Morphological and Physiological Responses Induced by Protein Hydrolysate-Based Biostimulant and Nitrogen Rates in Greenhouse Spinach

Abstract

Plant-derived protein hydrolysates (PHs) are gaining prominence as biostimulants due to their potential to improve yield and nutritional quality even under suboptimal nutrient regimens. In this study, we investigated the effects of foliar application of a legume-derived PH (0 or 4 mL L−1) on greenhouse baby spinach (Spinacia oleracea L.) under four nitrogen (N) fertilization levels (0, 15, 30, or 45 kg ha−1) by evaluating morphological and colorimetric parameters, mineral composition, carbohydrates, proteins, and amino acids. The fresh yield in untreated and biostimulant-treated spinach plants increased in response to an increase in N fertilization from 1 up to 30 kg ha−1, reaching a plateau thereafter indicating the luxury consumption of N at 45 kg ha−1. Increasing N fertilization rate, independently of PH, lead to a significant increase of all amino acids with the exception of alanine, GABA, leucine, lysine, methionine, and ornithine but decreased the polyphenols content. Interestingly, the fresh yield at 0 and 15 kg ha−1 was clearly greater in PH-treated plants compared to untreated plants by 33.3% and 24.9%, respectively. This was associated with the presence in of amino acids and small peptides PH ‘Trainer®’, which act as signaling molecules eliciting auxin- and/or gibberellin-like activities on both leaves and roots and thus inducing a “nutrient acquisition response” that enhances nutrients acquisition and assimilation (high P, Ca, and Mg accumulation) as well as an increase in the photochemical efficiency and activity of photosystem II (higher SPAD index). Foliar applications of the commercial PH decreased the polyphenols content, but on the other hand strongly increased total amino acid content (+45%, +82%, and +59% at 0, 15, and 30 kg ha−1, respectively) but not at a 45-kg ha−1-rate. Overall, the use of PH could represent a sustainable tool for boosting yield and nitrogen use efficiency and coping with soil fertility problems under low input regimens.

Nitrogen Use and Uptake Efficiency and Crop Performance of Baby Spinach (Spinacia oleracea L.) and Lamb’s Lettuce (Valerianella locusta L.) Grown under Variable Sub-Optimal N Regimes Combined with Plant-Based Biostimulant Application

“…The plant-based biostimulant used in this test was Trainer ® , a legume-derived protein hydrolysate (containing free amino acids and signaling molecules such as small soluble peptides), for which previous researches have already demonstrated its ability to boost crops' resources use efficiency (RUE) [15,52]-especially N uptake and assimilation [39]-as well as productivity [6,32] and quality [53,54]. Our results highlighted the ability of LDPH to enhance yield of both baby spinach and lamb's lettuce (+24.6% and +13.5% for plant sprayed with Trainer ® compared to control plants, respectively), which is in line with Carillo et al's [35] findings on spinach, and Di Mola et al [42,43] on other two important leafy greens (lettuce and baby leaf rocket) cultivated under variable N regimes. The positive effects of the foliar application of LDPH, irrespective of the N fertilization treatments, were more pronounced in spinach than in lamb's lettuce, demonstrating a species-specific response [15,55], especially that the same commercial plant-based biostimulant was used.…”

“…Interestingly, in our study the marketable fresh yield of LDPH-treated spinach and lamb's lettuce grown under N50% was similar to those grown under N100% (especially the non-treated plants). A number of biochemical and physiological aspects may have contributed to this result, including (i) a higher chlorophyll content (a, b and total) and SPAD index in biostimulant-treated than in non-treated plants, and (ii) improved leaf status in terms of nitrate content, triggering a more efficient translocation of assimilates to potential photosynthetic sinks, thus boosting plant growth and yield [35,42,43]. Moreover, several authors attributed the stimulation action and the increased N assimilation in response to LDPH application to multiple mechanisms of action involving (i) the hormones-like activities (i.e., auxin and giberrellins-like activities), (ii) the increase in the activity of the key enzymes glutamine synthetase and nitrate reductase, and (iii) the upregulation of specific genes responsible in N assimilation and pigment synthesis [27,33,[56][57][58].…”

Appraisal of Combined Applications of Trichoderma virens and a Biopolymer-Based Biostimulant on Lettuce Agronomical, Physiological, and Qualitative Properties under Variable N Regimes

“…The product has a density of 1.21 kg L −1 , a pH (1:5) of 4.7, an electrical conductivity; EC (1:5) of 20 mS cm −1 , 25 g kg −1 of organic N as peptides and free amino acids, 160 g kg −1 of organic C, lignosulphonates, and micronutrients, such as iron, manganese, zinc, copper, and molybdenum, in the following concentrations 10.0, 7.0, 3.0, 1.0, and 0.2 g kg −1 , respectively [46]. Peptides and free amino acids were obtained through enzymatic hydrolysis of a vegetal source of proteins, as reported by Carillo et al [7]. The peptides in the product have a high biological activity being signaling molecules (e.g., lateral root promoting peptides-LRPP).…”

“…The intensification in agricultural production appears to be the only useful strategy to meet the rapidly growing food demand in the future, although this imposes stress to the agroecosystem [1], presents serious problems to the ecosystem and health [2][3][4], since it requires high-input resource cropping systems (such as greenhouse horticulture), that are not ecologically sustainable [5]. In actual fact, greenhouse farming systems use the highest amount of synthetic nitrogen (N) fertilizers per unit area of cultivated produce than any other cropping system [6][7][8][9].…”

“…However, to date, the efforts to reduce N fertilizer use while at the same time attempting to increase NUE have been proven ineffective. This can be attributed to the inability of crop plants to adapt to low N availability conditions, which limit the activation of the physiological processes necessary for increasing crop production [7,15].…”