Light Shock Stress after Outdoor Sunlight Exposure in Seedlings of Picea abies (L.) Karst. and Pinus sylvestris L. Pre-Cultivated under LEDs—Possible Mitigation Treatments and Their Energy Consumption

Marco Hernandez Velasco,Anders Mattsson

doi:10.3390/f11030354

Abstract

Year-round cultivation under light emitting diodes (LEDs) has gained interest in boreal forest regions like Fenno-Scandinavia. This concept offers forest nurseries an option to increase seedling production normally restricted by the short vegetation period and the climate conditions. In contrast to some horticultural crops which can be cultivated entirely under LEDs without sunlight, forest seedlings need to be transplanted outdoors in the nursery at a very young age before being outplanted in the field. Juvenile plants are less efficient using absorbed light and dissipating excess energy making them prone to photoinhibition at conditions that usually do not harm mature plants. The outdoor transfer can cause stress in the seedlings due to high sunlight intensity and exposure to ultraviolet (UV) radiation not typically present in the spectra of LED lamps. This study tested possible treatments for mitigating light shock stress in seedlings of Picea abies (L.) Karst. and Pinus sylvestris L. transplanted from indoor cultivation under LEDs to outdoor sunlight exposure. Three sowings were carried out in 2014 (May and June) and 2015 (May) cultivating the seedlings during five weeks under LED lights only. Afterwards, higher light intensity or UV radiation treatments were applied during one week in order to adapt the seedlings to natural outdoor conditions. After transplanting a transition phase was introduced using shading cloths for one or three weeks as outdoor treatments for light shock mitigation. Chlorophyll fluorescence (ChlF) levels and CO2 assimilation rates were measured before transplanting and followed outdoors during 5 weeks. The ChlF results revealed stress symptoms in the photoreceptors during the first days after transplanting. After five weeks outdoors the ChlF levels had recovered and the light saturation points had shifted, allowing higher CO2 assimilation rates. By the end of the vegetation period the morphological attributes showed no major differences between treatments.

Highlights

Cultivation of forest seedlings under light emitting diodes (LEDs) followed by transplanting to outdoor conditions is a technology receiving increasingly more attention [1,2,3,4,5,6]
The results of this study suggest that the seedlings of both species managed to cope with the light shock stress and adapt to the outdoor conditions, additional studies within this field are required and of great importance
The applied treatments did not significantly improve the adaptation of the tree seedlings cultivated only under LED lights when transplanted to outdoor conditions

Summary

Introduction

Cultivation of forest seedlings under light emitting diodes (LEDs) followed by transplanting to outdoor conditions is a technology receiving increasingly more attention [1,2,3,4,5,6]. In horticulture this technology has been used during many years for producing for example flowers and vegetables [11,12,13]. The concept is often based on the assumption that further growth after transplanting to a larger container is conducted under artificial light without exposing the plants to intense direct sunlight. Only a part of the seedlings cultivated under artificial lights can be directly transferred outdoors when the weather conditions are favorable. The rest has to be cold stored, usually in carboard boxes, until the transplanting window opens again [14,15,16]

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