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Article

Seed Pre-Soaking with Melatonin Improves Wheat Yield by Delaying Leaf Senescence and Promoting Root Development

by
Jun Ye
1,2,3,†,
Wenjia Yang
1,4,†,
Yulin Li
1,5,
Shiwen Wang
1,2,4,*,
Lina Yin
1,2,4 and
Xiping Deng
1,2,5
1
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Northwest A&F University, Yangling 712100, China
2
Institute of Soil and Water Conservation, Chinese Academy of Science and Ministry of Water Resources, Yangling 712100, China
3
Inner Mongolia Academy of Agricultural & Animal Husbandry Sciences, Zhaojun Road No.22, Hohhot 010031, China
4
College of Natural Resources and Environment, Northwest A&F University, Yangling 712100, China
5
College of Life Sciences, Northwest A&F University, Yangling 712100, China
*
Author to whom correspondence should be addressed.
†
These authors are equal contribution to this study.
Agronomy 2020, 10(1), 84; https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy10010084
Submission received: 12 December 2019 / Revised: 23 December 2019 / Accepted: 27 December 2019 / Published: 7 January 2020
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Abstract

:
The effects of exogenous application of melatonin (MEL) on promoting plant growth and alleviating environmental stresses are already known, but the potential value in crop production is still poorly understood. In this study, the effects of seed pre-soaking with MEL on winter wheat (Triticum aestivum L.) growth and yield were investigated in a continuous two-year pot experiment and another year of field experimentation. Results showed that seed pre-soaking with different concentrations of MEL (10, 100 and 500 μM) for 24 h increased grain yields per plant from 29% to 80% in pot experiment and increased grain yield per area from 4–19% in field experiment, compared with the controls. Further analysis showed that the beneficial effects of MEL on improving wheat grain yield can be ascribed to: (1) increased spike number by enhancing tiller number; (2) enhanced carbon assimilation capacity by maintaining large leaf area, high photosynthetic rate and delaying leaf senescence; (3) promoted growth in root system. The result of this study suggests that MEL could be considered as an effective plant growth regulator for improving grain production in winter wheat.

Graphical Abstract

1. Introduction

Food security is one of the major policy concerns in the world because of its large population that needs to be fed, while arable land resources are limited and decreasing. That is also the case in many developing countries where food shortage is still a critical problem [1]. Therefore, any strategies for enhancing the production of these basic foodstuffs would have massive ramifications. Plant growth regulators are widely used in modern agricultural production, and their application plays an important role in increasing and securing crop yield [2]. Therefore, finding new plant growth regulators are an effective approach to improving crop yield in agriculture production [3].
Melatonin (N-acetyl-5-methoxytryptamine, MEL) is ubiquitous in living organisms, from bacteria to mammals [4]. Numerous publications have reported the presence of MEL in plants and plant products with a wide range of concentrations from pictograms to micrograms per gram of tissue [5,6,7]. The primary function of MEL in plants is to serve as the first line of defense against internal and environmental oxidative stressors, and exogenous application of MEL showed significant effects in enhancing tolerance to environmental stresses, such as heavy metals, drought, salinity, ultraviolet radiation, chilling, heat, pathogens and herbicides [8,9,10,11]. In addition, MEL has also been reported to play important roles in regulating plant growth and development, such as rooting, flowering and ripening [5,12,13,14,15]. Plants could absorb the exogenous MEL from roots, cotyledons, leaves and seeds in the case of various plant species [5,16]. Therefore, the potential usage of MEL in agriculture production has interested people in recent years [10,12,17].
Considering its beneficial effects on plant stress tolerance, the potential value as a plant growth regulator and usage in agriculture have been investigated in different studies. Discoveries to date indicate that the beneficial effects of MEL in promoting crop production include: (i) increasing the plant germination rate [16]; (ii) enhancing the capacities of plants to resist a variety of environmental stresses [12,18,19]; (iii) maintaining the chlorophyll content, promoting photosynthetic rate and delaying leaf senescence [10,20]; and (iv) promoting the development of the root system, which could lead to a greater absorptive capacity of nutrients from the soil [5,21].
In addition to the confirmed function on alleviating plant stress tolerance, the effect of exogenous applied MEL on improving the crop yield has also been confirmed in recently studies. In soybeans, seed coating with MEL significantly increased yield over 30% in a pot experiment [10]. In field experiments, the yields of corn, cucumber and mung bean primed with MEL were about 10–25% greater in comparison to those primed without MEL [7,16,17]. MEL can also increase the yield of vegetable plants. Tomato plants that grew from seeds that had been soaked with MEL prior to germination had much higher yields than that without MEL soaked [22]. In addition to improving the yield by exogenous application of MEL, the yield can also be modified by endogenous MEL contents; overexpression of rice serotonin N-acetyltransferase in rice plants confers increased yield with high endogenous MEL contents. On the contrary, low MEL production by suppression of either serotonin N-acetyltransferase or N-acetylserotonin methyltransferase in rice caused seedling growth retardation with yield penalty [23,24].
Previous studies have shown that MEL can improve crop yield, but its specific performance and the underlying mechanism on crop grain yield are still poorly understood. Firstly, the effects of exogenous application of MEL on crop grain yield have just been investigated in a few plant species, and mostly staple crops were not involved. Secondly, the mechanisms of MEL on improving crop yield are yet unclear. Therefore, the objective of this study was to investigate the performance and the underlying mechanisms of exogenous application of MEL on grain yield in winter wheat (Triticum aestivum L.), which is a crucial staple crop for humans [25].

2. Materials and Methods

2.1. Experiment 1: Effects of Seed Pre-Soaking with Melatonin (MEL) on Plant Growth and Grain Yield in the Pot Experiment

2.1.1. Plant Materials, Growth Conditions and MEL Treatments

The pot experiments were conducted during 2013–2015 year at Institute of Soil and Water Conservation, Chinese Academy of Sciences (34°16′56.24″ N, 108°4′27.95″ E; 460 m above sea level), Shaanxi Province, China. Seeds of wheat cultivar “Xinong 9871” were sown in pots containing 15 kg of air-dried brown soil (30 cm in diameter and 33 cm in depth). As base fertilizers, N/P(P2O5)/K(K2O) were presented at concentrations of 0.22, 0.15 and 0.05 g kg−1 dried soil, respectively. Before sowing, wheat seeds were surface-sterilized with 1% (v/v) sodium hypochlorite solution for 2 min and thoroughly rinsed with distilled water. Then, they were soaked in different concentrations of MEL solution or water (as control) for 24 h under dark condition. Four treatments were applied: (1) Control (CK), (2) 10 μmol/L MEL solution (MEL 10), (3) 100 μmol/L MEL solution (MEL 100) and (4) 500 μmol/L MEL solution (MEL 500). The MEL solutions were prepared as follows: 0.23g MEL was dissolved in 10 mL ethyl alcohol as a stock solution (100 mmol/L). The stock solution was diluted with distilled water to yield 10, 100 and 500 μmol/L. The treated seeds were sown on 25 October and harvested on 25 May. Fifteen seeds were sown per pot at a depth of 3 cm, and 6 uniform plants remained in each pot when the fourth leaf appeared. All pots were placed under a rain shed in the field. During the growth period, the soil water contents were maintained between the 75–95% of maximum pot capacity [26]. The average air temperature and sunshine hours in the wheat growing season during 2013–2016 (2013–2015: pot experiment; 2015–2016: field experiment) are shown in Figure 1.

2.1.2. Grain Yield and Dry Matter Accumulation

The grain yield (calculated based on per plant) in the pot experiment was investigated in two continuous seasons (2013–2014, 2014–2015). After maturity, the plants were harvested. Yield components, including spike number plant−1, grain number spike−1, thousand-grain weight, and harvest index, were investigated. Harvest index was calculated as: Harvest index = total grain weight (dry weight)/total aboveground biomass (dry weight). The growth (dry matter accumulation) was investigated at four growth stages (elongation, flowering, grain filling and physiological maturity) during 2014–2015 seasons. Plants were harvested and separated into shoot and root. The roots of each pot were carefully rinsed with water. All plant components were dried at 80 °C to constant weight and weighed, and the root/shoot ratio was calculated. The tiller number was investigated during the elongation stage and spike number was investigated when harvest was carried out.

2.1.3. Leaf Area and Photosynthetic Characters

Leaf area was measured at the beginning of grain-filling stage: leaf area = leaf length × leaf width × 0.835 [27]; Photosynthetic traits were measured in 10-day intervals from flowering to grain filling stage both for 2013–2014 and 2014–2015 growing seasons. The photosynthetic rate of flag leaf was measured between 09:00 and 11:00 by a portable photosynthesis system (LI-6400XT; LI-COR Biosciences, Lincoln, NE, USA). The chlorophyll concentration was determined through measuring the SPAD value by a SPAD meter (SPAD-502, Konica-Minolta, Tokyo, Japan). Chlorophyll fluorescence parameter was measured with a pulse amplitude modulated chlorophyll fluorescence system (Imaging PAM, Walz, Effeltrich, Germany). Maximal quantum yield of PSII photochemistry (Fv/Fm) was obtained using Imaging Win software (Version 2.40, Walz, Effeltrich, Germany).

2.2. Experiment 2: Effects of Seed Pre-Soaking with MEL on Wheat Yield in Field Experiment

In order to further confirm the effects of the MEL on improving wheat grain yield, a field experiment was conducted during the 2015–2016 growing season. The seed pre-soaking with MEL was the same as that in the pot experiment. The random complete block design was applied with three replications, and each plot was 5 × 10 m with row space of 0.15 m. The fertilizers were supplied with 180 Kg N ha−1, 140 kg P2O5 ha−1 and 75 Kg K2O ha−1 (recommended dosage in this area) before sowing. The sowing rate was 150 kg ha−1. Wheat seeds were sowed on 15 October 2015 and harvested on 25 May 2016. The grain yield was measured from an area of 4 m−2. The spike number ha−1, grain number per spike−1, thousand-grain weight and harvest index were also measured.

2.3. Statistical Analysis

The effects of treatment, year and the interactions between the treatments and years were calculated by combined ANOVA in SPSS 19.0 software (IBM Company, Chicago, IL). The comparisons among different treatments were made by Duncan’s multiple range tests. Statistical comparisons were significant when p < 0.05. Path coefficient analysis was carried out to partition the correlation coefficients of yield components into direct and indirect effects on grain yield [28].

3. Results

3.1. The Effects of MEL Application on Grain Yield in Pot Experiment

Seed pre-soaking with MEL significantly improved wheat grain yield (calculated based per plant) in both two growing seasons in the pot experiment (Table 1). The MEL treatment increased yield from 29% to 40% in the 2013–2014 growing season and from 45% to 79% in the 2014–2015 growing season, compared with control. In the two-year pot experiments, the highest yield was found at 100 μM MEL treatments. The analysis of variance for yield in the first two growing seasons showed that year and MEL treatment had significant effects on grain yield, whereas interaction effects between year and MEL treatment were not significant. Path coefficient analysis was conducted to describe the effects of yield components (Table 2). Spikes number per plant had the highest positive direct effect on yield (1.247), followed by grains number, spike−1 (1.056) and thousand-grain weight (0.414). The correlation coefficient between the grain yield and the spike number, grain yield and grain number spike−1, the grain yield and grain weight were 0.335, 0.37 and 0.28, respectively.

3.2. The Effects of MEL Application on Tiller Number and Dry Matter Accumulation During the Growth Process in the Pot Experiment

MEL treatment increased the tiller number during the elongation stage, and increased the spike number per plant regarding harvest in the pot experiment (Table 1). Tiller number plant−1 was increased by 25% and 56% under 100 μM MEL treatments in two growing seasons, and the spike number was increased by 17% and 60% under 100 μM MEL treatments in two growing seasons. In pot experiment, both shoot and root dry weights were enhanced in MEL-treated plants during growth stage (Figure 2). The higher dry matter accumulation was found at 100 μM MEL treatments. The shoot and root dry matter accumulations were increased by 57% and 62% during the mature period, respectively. The different extents of increase between shoot and root dry matters by MEL also lead to increased root/shoot ratio. Compared with the control plants, the ratio of root/shoot increased by 18% during the jointing stage, 20% during the flowering stage and 21% during the filling stage by 100 μM MEL treatments.

3.3. The Effects of MEL Application on Leaf Area and Photosynthetic Characters During the Grain Filling Stage in the Pot Experiment

There were significant differences in leaf area plant−1 among treatments (Figure 3). MEL-treated plants showed a tendency of increased leaf area under pot cultural conditions during both two growing seasons. The larger leaf area was found at under 100 μM MEL treatments, especially during the 2014–2015 season. The leaf area plant−1 under 100 μM MEL treatments was 32% and 53% larger compared with control plants during the flowering stage for 2013–2014 season and 2014–2015 season, respectively.
The photosynthetic rate of flag leaf begins to decrease after initial flowering, but the MEL-treated plants maintained higher photosynthetic rate compared with control plants (Figure 4). The higher photosynthetic rate was found in 100 μM MEL-treated plants. The photosynthetic rate in flag and the top third leaf under 100 μM MEL treatments were 25% and 42% higher than that of control plants after 40 days flowering in 2013–2014 season, and they were 19% and 36% higher than that of control plants in 2014–2015 season.
Leaf chlorophyll concentration (assessed by the value of SPAD) began to decrease after initial flowering, but it was maintained at higher levels in MEL-treated plants than in control plants both in flag and the top third leaf (Figure 5). For example, the leaf chlorophyll concentration in flag leaf of the control plants decreased from 53 at initial flowering to 38 after 40 days flowering during the 2013–2014 season; however, the leaf chlorophyll concentration in 100 µM MEL-treated plants decreased from 56 at the initial flowering to 44 after 40 days flowering. A similar result was found in top third leaf that the leaf chlorophyll concentration was maintained at higher levels in plants treated with 100 µM MEL than in control plants after flowering. The same tendency was shown during the 2014–2015 season. In general, the MEL treated plants maintained higher leaf chlorophyll concentrations than those of without MEL-treated plants during the flowering and grain filling stage, suggesting that the MEL treatment could enhance the capacity of photosynthesis and delay the leaf senescence.
Figure 6 showed the effects of MEL treatments on Fv/Fm in the flag and top third leaf during flowering and grain filing stages. There were no significant differences in Fv/Fm among the treatments after 0, 10 and 20 days of initial flowering during 2013–2014 season. The ratio of Fv/Fm significantly decreased after 30 and 40 days of initial flowering, but the MEL-treated plants maintained higher Fv/Fm than that of control plants both in flag and the top third leaf, especially under 100 µM MEL treatments. The same tendency was found under MEL treatment for both flag and the third leaf during the 2014–2015 season.

3.4. The Effects of MEL Application on Grain Yield in Field Experiment

In order to further confirm the effects of MEL application on wheat yield under field conditions, a field experiment was conducted during the 2015–2016 growing season. As shown in Table 3, grain yields of MEL-treated plants were significantly higher than those of the control. The grain yield increased by 4–19% compared with control in different concentrations of MEL application. The highest grain yield was shown at 500 μM MEL treatment. The further investigation showed that MEL-treatment increased the spike number, grain number per spike and grain weight, and increased the yield.

4. Discussion

Seed pre-soaking with MEL achieved high grain yield during both two growing seasons in the pot experiment. The effect of MEL application on wheat grain yield also showed a dosage effect. The high grain yield could be ascribed to the high spike number per plant, high grain number per spike and high grain weight (Table 1 and Table 2). In the pot experiment, MEL treatment increased the grain yield by 29–80%, compared with no MEL treatment. In the field experiment, MEL treatment increased the grain yield by 4–19%, compared with controls. Similar extents of yield increase have been reported in corn (20%) and mung bean (30%) by previous studies [7,10,17]. Taken together, these results show that there is a great value MEL has, for improving the wheat production.
In agriculture production, crop yield depends on both the dry matter accumulation during the whole season and the dry matter distribution (parting) when harvesting [29]. In this study, MEL-treated plants maintained the large dry matter accumulation; these results are supported by the observations that MEL promotes vegetative growth [30]. Dry matter distribution, presented by harvest index, was also enhanced by Mel treatment. The yield components analysis showed that MEL increased the yield by increasing both spike number and grain number (Table 1). Spike number depends on the tiller number and its survival after elongation [31]. In this study, the increased number of spikes could be ascribed to the MEL-treated induced increase in the tiller number, but not survival rate. And grain number is another reason that contributes to the increased yield in this study (Table 1).
Seeds of plants contain a large amount of MEL, and seeds’ endogenous MEL concentration could be significantly increased by exogenous MEL treatment [32,33]. In this study, MEL treatment increased the tilling number, which resulted in increased spike number. It has been reported that promoting shoot growth in rapidly developing tissues, particularly during germination and seedling development, is one of typical characteristics of MEL [20]. Increased photosynthetic rate could result in sufficient carbohydrate supplementation, which would be beneficial to tiller growth. Although how MEL regulates the tiller emergence is ambiguous now, the underlying mechanisms of MEL promoting seed germination and seedling growth are clearer. During seed germination, the endogenous indoleacetic acid (IAA) was found to be increased by exogenous MEL [16]. More evidence showed that there is significant crosstalk between MEL and other plant growth regulators, including cytokinin, salicylic acid, jasmonic acid, gibberellins (GA), abscisic acid (ABA) and ethylene [7,34,35]. As such, during the early stage of germination, MEL down-regulated the ABA biosynthesis gene and up-regulated the ABA catabolism gene; meanwhile, MEL up-regulated the GA biosynthesis gene, which resulted in a rapid decrease in ABA contents and increase in GA contents [36]. Our current study supports that MEL prompting the shoot growth during the germination and seedling growth stage, which could increase the spike number when harvesting. In addition to increasing the tiller number, increasing the grain number and grain weight could be other main factors that influence grain yield when harvesting. In wheat production, maintaining high photosynthetic rate during the flowing and grain filling stages contributes to enhanced grain number and grain weight [37]. In addition, in the later filling stage, delay the leaf senesces is prominent for achieving high yield [38]. In this study, the MEL treated plants maintained high photosynthetic rate and delayed the leaf senescence during grain filling stage. Although the function of MEL in enhancing the photosynthetic rate is largely unclear, the available evidence suggests that MEL is important not only in helping to combat the excess reactive oxygen species (ROS) produced in actively photosynthesizing tissues, but that MEL is involved in perception and response to different intensities and wavelengths of light [34,35]. In addition to directly enhanced photosynthesis rate, the facts that MEL application delays leaf senesces and protects the chlorophyll from degradation have also been widely reported in different studies, and one of the important reasons is that MEL enhances the plant antioxidant ability either directly or by activating the plant enzymatic or non-enzymatic antioxidant system, decreased the H2O2 induced cell death and protein degradation [20,39]. Recently, MEL was found to up-regulate ABA catabolic genes and down-regulate ABA biosynthetic genes, resulting in a rapid reduction in ABA, which was involved in MEL-regulated leaf senescence [36]. Under drought and heat stress, MEL suppressed leaf senescence through block ABA signal pathway and promote the cytokinin pathway [19,40]. Although in this study, there was not drought nor heat treatment, the plant will unavoidably suffer these stresses during the growing season in natural conditions. In addition to enhancing photoassimilate, photoassimilate partitioning also affects the grain yield. Although there are no specific studies on the effect of MEL on the photoassimilate partitioning, the exogenous MEL regulated the carbon assimilation and degradation were found at transcript level. Such sugar metabolism-related genes were altered by MEL and contributed the plant growth and yield in soybeans [10]. The increase in harvest index by MEL application was also found in this study, suggesting that MEL is involved in alteration of photoassimilate partitioning.
In addition to the directly effects of MEL on shoot growth, MEL regulating the root development has also been confirmed in early 2000s [41]. MEL has been found to promote root branching, particularly the adventitious root formation in several species [5]. In crop production, one beneficial function of MEL on improving crop production is to promote the development of strong root system, which could lead to a greater absorptive capacity of nutrients and water from the soil [5,21]. For cucumbers, MEL stimulated the root germination and vitality and increased the root/shoot ratio; therefore, MEL may have an effect on strengthening cucumber root growth [42]. In this study, MEL-treated plants maintained high root/shoot ratio during the jointing, flowing and filling stages (Figure 2). The high root/shoot ratio could contribute to achieving high yield by increasing the nutrients and water uptake, which could also be involved in regulating the wheat growth and yield. Recently, more evidence has supported that auxin participates in the MEL modulation of root growth [5,43,44]. Exogenously applied MEL stimulates root growth and raises IAA in root of etiolated seedling of Brassiea jucea (reported recently) [45]. In addition, Chen et al. [46] found that H2O2 acts downstream of MEL to induce lateral root formation.
In this study, MEL treatment increased the grain yield by 29–80% in pot experiment, but only by 4–19% in field experiment. Further analysis showed that the maximum spike number per plant was increased by 15% and 57% in continuous two years pot experiment, but maximum spike number per area just increased by 5.5% in field experiment. Spike number has a maximum contribution to increase the yield among three yield components that were analyzed in this study (Table 2). Therefore, a lesser increase in spike number in a field experiment than in a pot experiment may lead to lesser increase in wheat yield in field experiment. In the pot experiment, the increase in tiller number was not limited by spaces because of the big edge effect; it was limited in field experiment because of the population effect. Thus, when yield was presented base on per plant in pot, it could enlarge the effect of MEL on promoting the grain yield. In addition, although the alleviating stress tolerance was not considered in this study, the wheat could also suffer stresses during the growing season; the contribution of MEL through alleviating the stresses cannot be ignored. The difference in environment and stress during the two years could also lead to different effects of MEL on enhancing yield in two-year experiments (Table 1). Besides, the differences in climate in three years may also present other reasons for the inconsistency of the yield increase between the pot and filed studies.
Seed pre-soaking with MEL largely increased the grain yield of winter wheat both in the pot and field experiments. Wheat is the largest staple crop in the world, yielding more than 7.5 million tons per year [47]. Therefore, a small increase in wheat yield will result in a big effect on food security in the word. Based on our current study and previous research about the mechanism of the MEL on plant development [29,34,35,36,37,40,45], a potential mechanistic diagram of MEL on improving wheat grain yield is proposed for future research reference. (1) During the germination and seedling growth stages, the MEL could crosstalk with IAA and GA, increasing the tiller number and seedling growth, which leads to an increase in spike number. (2) During the flowering and grain filling stages, MEL may crosstalk with ABA and cytokinin, eliminate ROS and delay leaf senescence, and enhance carbon assimilation capacity, which leads to increased grain weight and grain number. (3) In the root, MEL may crosstalk with auxin and H2O2 and promote root/shoot ratio (Figure 7). It is worth noticing that in the current study, when the seeds were pre-soaked with MEL, the priming effect could last the whole growing season, which was finally reflected by improved yield, suggesting that MEL has great prospects in agricultural production through exogenous application or even cultivated the plant with high MEL content.

Author Contributions

Design of the two experiments, S.W. and L.Y.; provided guidance during the experimental process, X.D.; performed experiment 1, J.Y.; performed experiment 2 and collected data, W.Y. and Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key Technology R&D Program (2015BAD22B01), National Basic Research Program of China (2015CB150402), National Nature Science (51479189) and the 111 project of the Chinese Education Ministry (B12007).

Conflicts of Interest

Authors declare no conflict of interest.

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Figure 1. Monthly average air temperature and sunshine hours during the three growing seasons of the experimentation (October to June in 2013–2014, 2014–2015 and 2015–2016).
Figure 1. Monthly average air temperature and sunshine hours during the three growing seasons of the experimentation (October to June in 2013–2014, 2014–2015 and 2015–2016).
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Figure 2. The effects of MEL application on shoot dry weight (A), root dry weight (B) and root/shoot ratio (C) at different growth stages (jointing, flowering, grain filling and maturity stages). Four MEL concentrations were applied: 0 μM (CK), 10 μM (MEL 10), 100 μM (MEL 100) and 500 μM (MEL 500). Values are means ± SEs of twelve replicates. Significant differences between different treatments are indicated by different letters (p < 0.05).
Figure 2. The effects of MEL application on shoot dry weight (A), root dry weight (B) and root/shoot ratio (C) at different growth stages (jointing, flowering, grain filling and maturity stages). Four MEL concentrations were applied: 0 μM (CK), 10 μM (MEL 10), 100 μM (MEL 100) and 500 μM (MEL 500). Values are means ± SEs of twelve replicates. Significant differences between different treatments are indicated by different letters (p < 0.05).
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Figure 3. The effects of MEL application on leaf area at initial flowering stage in the 2013–2014 (A) and 2014–2015 (B) growing seasons. Values are means ± SEs of six replicates. Significant differences are indicated by different letters (p < 0.05).
Figure 3. The effects of MEL application on leaf area at initial flowering stage in the 2013–2014 (A) and 2014–2015 (B) growing seasons. Values are means ± SEs of six replicates. Significant differences are indicated by different letters (p < 0.05).
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Figure 4. The effects of MEL application on photosynthetic rate after 0, 10, 20, 30 and 40 days of initial flowering. (A,B) are the photosynthetic rate of flag leaves in the 2013–2014 and 2014–2015 growing seasons, and (C,D) are for the top third leaves in two growing seasons. Values are means ± SEs of six replicates. Significant differences between different treatments on the same day of the experimental period are indicated by different letters (p < 0.05).
Figure 4. The effects of MEL application on photosynthetic rate after 0, 10, 20, 30 and 40 days of initial flowering. (A,B) are the photosynthetic rate of flag leaves in the 2013–2014 and 2014–2015 growing seasons, and (C,D) are for the top third leaves in two growing seasons. Values are means ± SEs of six replicates. Significant differences between different treatments on the same day of the experimental period are indicated by different letters (p < 0.05).
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Figure 5. The effects of MEL application on chlorophyll content (SPAD units) after 0, 10, 20, 30 and 40 days of initial flowering. (A,B) are the chlorophyll contents of flag leaves in the 2013–2014 and 2014–2015 growing seasons, and (C,D) are for the top third leaves in two growing seasons. Values are means ± SEs of six replicates. Significant differences between different treatments on the same day of the experimental period are indicated by different letters (p < 0.05).
Figure 5. The effects of MEL application on chlorophyll content (SPAD units) after 0, 10, 20, 30 and 40 days of initial flowering. (A,B) are the chlorophyll contents of flag leaves in the 2013–2014 and 2014–2015 growing seasons, and (C,D) are for the top third leaves in two growing seasons. Values are means ± SEs of six replicates. Significant differences between different treatments on the same day of the experimental period are indicated by different letters (p < 0.05).
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Figure 6. The effects of MEL application on Fv/Fm ratio after 0, 10, 20, 30 and 40 days of initial flowering. (A,B) are the Fv/Fm ratio of flag leaves in the 2013–2014 and 2014–2015 growing seasons, and (C,D) are for the top third leaves in two growing seasons. Values are means ± SEs of four replicates. Significant differences between different treatments on the same day of the experimental period are indicated by different letters (p < 0.05).
Figure 6. The effects of MEL application on Fv/Fm ratio after 0, 10, 20, 30 and 40 days of initial flowering. (A,B) are the Fv/Fm ratio of flag leaves in the 2013–2014 and 2014–2015 growing seasons, and (C,D) are for the top third leaves in two growing seasons. Values are means ± SEs of four replicates. Significant differences between different treatments on the same day of the experimental period are indicated by different letters (p < 0.05).
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Figure 7. The potential mechanism diagram of MEL on improving wheat grain yield based on the current and previous studies. Three potential mechanisms are involved in MEL improving the wheat yield. (1) During the germination and seedling growth stages, the MEL could promote the auxin and gibberellins (GA) synthesis, increasing the tiller number and seedling growth, which leads to an increase in spike number. (2) During the flowering and grain filling stages, MEL may block abscisic acid (ABA) synthesis and promote cytokinin synthesis, eliminate reactive oxygen species (ROS) and delay leaf senescence and enhance carbon assimilation capacity, which leads to increased grain weight and grain number. (3) In the root, MEL may promote auxin accumulation and eliminate H2O2 and promote root/shoot ratio.
Figure 7. The potential mechanism diagram of MEL on improving wheat grain yield based on the current and previous studies. Three potential mechanisms are involved in MEL improving the wheat yield. (1) During the germination and seedling growth stages, the MEL could promote the auxin and gibberellins (GA) synthesis, increasing the tiller number and seedling growth, which leads to an increase in spike number. (2) During the flowering and grain filling stages, MEL may block abscisic acid (ABA) synthesis and promote cytokinin synthesis, eliminate reactive oxygen species (ROS) and delay leaf senescence and enhance carbon assimilation capacity, which leads to increased grain weight and grain number. (3) In the root, MEL may promote auxin accumulation and eliminate H2O2 and promote root/shoot ratio.
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Table
Table 1. The effects of MEL application on tiller number, spike number per plant, thousand-grain weight, grain number per plan, harvest index and grain yield per plant in pot experiment during 2013 to 2015 growing season.
Table 1. The effects of MEL application on tiller number, spike number per plant, thousand-grain weight, grain number per plan, harvest index and grain yield per plant in pot experiment during 2013 to 2015 growing season.
YearTreatmentTiller NumberSpike Number Per PlantThousand -Grain Weight (g)Number Grain Per SpikeHarvest IndexGrain Yield (g per plant −1)
2013–2014CK3.8 c2.7 b42.0 a46 b0.47 b5.0 b
MEL 104.3 b3.3 a39.6 a55 a0.55 a6.5 a
MEL 1004.8 a3.1 a41.3 a56 a0.54 a7.1 a
MEL 5004.5 ab3.1 a40.7 a54 a0.50 ab6.6 a
2014–2015CK4.9 c3.8 c35.9 a31 c0.44 b4.1 c
MEL 106.5 b5.2 b35.8 a32 b0.55 a6.0 b
MEL 1007.7 a6.0 a37.1 a33 a0.55 a7.5 a
MEL 5006.5 b5.3 b36.0 a32 b0.53 a6.2 b
Source of variationdf
Year117.46 **24.83 **9.42 **35.20 **24.16 **4.86 *
Treatment30.584.02 **2.80 *3.30 *0.7444.55 **
Year × Treatment34.05 **7.46 **1.424.09 **3.85 *2.65
Four MEL concentrations were applied: 0 μM (CK), 10 μM (MEL 10), 100 μM (MEL 100) and 500 μM (MEL 500). Values are means of fifteen replicates during 2013 to 2014 growing season and thirteen replicates during the 2014 to 2015 growing season. Significant differences between different treatments are indicated by different letters as a, b or c (p < 0.05). * and ** indicates significant differences at p < 0.05 and 0.01.
Table
Table 2. Direct and indirect effects on grain yield (GY) via thousand-grain weight (GW), grain number per spike (GNS) and spike number per plant (SNP) in a two-year pot experiment.
Table 2. Direct and indirect effects on grain yield (GY) via thousand-grain weight (GW), grain number per spike (GNS) and spike number per plant (SNP) in a two-year pot experiment.
CharactersDirect EffectIndirect Effect ViaTotal Correlation With GY
GWGNSSNP
GW0.414-0.436−0.5640.286
GNS1.0560.171-−0.8570.37
SNP1.247−0.187−0.725-0.335
Table
Table 3. The effects of MEL application on grain yield, spike number, grain number per plant, thousand-grain weight and harvest index in field experiment during 2015–2016 growing seasons.
Table 3. The effects of MEL application on grain yield, spike number, grain number per plant, thousand-grain weight and harvest index in field experiment during 2015–2016 growing seasons.
TreatmentGrain Yield (kg ha−1)Spike Number (10 4 ha−1)Grain Number Per SpikeThousand-Grain Weight (g)Harvest Index
CK5242 d457 c42.1 b47.0 b0.47 a
MEL 105453 cd461 bc43.2 a48.3 a0.49 a
MEL 1005843 b477 ab43.6 a46.5 bc0.48 a
MEL 5006243 a482 a43.7 a45.8 c0.48 a
Four MEL concentrations were applied: 0 μM (CK), 10 μM (MEL 10), 100 μM (MEL 100) and 500 μM (MEL 500). Values are means of three replicates. Significant differences between different treatments are indicated by different letters as a, b or c (p < 0.05).

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Ye, J.; Yang, W.; Li, Y.; Wang, S.; Yin, L.; Deng, X. Seed Pre-Soaking with Melatonin Improves Wheat Yield by Delaying Leaf Senescence and Promoting Root Development. Agronomy 2020, 10, 84. https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy10010084

AMA Style

Ye J, Yang W, Li Y, Wang S, Yin L, Deng X. Seed Pre-Soaking with Melatonin Improves Wheat Yield by Delaying Leaf Senescence and Promoting Root Development. Agronomy. 2020; 10(1):84. https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy10010084

Chicago/Turabian Style

Ye, Jun, Wenjia Yang, Yulin Li, Shiwen Wang, Lina Yin, and Xiping Deng. 2020. "Seed Pre-Soaking with Melatonin Improves Wheat Yield by Delaying Leaf Senescence and Promoting Root Development" Agronomy 10, no. 1: 84. https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy10010084

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