Effects of Light Intensity on Growth and Quality of Lettuce and Spinach Cultivars in a Plant Factory

Abstract

The decreased quality of leafy vegetables and tipburn caused by inappropriate light intensity are serious problems faced in plant factories, greatly reducing the economic benefits. The purpose of this study was to comprehensively understand the impact of light intensity on the growth and quality of different crops and to develop precise lighting schemes for specific cultivars. Two lettuce (Lactuca sativa L.) cultivars-Crunchy and Deangelia-and one spinach (Spinacia oleracea L.) cultivar-Shawen-were grown in a plant factory using a light-emitting diode (LED) under intensities of 300, 240, 180, and 120 μmol m^-2 s^-1, respectively. Cultivation in a solar greenhouse using only natural light (NL) served as the control. The plant height, number of leaves, and leaf width exhibited the highest values under a light intensity of 300 μmol m^-2 s^-1 for Crunchy. The plant width and leaf length of Deangelia exhibited the smallest values under a light intensity of 300 μmol m^-2 s^-1. The fresh weight of shoot and root, soluble sugar, soluble protein, and ascorbic acid contents in the three cultivars increased with the increasing light intensity. However, tipburn was observed in Crunchy under 300 μmol m^-2 s^-1 light intensity, and in Deangelia under both 300 and 240 μmol m^-2 s^-1 light intensities. Shawen spinach exhibited leaf curling under all four light intensities. The light intensities of 240 and 180 μmol m^-2 s^-1 were observed to be the most optimum for Crunchy and Deangelia (semi-heading lettuce variety), respectively, which would exhibit relative balance growth and morphogenesis. The lack of healthy leaves in Shawen spinach under all light intensities indicated the need to comprehensively optimize cultivation for Shawen in plant factories to achieve successful cultivation. The results indicated that light intensity is an important factor and should be optimized for specific crop species and cultivars to achieve healthy growth in plant factories.

Keywords: LED light intensity; indoor cultivation; nutrient content; photosynthesis; plant development; tipburn.

Figure 1

Lettuce cultivars Crunchy (A,B) and Deangelia (C,D) and spinach cultivar Shawen (E,F) grown in a solar greenhouse with natural light (NL) and a closed-type plant factory with light intensities of 300, 240, 180, and 120 μmol m⁻² s⁻¹ (indicated by LI300, LI240, LI180, and LI120, respectively).

Figure 2

Plant height of two lettuce cultivars Crunchy (A) and Deangelia (B) and one spinach cultivar Shawen (C) grown in a solar greenhouse (NL) and a closed-type plant factory with light intensities of 300, 240, 180, and 120 μmol m⁻² s⁻¹ (indicated by LI300, LI240, LI180, and LI120, respectively).

Figure 3

Plant width of two lettuce cultivars Crunchy (A) and Deangelia (B) and one spinach cultivar Shawen (C) grown in a solar greenhouse (NL) and a closed-type plant factory with light intensities of 300, 240, 180, and 120 μmol m⁻² s⁻¹ (indicated by LI300, LI240, LI180, and LI120, respectively).

Figure 4

Number of leaves of two lettuce cultivars Crunchy (A) and Deangelia (B) and one spinach cultivar Shawen (C) grown in a solar greenhouse (NL) and a closed-type plant factory with light intensities of 300, 240, 180, and 120 μmol m⁻² s⁻¹ (indicated by LI300, LI240, LI180, and LI120, respectively).

Figure 5

Tipburn in Crunchy under light intensity of 300 μmol m⁻² s⁻¹ (A) and in Deangelia under light intensities of 300 and (B) and 240 (C) μmol m⁻² s⁻¹. Red arrow points to the exact location of the tipburn. Leaf curling in Shawen (D).

Figure 6

Leaf length and leaf width of two lettuce cultivars Crunchy (A,B) and Deangelia (C,D) and one spinach cultivar Shawen (E,F) grown in a solar greenhouse (NL) and a closed-type plant factory with light intensities of 300, 240, 180, and 120 μmol m⁻² s⁻¹ (indicated by LI300, LI240, LI180, and LI120, respectively).

Figure 7

Fresh weight of shoot and root of two lettuce cultivars Crunchy (A,B) and Deangelia (C,D) and one spinach cultivar Shawen (E,F) grown in a solar greenhouse (NL) and a closed-type plant factory with light intensities of 300, 240, 180, and 120 μmol m⁻² s⁻¹ (indicated by LI300, LI240, LI180, and LI120, respectively). Bars with different letters within each panel indicate significant differences at p < 0.05 according to Duncan’s test.

Figure 8

Fv/Fm and PI of two lettuce cultivars Crunchy (A,B) and Deangelia (C,D) and spinach cultivar Shawen (E,F) grown in a solar greenhouse (NL) and a closed-type plant factory with light intensities of 300, 240, 180, and 120 μmol m⁻² s⁻¹ (indicated by LI300, LI240, LI180, and LI120, respectively).

Figure 9

Soluble sugar content of two lettuce cultivars Crunchy (A) and Deangelia (B) and one spinach cultivar Shawen (C) grown in a solar greenhouse (NL) and a closed-type plant factory with light intensities of 300, 240, 180, and 120 μmol m⁻² s⁻¹ (indicated by LI300, LI240, LI180, and LI120, respectively). Bars with different letters within each panel indicate significant differences at p < 0.05 according to Duncan’s test.

Figure 10

Soluble protein content of two lettuce cultivars Crunchy (A) and Deangelia (B) and one spinach cultivar Shawen (C) grown in a solar greenhouse (NL) and a closed-type plant factory with light intensities of 300, 240, 180, and 120 μmol m⁻² s⁻¹ (indicated by LI300, LI240, LI180, and LI120, respectively). Bars with different letters within each panel indicate significant differences at p < 0.05 according to Duncan’s test.

Figure 11

Ascorbic acid content of two lettuce cultivars Crunchy (A) and Deangelia (B) and one spinach cultivar Shawen (C) grown in a solar greenhouse (NL) and a closed-type plant factory with light intensities of 300, 240, 180, and 120 μmol m⁻² s⁻¹ (indicated by LI300, LI240, LI180, and LI120, respectively). Bars with different letters within each panel indicate significant differences at p < 0.05 according to Duncan’s test.

Figure 12

(A) The supplemental LED lighting system for plants. (B) Computer-based control system (C) The illumination spectrum used for LED treatments.