LED Light Quality of Continuous Light before Harvest Affects Growth and AsA Metabolism of Hydroponic Lettuce Grown under Increasing Doses of Nitrogen.

Yubin Zhang,Lingyan Zha,Mingjie Shao,Wenke Liu,Qichang Yang,Chengbo Zhou

doi:10.3390/plants10010176

Abstract

To study the effects of light quality of continuous light before harvest on the growth and ascorbic acid (AsA) metabolism of lettuce (Lactuca sativa L.) grown under relative high nitrogen level, lettuce plants grown under different nitrogen levels (8, 10 and 12 mmol·L−1) were subjected to continuous light with different red: blue light ratios (2R:1B and 4R:1B) before harvest. The results showed that the shoot fresh weight of lettuce under 12 mmol·L−1 nitrogen level was significantly higher than that under other treatments. There were no significant differences in shoot dry weight, root fresh weight, root dry weight, soluble sugar content, nitrate content and AsA content in leaves among the treatments at different nitrogen levels. The content of AsA in leaves was significantly higher than that in petioles before and after continuous light. Under the same nitrogen level, the fresh weight of lettuce under continuous light quality 4R:1B was significantly higher than that under other treatments. The content of AsA in lettuce leaves increased in different degrees after continuous light before harvest. High yield and AsA content could be obtained by 72 h continuous light with red and blue light 4R:1B at 12 mmol·L−1 nitrogen level. After continuous light, the content of AsA increased significantly due to the increase of the ratio of red light and nitrogen level, which increased the activities of L-galactono-1,4-lactone dehydrogenase (GalLDH) and dehydroascorbic acid reductase (DHAR) involved in AsA synthesis and in the recycling of DHAR to AsA respectively.

Highlights

Nitrogen (N) is an element which is a component of proteins, nucleic acid, hormones and many other important substances in plants, which plays a significant role in plant metabolism [1]
The treatment of nitrogen level had no significant effect on the root fresh weight (RFW) and root dry weight (RDW) before continuous light treatment
shoot fresh weight (SFW), shoot dry weight (SDW), RFW and the RDW of lettuce was significantly increased by continuous light treatment compared with that before treatment

Summary

Introduction

Nitrogen (N) is an element which is a component of proteins, nucleic acid, hormones and many other important substances in plants, which plays a significant role in plant metabolism [1]. The application of extensive N fertilizers, may lead to the consumption of a large amount of carbohydrates, resulting in the decrease of plant biomass under high nitrogen supply, and the growth and development of plants are limited [2]. This phenomenon is even more prominent under low light conditions, mainly manifested in low AsA content and excessive accumulation of nitrogen [3]. The production of hydroponic leafy vegetables rich in ascorbic acid (AsA) and low nitrate content is critical for the improvement of the cultivation systems towards the production of high-quality products for the consumers. Sufficient nitrogen could promote the photosynthesis of plants, which is closely related to the light environment of plants [8]

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