Increasing Duration and Intensity of Supplemental Lighting during Nighttime to Promote Growth and Photosynthesis in Cymbidium Plants

Abstract

These studies were conducted to investigate the influence of supplemental lighting timing in a day (Expt. 1), and to determine how the duration and light intensity during nighttime supplemental lighting (NSL) influence the growth and photosynthesis of the Cymbidium plants (Expt. 2). Nine-month-old Cymbidium Yang Guifei and Wine Shower plants were treated with three different supplemental lighting timings: from 22:00 to 02:00 HR (middle of the night, MN)

from 17:00 to 21:00 HR (day extension, DE)

and from 07:00 to 09:00 HR plus from 17:00 to19:00 HR (day extension which was divided into two sections, 1/2DE), and with non-supplemental lighting (8/16 h

short day, SD) for 4 months. All of the supplemental lighting treatments were provided by 100% red LEDs (peak at 640 and 660 nm) with 150 µmolm-2s-1 and 800 µmolmol-1 of CO2 was supplied during nighttime. In both cultivars, pseudobulb diameter under supplemental lighting treatments (MN, DE, and 1/2DE) appeared to be greater irrespective of supplemental lighting timings than under SD. Photosynthetic assimilation rate (An) also showed the similar response as growth. These results indicate that supplemental lighting timing application is not required for promoting growth and photosynthesis in these two Cymbidium plants. Furthermore, to accelerate growth and photosynthesis of Cymbidium plants, it is related with the total light integral. Thus, in Expt.2, 2-month-old Cymbidium plants with the same cultivar in Expt. 1 were used to determine how NSL duration and light intensity, which affects NSL light integral, influence growth and photosynthesis. Plants were treated with five NSL durations of 2, 4, 6, 8, and 16 h and three light intensities of 10, 100, and 200 µmolm-2s-1 for 4 months, which provided 13 NSL light integrals ranging from 0 to 11.52 molm-2d-1. After 4 months of NSL treatment, plants in all treatments were then grown in the same ambient photoperiod to identify the residual effects on subsequent growth. In both cultivars, pseudobulb diameter, number of leaves, leaf width, and biomass accumulation significantly increased as NSL duration and light intensity increased. In case of pseudobulb diameter, it increased by 33% and 43% in Yang Guifei and Wine Shower, respectively, as NSL light integral increased from 0 to 11.52 mol∙m-2∙d-1. However, relative chlorophyll content (SPAD value) significantly decreased in Yang Guifei by increasing NSL light integral during NSL treatment but, it recovered after the treatments ended. An during NSL treatment was promoted according to the increase of NSL duration and light intensity in both cultivars. Also, daily An was increased with increasing NSL light integral and showed positive correlation with pseudobulb diameter. These experiments indicate that supplemental lighting during nighttime with high light integral accelerates growth with greater photosynthetic assimilation rate.