LED pre-exposure shines a new light on drought tolerance complexity in lettuce (Lactuca sativa) and rocket (Eruca sativa)

Highlights

• Exposure to varying Red:Blue LED light ratios promoted morphological and physiological development of lettuce and rocket.

• LED treatments which promoted highest antioxidant activity and ABA concentration resulted in decreased drought tolerance.

• Increased stomatal number and aperture area due to LED pre-exposure correlated with drought sensitivity only in rocket.

• Photomorphogenic and drought tolerance responses were species-specific.

Abstract

We investigated the morphological and physiological bases of drought tolerance in rocket (Eruca sativa) and lettuce (Lactuca sativa) seedlings as induced by pre-exposure to different ratios of red (R) and blue (B) LED light. Seedlings were grown for 17–18 days under white (control), 100R:0B, 75R:25B, 50R:50B, 25R:75B, or 0R:100B LED lighting at 156 μmol/m²/s. All seedlings were moved under white fluorescent lights 17–18 days after sowing. Half the seedlings were then droughted for 7 days, followed by re-irrigation for 7 days, while the other seedlings remained fully irrigated. Exposure to combined red and blue wavelengths promoted morphological development (leaf area, number of open leaves, leaf specific weight), as well as pigment (chlorophyll, carotenoids) and secondary metabolite (flavonoid) biosynthesis in both species. Morphological and physiological characteristics were more enhanced by monochromatic red or blue light exposure in lettuce than in rocket. Rocket grown under combined R:B lighting, and lettuce grown under 100 % red or blue LED light suffered most from drought (in the form of lower relative water content (RWC) and increased membrane leakage), despite increased ABA concentrations and antioxidant activity at mid-drought. In lettuce, but not in rocket, there was a positive residual effect of LED pre-treatment on fresh weight of droughted and re-irrigated plants. Increased concentrations of antioxidant metabolites did not correlate with tolerance to drought. All combined R:B light treatments increased stomatal density in both rocket and lettuce. All LED regimes which resulted in increased stomatal aperture area per unit leaf area (AALA) (μm2 aperture/cm² leaf) resulted in decreased RWC at the end of drought. Stomatal AALA was negatively correlated with end-drought RWC (r = -0.89, p = 0.02) in rocket, but not in lettuce. The relationship between stomatal development and antioxidant activity on drought tolerance appears to be species-specific. The benefits provided to two leafy green species when grown under LED lights must be balanced against the resulting vulnerability to drought.

Introduction

The complex mechanisms by which plants tolerate or respond to drought involve chemical signaling and gene expression at the molecular level and altered morphology and physiology at the organism level. Drought, defined as the lack of sufficient water in the plant root zone to sustain maximum growth and productivity (Deikman et al., 2012), is one of the leading contributors to global crop loss (Lesk et al., 2016). Plant tolerance to drought can result from genetic adaptation and/or to horticultural practice and is expressed both physiologically and morphologically through altered gene expression, modified hormonal levels, metabolite biosynthesis and antioxidant activity (Zhang et al., 2014). Seeds, seedlings, and mature plants can be treated with compounds to induce protection against drought (Ashraf et al., 2011), and the environment itself (i.e. light quantity and quality or soil structure and chemistry) can be modified to provide protection (Kammann et al., 2011).

Common physiological adaptations to improve water use efficiency upon drought stress perception include reducing leaf area growth and increasing root growth, modifying cellular relative water content via osmotic adjustment, and abscisic acid-induced stomata closure (Bartels and Sunkar, 2005; Taiz and Zeiger, 2006). An equilibrium exists between reactive oxygen species (ROS) generation, which occurs even under healthy conditions, and ROS scavenging. Biotic or abiotic stresses disrupt this equilibrium and result in damaging levels of ROS. ROS concentrations increase under drought and become most toxic in organelles such as mitochondria and chloroplasts which facilitate a high rate of electron flow (Gill and Tuteja, 2010). A common drought stress response is therefore the induction of antioxidative enzymes and metabolites to neutralize the damaging effects of ROS (Bernstein et al., 2010). The specific antioxidant compounds (enzymatic and non-enzymatic) upregulated during stress conditions, and the degree of their expression are influenced by the developmental state of the plant, the duration of the stress, and subcellular localization of ROS accumulation (Gill and Tuteja, 2010).

Drier and hotter climate conditions are predicted in the coming decades, so it is crucial to improve water use efficiency with techniques that confer drought tolerance on current crop varieties, in addition to breeding more stress-resistant varieties. Developing new water-saving cultivation strategies, such as controlled-environment agriculture, can also reduce overall water use to meet current and future crop demands (Taulavuori et al., 2017; Bantis et al., 2018).

Blue (400−500 nm) and red (600−700 nm) light wavelengths, which are selectively absorbed by photosynthetic pigments, influence the development and efficiency of photosynthetic machinery, as well as phytochemical synthesis and plant architecture (Chen and Yang, 2018). Specialized photoreceptors for red- (phytochromes; Mathews, 2006) and blue-light (cryptochromes, phototropins, zeaxanthin; Ahmad et al., 2002; Valverde et al., 2004) regulate specific responses to the nature of incident light either separately or in a co-regulatory manner (Fraikin et al., 2013; Christie, 2007).

Expression of both red and blue photoreceptors above a critical threshold is needed for proper activation of specific morphological and physiological responses to light, such as inhibition of hypocotyl elongation (Hernández and Kubota, 2016), or enhancement of flavonoid biosynthesis via chalcone synthase (CHS) (Wade et al., 2001), a phenomenon known as photoreceptor “co-action” (Mohr, 1994). Flavonoids are involved in the absorption of high-energy wavelengths, ROS scavenging, and developmental regulation in the form of phytohormone transport and protein kinase activation (Brunetti et al., 2018). Guard cells, which open and close stomata, are particularly sensitive to blue light (Frechijia et al., 1999). Optimal plant growth is therefore promoted by the combined exposure to blue and red light, as opposed to monochromatic light (Hogewoning et al., 2010; Hernández and Kubota, 2016).

Light-emitting diodes (LEDs) have become increasingly popular among agronomists and researchers because of their ability to efficiently modify plant growth and development by providing specific wavelengths of photosynthetically active radiation (PAR), especially in the blue and red ranges.

While not all of the spectrum produced by fluorescent/incandescent lighting is equally utilized by plants, use of blue-light LEDs can target processes leading to increases in biomass and in polyphenol, antioxidant, and carotenoid concentrations across various plant species (Johkan et al., 2010; Kopsell and Sams, 2013). Supplemental treatment with red LED light resulted in increased beta-carotene in pea (Wu et al., 2007), polyphenolics in lettuce (Samuolienė et al., 2012) and yield in tomato (Xu et al., 2012). Phytomedicinal properties can also be increased, as pigments upregulated by both blue and red light are associated with anti-carcinogenic activity and enhanced resistance to chronic disease (Kopsell et al., 2014).

Spectral effects on carotenoid biosynthesis are of considerable interest because of their antioxidant activity (Hannoufa and Hossain, 2012). In addition, the carotenoid zeaxanthin is a metabolic precursor of abscisic acid (ABA) (Nambara and Marion-Poll, 2005). ABA facilitates communication of stress from roots to shoots, controls stomatal closure to optimize water use efficiency, and regulates expression of late embryogenesis abundant (LEA) proteins – all of which protect cells from dehydration (Hanin et al., 2011; Vishwakarma et al., 2017). Red light promotes transcription of phytoene synthase (PSY), the enzyme which catalyzes the first committed step of the carotenoid biosynthesis pathway (Cazzonelli and Pogson, 2010) and blue-light-induced stomata opening is facilitated by and increases with zeaxanthin concentration (Frechijia et al., 1999; Taiz and Zeiger, 2006). Altering light quality by using LEDs could therefore enhance stress tolerance by increasing the production of ABA precursors and antioxidant compounds.

Rocket (Eruca sativa), also known as arugula, and lettuce (Lactuca sativa) leaves are popular in salads and are two of the crops most commonly grown indoors and under LEDs (Agrilyst, 2017). However, the artificial environments of indoor farms are susceptible to failures of support systems for temperature and light control, as well as for crop irrigation (Goddek et al., 2019). The effect of LED pre-exposure on drought tolerance has not received much attention (Ahmadi et al., 2019). We investigated the effect of exposure to different ratios of red and blue light on the growth efficiency, nutritional quality, and drought tolerance of the two leafy greens rocket and lettuce.