1. Introduction
Agriculture is facing various crises that are worsening with time. Increasing food production to meet or feed the mounting population is a foremost challenge. This can be accomplished by further use of farm lands for an overall hike in food production or technically enhancing the yields from pre-existing lands by application of fertilizers or implementation of novel approaches; for instance, precision farming systems viz., cutting-edge irrigation arrangements, and ecologically accomplishable crop revolutions [
1]. Crop diseases decrease yield, resulting in a prominent crisis to food security, creating a global malnutrition spree affecting nearly 815 million people [
2]. Henceforward, natural fertilizers are well thought out as probable as well as safe alternatives to chemical fertilizers [
3]. Additionally, the presence of several horticultural important traits in the wild gene pool makes them suitable as potential breeding candidates for crop improvement [
4]. In this aspect, plant secondary metabolites are being emphasized for their disease regulator competences and combined in more than a few defense control programs [
5].
The marine ecosystem serves as a rich source of bioactive compounds, such as sulfated polysaccharides, terpenoids, phenolics, lactones, sterol and fatty acids, possessing pharmacological and plant growth-stimulating properties [
6]. Seaweeds form a key portion of these bioactive natural composites, with over 9000 species, known for their biostimulator potentials. Additionally, seaweed products as biostimulants that can enhance crop production are also being focused on. Biostimulants are materials supplementary to fertilizers, which endorse plant growth at lower concentrations [
7]. In addition, seaweeds are extensively applied in the fields of agriculture and horticulture to improve quality and quantity, and the results are promising [
8]. Innumerable seaweeds are being applied as liquid fertilizers to upsurge crop yields, as they are rich in macro-nutrients, besides trace elements essential for the development and enrichment of plants. Commercial seaweed products are also being marketed successfully [
9]. Besides being inexpensive, the seaweed extracts have surplus allelopathic chemicals that promote seed germination as well as emergence rates. Seaweed extracts are known to have a positive impact on prime stages of plant ontogenesis—starting from seed germination to seedling growth [
10]. Furthermore, seaweeds are reported with higher amounts of growth hormones, attributed to their plant biostimulant activities [
11].
Seed germination is a decisive procedure in plant growth, and the enrichment of germination potentialities of a seed can eventually enable a surge in crop yields, and is dependent on numerous chemical factors (soil moisture salinity, metal, mineral composition) [
12]. The emergence of seed is promoted by various methods for enhanced agricultural yields, like exposing them to biostimulants or growth promoting hormones by the process of seed priming. As the very first stage of plant growth, germination is defined as an outcrop of the radicle from the tissues enfolding the seed [
13]. As germination rates may vary among species, the analyses of germination rates might be directly proportional to the growth rates and consequently, their yields [
14,
15].
Cherry tomato,
Solanum lycopersicum (L.) var.
cerasiforme Mill. is a widespread, table purpose tomato variety, bearing bright red color, and small fruits. It is a probable ancestor of a cultivated tomato variety with small fruits bright red in color, resembling a cherry and tasting excellent [
16]. They are also favorable candidates in breeding programs for their genetic diversity, offering the selection of parental traits along with extensive geographic ranges. [
17]. With the debarring effects of crop growth promoting chemicals that alter soil ecology and have hazardous environmental and health impacts, researchers are concentrating on the allegation of naturally benign substitutes to increase yields, while offering effective crop protection. The favorable agronomic traits of cherry tomato (an intermediary genetic admixture flanked by wild currant-type tomatoes and domesticated garden tomatoes), such as higher nutrient composition, offer plans for balanced utilization to unravel indigenous complications encompassing crop adaptation to climatic variations or, to endorse functional food consumption [
18].
Seed priming of native seed species can evoke ecological restoration, stimulating the expression of dormant genes responsible for the expression of favorable agronomic traits. Since the sources of natural varieties are collected from the wild and their sources are limited, there is a persistent requirement for novel methods for seed-based restoration technologies. In this aspect, seed priming could be a strategy for sustainable seedling establishment, plant growth, and restoration of native seed. Hence, this research intended to discover the bio stimulator potentials of liquid seaweed extract of green alga, U. flexuosa, along with screening of their phytochemical and elemental composition on cherry tomato.
4. Discussion
The indiscriminate application of fertilizers has not only intoxicated the environment, but also lost their efficiency. Alternative naturally benign bases of fertilizers, sourced from biological sources such as plants, animals and micro-organisms, have paved the way for the practice of “organic farming”. Many eco-friendly bioactive compounds from seaweeds have been widely used in the agricultural field as plant growth promoters. Seaweeds are reported for their copious amounts of novel as well as assorted range of marine secondary metabolites [
30]. Global population growth has seen leaps and bounds in the recent years, posing food insecurity [
31]. With the foremost necessity of augmentation of crop production, farmers are in stress to improve yields of agriculturally important crops. As an imperative crop, the germination capability of tomato seed is valued to be around 70%. Seed emergence is mainly prejudiced by the equipoise, flanked by the growth skills of the embryo, in addition to the mechanical resistance of the endosperm, which should be debilitated for germination [
32]. Seaweeds are being sought out as potential enhancers of crop growth and yield and are replacing chemical fertilizers owing to higher efficiencies, broader action range, eco-friendly nature, and cost-effective feature. Seaweeds, reported with outstanding plant growth promoting potentials, increased plant height, root as well as shoot lengths, consequently, are designated as plant growth biostimulants, as reviewed by Khan et al. [
7] and Craigie [
33] As a crucial and initial plant growth activity, the evaluation of a seed’s germination and associated parameters can help in determining the rate of a crop success, in terms of yield and economy [
34]. As contemplation, the current investigation was performed to determine the plant growth stimulant activities of green seaweed
U. flexuosa (Chlorophyceae).
As the germination of a seed counts on various physical aspects, together with nutrient composition [
12], preliminary tests of the extracts were performed by analyzing the pH and electrical conductivity (EC) of the extracts. The nutrient content of a solution, in terms of salts and electrolyte concentration can be determined by measuring their EC. The EF-LSE of
U. flexuosa was found to possess a neutral pH and an optimum EC that indicates the presence of salts, for instance, boron, zinc, magnesium, calcium and other essential plant nutrients in a nutritive solution [
35]. Higher rates of EC of nutrient or fertilizer solutions are proven with the stimulation of favourable agronomic traits, such as increase in nutritional quality, colour gradient and quality of tomato fruits [
36,
37]. However, solutions outside the optimum EC had an inhibitory effect on plant growth activities [
38]. A nutrient solution within optimum EC was found optimal for the growth stimulation of lettuce in glasshouse conditions [
39]. Henceforth, the EF-LSEs were designated as ideal to be tested for biostimulant potential by means of seed priming.
Additional experiments were carried out to determine the phytochemical as well as the elemental composition of the EF-LSEs. A preliminary phytochemical screening of the EF-LSE was done, which exposed the existence of more than a few compounds, such as carbohydrates, protein, phenols, flavonoids, saponins, tannins, and coumarins. Carbohydrates from different seaweeds were found to act as growth promoters of several crops such as tomato, soybean, duckweed and mung bean [
40,
41,
42]. Proteins from seaweeds are recorded for their enhanced plant biostimulant activities in mung bean [
43], and cherry tomato plants [
44]. Proteins help plants to alleviate stress and increase their tolerance levels against abiotic stress like heat, cold, salt and even heavy metals [
45].
Seaweeds are rich in phenolic compounds with varied bioactive properties [
46,
47]. Rajauria et al. [
48] identified and characterized eight phenolic compounds from brown Irish seaweed
Himanthalia elongate, which exhibited strong antioxidant activities. Chanthini et al. [
6] correlated the levels of phenolic compound concentration with their antifungal potential.
U. flexuosa had a considerate amount of phenols (1 mg/g of dry weight). Farasat et al. [
49] detected higher phenolics as well as flavonoid levels from
U. flexuosa and other edible green seaweeds. Besides,
Ascophyllun nodosum extracts were able to increase the levels of phenols and flavonoids together, post application [
50]. Saponins showcase a wide array of biological activities that play a pivotal role in plant growth as well as defense [
51]. Coumarins also play a crucial part in plant development. These compounds have been proven with plant growth promotion capabilities alone and also in combination with phytohormones in faba bean [
52]. Besides, the coumarin compounds were able to stimulate seed germination and seedling growth of wheat and sorghum seeds at optimum concentrations [
53].
Tuhy et al. [
54] testified that plant biomass surged by treatment with seaweed-derived micronutrients. The composition and functioning, together with the yield of all the plants, are reliant on their chlorophyll contents [
55]. The chlorophyll content of
U. flexuosa ranged up to 0.9 mg/g, which is relatively high among several other green seaweeds. This was also in agreement with the results published by Rathod [
56]. The elemental composition performed revealed the presence of seven elements (O, Na, Mg, S, Cl, K and Ca) present on cell wall surface. The seaweed was 56.25%
w/v of oxygen and along with high phenol content may be regarded as excellent candidates of antioxidizing agents [
57]. Plant growth promoting elements such as sodium and potassium present in the cell wall of the seaweed makes them appropriate biofertilizers, besides a broad-spectrum of applications in the agricultural sector [
58]. Several other mineral compounds such as chlorine, magnesium and calcium that are critical plant micronutrients are extant in the cell surface.
Tomato seeds treated with EF-LSEs displayed a positive response with respect to early germination, mean germination time, germination percentage, energy as well as better seed vigour index. Seed priming treatments achieved with quite a few plant derivatives such as plant hormones have been operational in the enhancement of seed germination of
Angelica glauca, a threatened medicinal herb [
59] as well as endive and chicory [
60]. Seaweed extracts have been proven to show development enhancing properties on plant as well as seeds of various plants [
61,
62]. Also, priming seeds promoted early emergence of brinjal and tomato seeds compared with un-primed [
63].
The seeds primed with EF-LSEs of
U. flexuosa was analysed in different concentrations, comparing with the standard SFS solution and the control. The EF-LSE treated seeds displayed an increased germination percentage, exhibiting a lower mean germination time (MGT), taking only 2.2 days to emerge.
Codium tomentosum, a green seaweed extract-treated aubergine seeds displayed lower MGT [
64]. The potential of EF-LSEs to stimulate seed germination was reported long back in ornamental plants [
65], green Chilies and Turnip [
66]. Kavipriya et al. [
67] also reported that priming of green gram seed with different seaweed extracts such as
Ulva lactuca and
Caulerpa scalpelliformis induced faster seed germination. Rapid seed emergence was recorded by priming the red gram seeds with the extracts of
Sargassum myriocystum [
68]. Furthermore, the positive effects of seaweed on the germination of green [
69] and black gram [
70] were also noted.
The comparative increase in seed emergence is correlated with seed eminence that is appeared to be augmented by the treatment of EF-LSEs. In addition, Amabika and Sujatha [
68] proved that seed quality can be assessed by determining their seedling vigour index. Higher SVI of EF-LSE treated seeds implies an upliftment in seed quality. This was also evident from the results of other parameters of EF-LSE treated seeds, exhibiting higher seedling-plant height, radicle-root, as well as plumule-shoot lengths and dry-wet weight, in comparison with the control. Similar results of increased SVI from seed priming with EF-LSEs of
U. lactuca, U. reticulata, Padina pavonica,
S. johnstonii were correlated with that of increased seed germination and growth rates of brinjal and tomato, along with chilli [
63].
As the primary developmental plant growth phase, the radicle and plumule are of prominent importance to determine the foundation of a plant. Seeds with an eminent radicle and plumule grow hastily, besides having an amplified competence [
71]. Longer radicle lengths are also indicators of greater plant establishment efficiency. Seeds that produce shortened radicle-plumule might have issues in nutrient conduction to the embryo [
72]. The EF-LSE primed seeds exhibited higher lengths of radicle and plumule, which improved with higher concentrations. EF-LSEs of seaweeds,
S. wightii and U.
lactuca, were demonstrated in their latent seed germination, besides plant growth promotion capabilities [
73,
74]. Similar increase of radicle-plumule lengths of tomato seeds was observed with the EF-LSEs of
C. sertularioides and
S. liebmannii [
75].
The growth enhancement displayed by the EF-LSE primed seeds is owed to the occurrence of essential plant macro- and micro-nutrients, in addition to phytohormones. Di Filippo-Herrera et al. [
76] reported the biostimulant activity of red seaweeds (
Acanthophora spicifera,
Gelidium robustum, and
Gracilaria parvispora) and brown seaweeds (
Macrocystis pyrifera, Sargassum horridum and
Ecklonia arborea) primed on seeds of mung bean, which is primarily due to their nutritional and hormonal constituents. The fact that seaweeds stimulate plant growth has been documented by various researchers worldwide [
77,
78].
Similarly, EF-LSEs were earmarked for plant growth promotion by amplifying the growth of root and shoot of the tomato seeds, thereby displaying increased heights, compared to the control. Plant height was higher compared with seeds treated with SFS. Seaweed-treated seeds exhibiting higher root shoot lengths and ratio were previously reported in many studies [
74,
79,
80]. Unlike the plant assessment results, comparatively developed shoot lengths remained. This suggests that the distribution of photosynthates and other compounds that aid in plant growth has shifted towards the shoot or increased in the above ground area. This could pave the way for the increase of plant yield [
76].
Seed weight is considered an ecologically crucial character in plant progress, by way of influencing the establishment capacity of a seedling, as well as plant height and yield. This was proved by Wuff [
81], who found that the
Desmodium paniculatum seeds of higher biomass produced a high yield. In addition, the EF-LSE treated seeds displayed increased wet and dry weights equated with control, besides improving with an EF-LSE concentration. This was in agreement with the results published by Karthikeyan and Shanmugam [
82], who studied the effect of
Kappaphycus alvarezii extract on peanuts. Vijayakumar et al. [
83] also reported the increase in seed weight of
Capsicum annum by treatment with
Codium decorticatum EF-LSE. Increased seed weights produced plants with higher height, shoot mass, and yield [
84]. Seed weight increase might be due to the production and accumulation of storage oils and several proteins that might promote plant growth abilities [
85]. Seeds take in surrounding water, protoplasmic macromolecules by imbibition. The EF-LSEs are actively imbibed in the seed through capillary action, thus increasing biomass. Seeds with higher biomass have been reported to have better seedling growth. Sun et al. [
86]. reported that maize seeds with increased biomass had high yields.
The biomass of EF-LSE treated seeds increased with time, thus revealing that the higher imbibing rate occurs at later hours of priming. This is attributed to the active process of enzymatic breakdown and mitosis essential for emergence. The increased imbibing rate of seeds exposed to EF-LSEs is primarily due to the abundance of plant essential nutrients and phytohormone composition of the EF-LSEs.
U. lactuca and
P. gymnospora primed tomato seeds exhibited higher imbibition rates during later stages of priming and hence were reported as more successful and better candidates for developing effective biostimulants to improve the growth of tomato plants (Hernández-Herrera [
9]).
With respect to the biostimulant potentials of seaweed extract, the yield and quality parameters analyzed presented favorable results. The EF-LSE primed seeds observed an increase in yield, attributed to the presence of phyto-hormones and various plant growth promoting elements, as evident from the EDX analysis of the seaweed. Furthermore, the flavour of the cherry tomato fruit is directly proportional to the amount of TSS [
87]. The nutritional value of these tomatoes is based on ascorbic acid content, which was also enhanced by the EF-LSE. Lycopene content, which is the reason for the ripening of fruits at an optimum stage, was also enhanced by EF-LSE treatment. Phenolic compounds, an indication of plant innate defense system, were augmented on treatment with seaweed extracts. These results are in agreement with that of Murtic et al. [
88].
Since the cherry tomato gene pool, a wild relative of tomato, provides an opportunity to produce more nutritive and resilient tomato cultivar varieties, an attempt to preserve and conserve this inclusive gene pool in gene banks is critical [
89]. Additionally, these wild relative crop types are nutritional repositories, whose cultivation shall be enhanced to meet the increasing global food security targets. The establishment of these species can be further stimulated by seed priming techniques. The application of seaweed extracts as a seed priming agent towards the improvement of agronomic traits of cherry tomato have resulted in positive responses to amplified seed germination capabilities, germination energy, and augmented seedling establishment. In addition, seed priming effects have induced a long-lasting priming effect by altering plant and fruit physiology in a favorable way, by increasing their biochemical constituents and fruit yield. Hence, this study proves the potential of
U. flexuosa as a potential agricultural biostimulant that is both economic and effective.