction of the absorbed light distributed to photosystem II ( Figure 4 ), which are caused by lowering content of chlorophylls ( Figure 5 and Figure 6 ). This potential mechanism ( Figure 10 ) is in a good accordance with the decreasing content of chlorophylls in lettuce plants under the short photoperiod, which is shown in a number of works [ 28 , 30 , 39 ]; however, it should be noted that changes in the chlorophyll content are not shown in several works [ 29 , 40 ]. Increasing the chlorophyll content under the increased photoperiod can be caused by the direct light influence on the reduction of protochlorophyllide to chlorophyllide [ 41 , 42 ]; this hypothesis is in accordance with the negative influence of the increased photoperiod on the chlorophyll content under the constant total light integral for one day [ 30 ].…”

“…It can be expected that increasing the photoperiod should additionally increase the lettuce production; however, the literature results seem to be contradictory. Work [ 29 ] shows that the increased photoperiod does not significantly change the lettuce production, and can even, decrease the linear electron flow; in contrast, some studies [ 28 , 30 ] reported a positive influence of the increased photoperiod on the photosynthetic parameters and production of lettuce plants. In this work we investigated the effect of photoperiod on the photosynthetic parameters, respiration, content of chlorophylls, biomass, and dry weight.…”

“…The light intensity was 180 µmol m −2 s −1 ; the Thorlabs PM100D optical power meter (Thorlabs Inc., Newton, MA, USA) with an S120VC sensor (200–1100 nm) was used to control the light intensity. This light intensity was used on the basis of our previous works [ 22 , 29 ] and results by Iqbal et al [ 28 ]; the last work showed that light dependences of biomass and DW of leaves in lettuce plants were saturated under light intensity equaling to 200 µmol m −2 s −1 (14 and 17 h photoperiods, illumination by LEDs).…”

“…The influence of photoperiod on physiological, biochemical, and morphological parameters of lettuce seems to be intricate. According to Smirnov et al [ 29 ], an increased photoperiod weakly increases the biomass, DW, and leaf area, stimulates the non-photochemical quenching, suppresses LEF, and decreases the content of chlorophylls. In contrast, studies [ 28 , 30 ] show that the increased photoperiod significantly increases the biomass, DW, content of chlorophylls, stomata conductance, and photosynthetic CO 2 assimilation.…”

“…In contrast, the light intensity can potentially influence on effect of the photoperiod because the influence of duration of illumination on the chlorophyll content and photosynthesis is weaker in lettuce plants cultivated under 300 μmol m −2 s −1 light and stronger in those cultivated under 150 μmol m −2 s −1 light [ 28 , 31 ]. However, decreasing LEF following increasing photoperiodic conditions was also observed in lettuce plants cultivated under 180 μmol m −2 s −1 light [ 29 ].…”