Quantitative Morphological Analysis of Spinach Leaves Grown Under Light-Emitting Diodes or Sulfur-Microwave Lamps

Alternative lighting sources that offer advantages over traditional lighting regimes are being investigated for space-based plant growth applications. Light emitting diodes (LEDs) and sulfur-microwave lamps are among these candidate systems because of their potential for reduced power consumption and increased radiation use efficiency over conventional lighting systems such as high-pressure sodium (HPS) and cool white fluorescence (CWF). The effects of LEDs and microwave lamps on plant growth must also be characterized before they can be considered for spaceflight applications. Leaf morphology of spinach plants (Spinacia oleracea L. cv. Whitney) grown for 28 days under sulfur-microwave lamps or one of 4 LED arrays with peak output wavelengths of 660, 690, 700 and 725nm, (each supplemented with 470nm blue LEDs) was compared to control plants grown under HPS and CWF. Anatomical features were similar among plants grown under the control, microwave, 660 and 690nm LED lighting systems. In contrast, significant differences in leaf and petiole structures were evident in plants grown under the 700 and 725nm LED arrays compared to the controls. Leaf thickness and cross-sectional petiole area were 25% smaller in plants grown under the 700nm LED array and 60% smaller in plants grown under the 725nm LED array. Additionally, the ratio of leaf palisade mesophyll to spongy mesophyll decreased disproportionately in these plants. Under the 700nm LED array, the palisade mesophyll was 37% thinner than the controls, yet the spongy mesophyll was not significantly thinner. Under the 725nm LED array the palisade mesophyll was virtually undetectable, leaving nearly the entire leaf consisting of only spongy mesophyll. Structures designed for maximal light interception were minimized in plants grown under the far-red LED arrays while those devoted to gas exchange were less effected.