Morphological and physiological properties of indoor cultivated lettuce in response to additional far-red light

Abstract

Understanding the wavelength-dependence of plant responses is of particular importance to optimize production and quality. Far-red (FR) light is known to regulate phytochrome-mediated morphological and physiological plant responses, while limited attention has been paid to the benefits of including FR light to indoor plant cultivation. Lettuce (Lactuca sativa L. cv. ‘Tiberius’) plants were grown under a fully controlled environment, in which three different light treatments were arranged. The photosynthetic photon flux density (PPFD) was 200 μmol m−2 s−1 provided by a mixture of red and blue light (red/blue light ratio was 7:1) with peak intensities at 450 nm and 660 nm, respectively. In two of the treatments, 50 μmol m−2 s−1 FR light with peak intensity at 740 nm was supplemented during the day (FR-Day, 16 h) or end of the day (FR-EOD, 1 h). Plant total biomass was increased by 39% and 25% in the FR-Day and FR-EOD treatment, respectively, compared with the control. Adding FR light resulted in 27–49 % larger plant leaf area with an open plant canopy, which facilitated a better light interception and led to higher plant radiation use efficiency (RUE). Although temporarily adding FR light increased net leaf photosynthetic rates by 7–10 %, FR light treatments resulted in a significantly lower chlorophyll and total nitrogen content, as well as lower leaf absorbance, which led to lower maximum leaf photosynthetic rate. Such downregulation had limited effect on plant growth as the net leaf photosynthetic rates in their corresponding growth environments were similar between treatments. The extent of FR light induced leaf physiological modifications was lower in the FR-EOD treatment, in comparison with the FR-Day treatment. Our estimation showed that ˜36% and ˜94% of the enhanced dry mass production in the FR-Day and FR-EOD treatments, respectively, could be attributed to the FR light regulated RUE improvements, the other part enhanced dry mass production might be the consequence of additional light. We conclude that the acclimation process of plant morphology triggered by additional FR light plays a pivotal role for improving the production of indoor cultivated lettuce, and the enhanced production by additional FR light cannot be achieved by adding similar amount of red and blue light.