Stimulation effects of foliar applied glycine and glutamine amino acids on lettuce growth. -

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Stimulation effects of foliar applied glycine and glutamine amino acids on lettuce growth.

In this study, the growth and nutrient uptake of Romain lettuce (Lactuca sativa subvar Sahara) were evaluated under spray of glycine or glutamine at different concentrations of 0 (as control), 250, 500 and 1000 mg.L-1, as well as a treatment of 250 mg.L-1 glycine+250 mg.L-1 glutamine. The results showed that there was significant increase in leaf total chlorophyll content under Gly250+Glu250, Gly250 and Glu1000 mg.L-1treatments, and in leaf carotenoids content under 250 mg.L-1 glutamine spray compared with the control plants. Shoot fresh and dry weights were highest under 500 mg.L-1 glycine, whereas root fresh weight was highest under 250 mg.L-1 glycine spray. Foliar application of glycine and glutamine had no significant increase in leaf mineral concentrations except for iron, in which 1000 mg.L-1Gly spray resulted in significantly higher leaf Fe concentration compared with the control plants. Leaf vitamin C was increased at 250 and particularly 500 mg.L-1 spray of glycine and glutamine compared with the control. The results indicate that foliar application of glycine and glutamine amino acids can have beneficial effects on lettuce growth, as higher fresh yield, leaf chlorophyll content and vitamin C were obtained by low to moderate concentrations of glycine and/or glutamine amino acids.

Material and Methods:

Black plastic pots with a volume of 5 L were filled with the soil and then watered up to 80% soil field capacity. After three days, 10 seeds were sown to a depth of 1-2 cm in the potted soil.

After germination, plants were thinned to the three biggest plants per pot and after two weeks they were reduced to one plant per pot. The plants were irrigated (up to 80% soil field capacity) daily, in the afternoon. The greenhouse temperature was 25 ± 5°C, with a light intensity of 200 µmol m2 s-1and 70–75% air humidity. Different concentrations of glycine and glutamine amino acids including 0, 250, 500 and 1000 mg.L-1, as well as a treatment of 250 mg.L-1 glycine+250 mg.L-1 glutamine were sprayed on the lettuce plants. Pots were arranged in a completely randomized design with four replications, in which each pot represented a replicate. Different concentrations of glycine or glutamine amino acids (as treatments) were sprayed, 5 times on the plants, during a two month growth period. The first spray was done two weeks after seedling emergence, and the remaining sprays were done with one-week interval. Zero concentration of amino acids was considered as the control, and distilled water was sprayed on the control plants. Spray treatments were done in the morning, at one hour after sunrise, with a portable sprayer, by which the upper and lower surfaces of leaves were sprayed. A total amount of 90-100 mL of solution (by 5 sprays of each treatment) was applied on a single plant. At harvest, plant height was measured using a tape and SPAD  value of outer lettuce leaves was measured using a portable SPAD meter  with an average of 30 readings per plant, on middle part of leaves. Plants were cut at soil surface and roots were carefully separated from the soil particles by washing with tap water and then drying with tissue paper. The shoot and root fresh weights were measured using a digital scale. The shoot and root materials were then dried in an oven at 65°C for 48 h, and thereafter the plant’s dry weight was measured using a digital scale. Leaf chlorophyll (a, b and total) and carotenoids were determined using acetone extraction of 0.5 g of fresh leaf tissues. For determination of leaf vitamin C, 5 g of fresh leaves was crushed in a porcelain mortar in 10 mL of metaphosphoric acid (6%) and centrifuged at 4000 rpm for ten min. Five mL of the supernatant was transferred into an Erlenmeyer flask, and received 20 mL of metaphosphoric acid (3%). The titration of the extract was done by di-chlorophenol indophenols until the appearance of a rosa colour. The amount of vitamin C (mg 100-1 g FW) was calculated accordingly and based on a standard curve of L-ascorbic acid concentrations of 0, 10, 20 and 40 mg.L-1. Nitrogen (N) concentration of leaves was determined by the Kjeldahl method, potassium (K) using flame photometry, and magnesium (Mg), calcium (Ca), Iron (Fe) and zinc (Zn) were determined using atomic absorption spectrophotometer.

Results and Discussion

Comparison of means showed that application of different concentrations of glycine and glutamine amino acids tended to increase the total chlorophyll concentration of leaves; however, foliar application of glutamine at 1000 mg.L-1, and glycine at 250, 500 and 1000 mg.L-1, as well as Gly250+Glu250 treatment resulted in significantly higher total chlorophyll of leaves compared with the control plants (Table 3). Leaf carotenoids concentration was increased by foliar application of 250 mg.L-1 glutamine compared with the control plants, and there was no difference among other treatments.

Determination of leaf nutrient concentrations showed that only leaf Fe concentration was significantly affected by amino acids treatments. Leaf Fe was significantly increased under foliar application of 1000 mg L-1 glycine, whereas the increase in leaf Fe by other amino acid treatments was not statistically significant compared with the control plants (Table 3). Application of amino acids, via the root system or foliar feeding, has been known to improve the uptake and concentrations of leaf nutrients

in the present study, although the foliar application of amino acids showed no significant effects on other leaf nutrient concentrations, there was an increasing trend in most nutrients, under glycine or glutamine sprays (data not shown). Lettuce shoot fresh and dry weights were increased by foliar applications of glycine and glutamine; however, plants treated with 500 mg.L-1 glycine or Gly250+Glu250 had significantly higher shoot fresh weight compared with the control plants (Figure 1). Similarly, shoot dry weight was significantly improved with 500 and 1000 mg.L-1of glycine, as well as in Gly250+Glu250 treatment, compared

with the control plants (Figure 1). Better uptake and metabolism of nutrient elements can also play an important role in this regard, despite the leaf nutrient concentrations (except Fe) being non-significantly compared with the control plants. In celery, foliar application of a mix of amino acids, in concentrations of 500 and 700 ppm, increased plant height, shoot fresh and dry weights, leaf N concentration, leaf yield and leaf soluble carbohydrates (Shehata et al. 2011). Plant overall root length (Figure 2) was not changed by foliar application of different concentrations of glycine or glutamine compared with the control plants; however, the longest plant root was in plants sprayed with the Gly250+Glu250 treatment, which showed significantly longer roots compared with those plants that were treated with 1000 or 500 mg.L-1 glutamine. Root fresh weight (Figure 3) was increased only in plants treated with foliar application of 250 mg.L-1 glycine compared with the control. Plants sprayed with 250 mg.L-1of glutamine or Gly250+Glu250 showed higher root fresh weight

compared with those plants that were sprayed with 500 mg.L-1of glutamineDetermination of root length and root fresh weight revealed that foliar application of 500 and 1000 mg.L-1of glutamine was less effective than its 250 mg.L-1 or when 250 mg.L-1of glutamine was used with 250 mg.L1 of glycine. Reduced nitrate uptake due to the application of amino acids may induce restricted root growth. In the present study, the most pronounced effect of foliar application of glycine and glutamine amino acids was the leaf vitamin C concentration (Figure 4). Foliar application of glycine and glutamine, at concentrations of 250 and 500 mg.L-1, significantly increased leaf vitamin C content compared with the control plants. Nevertheless, the highest concentration of leaf vitamin C was in those plants treated with 500 mg.L-1of glycine or glutamine, and the lowest concentration was in the control plants and in those plants that were treated with 1000 mg.L-1of glycine (Figure 4).

L-ascorbic acid (or vitamin C) is one of the most important quality factors in leafy vegetable crops. This vitamin is a water-soluble compound and its content represents the overall quality of leafy vegetable products, as there is always a good correlation between its content and product quality. It is well known that the amount of vitamin C, as a secondary metabolite in agricultural products, significantly decreases with restricted growth conditions.

In the present study, promotion and optimization of vitamin C biosynthesis, as well as a reduction in its inactivation/degradation, are probably the mechanisms involved in its higher content under foliar application of glycine and glutamine amino acids. Enhanced chlorophyll concentration and photosynthesis are highly correlated to vitamin C content of plant tissues (Souri and Hatamian 2019), and such effects were also observed in the present study. It seems that the growth stimulation of these amino acids is concentration dependent. Such effects were also obtained by Fahimi et al. (2016) who showed in cucumber plants, that foliar application of Biomin (an amino acidchelated macro-micro-nutrients), at 1000 ppm, resulted in severe leaf chlorosis and necrosis symptoms, but foliar application of 500 ppm showed beneficial effects. Similarly, application of glycine at concentration of 40 mg.L-1 in nutrient solution reduced many growth characteristics of coriander plants, whereas lower concentrations of 5 and 10 mg.L-1 increased plant growth (Mohammadipour and Souri 2019a). The bio-stimulating effect of amino acids on plant growth has been a matter of interest in various studies, and their protection against stress could be mainly by this mechanism. Foliar application of putrescine (in concentration of 25, 50 and 100 mg L-1) and glutamine (at concentrations of 50, 100 and 200 mg.L-1), alone or in combination, on onion (Allium cepa L. ‘Giza 20’) plants, significantly increased plant growth characteristics, including bulb yield and quality (Amin et al. 2011). Similarly, in garlic, three foliar applications of a commercial product of “Amino Total”, containing a range of amino acids, significantly increased plant height, number of leaves per plant and marketable bulb yield (Shalaby and El-Ramady 2014).

Application of different concentrations of glycine (0, 5, 10, 20 or 40 mg.L-1) to coriander plants (Coriandrum sativum L.), via Hoagland nutrient solution, showed that leaf SPAD value, fresh and dry weights of shoots and roots were significantly increased by 10 mgL-1 glycine compared with the control plants. study showed that foliar application of moderate concentrations of glycine and/or glutamine can enhance lettuce growth, particularly leaf greenness, fresh weight and leaf vitamin C content. This achievement can highlight a no fertilizer approach to the production of leafy vegetable crops, which are healthy for people and society. This in turn can avoid many damages and negative effects of chemical fertilizers on soil physiochemical characteristics and its fertility.

Conclusion:

In the present study, foliar application of glycine or glutamine in some concentrations showed stimulating effects on lettuce growth. The overall response of lettuce to foliar application of glycine was better than those of glutamine for many traits. The beneficial effects of these amino acids on root growth were in low concentration of 250 mg.L-1 treatments. Beneficial effects of glycine or glutamine spray on leaf mineral content was also observed; however, only leaf Fe was significantly increased by 1000 mg.L-1 glycine spray compared with the control. Improvement in root and shoot growth, and leaf pigmentation and quality (vitamin C content) were mainly observed in low to moderate levels of these two amino acids. The results indicate that the application of these amino acids, as growth stimulants, has practical implications in the production of leafy vegetable crops such as lettuce and that their safe production is very important.

Citation:

Noroozlo, Y.A., Souri, M.K. and Delshad, M., 2019. Stimulation effects of foliar applied glycine and glutamine amino acids on lettuce growth. Open Agriculture, 4(1), pp.164-172.