Chemical and Biological Technologies in Agriculture

Co-inoculation of maize with Herbaspirillum seropedicae and humic substances increases the sizes of plant-associated bacterial populations and enhances grain yields under laboratory and field conditions. Root exudation is a key mechanism in the regulation of plant-bacterial interactions in the rhizosphere; humic matter supplementation is known to change the exudation of H⁺ ions and organic acids from maize roots. Our starting premise was that H. seropedicae and humic acids would modify maize seedling exudation profiles. We postulated that a better understanding of these shifts in exudate profiles might be useful in improving the chemical environment to promote better performance of plant growth-promoting bacteria delivered as bioinoculants. Thus, root exudates of maize were collected and analyzed by gas chromatography-mass spectrometry (GC-MS) and proton nuclear magnetic resonance (¹H NMR).

Nitrogenous compounds, fatty acids, organic acids, steroids, and terpenoid derivatives were the main structural moieties found in root exudates. Significant changes in exudation patterns occurred 14 days after the initiation of experiments. Quantities of fatty acids, phenols, and organic acids exuded by seedlings treated with humic acids alone differed from the quantities exuded in other treatments. Seedlings treated with H. seropedicae or H. seropedicae in combination with humic acids exuded a diversity of nitrogenous compounds, most of which had heterocyclic structures. Twenty-one days after initiating the experiment, seedlings treated with H. seropedicae alone exuded elevated quantities of steroids and terpenoid derivatives related to precursors of gibberellic acids (kaurenoic acids).

Changes in root exudation profiles induced by our treatments became most marked 14 and 21 days after initiation of the experiment; on those days, we observed (i) increased fatty acid exudation from seedlings treated only with humic acids and (ii) increased exudations of nitrogenated compounds and terpenes from seedlings treated only with H. seropedicae. Improved knowledge on the effects of bacterial inoculants and supplementation with humates on plant exudate composition may contribute substantially to improved understanding of plant metabolic responses and lead to new approaches in the use of selected compounds as additives in bioinoculant formulations that will modulate the cross-talk between bacteria and plants, thereby improving crop yields.

In this research, the effects of exogenous application of certain biostimulants [amino acid (AA), humic acid (HA), fulvic acid (FA), and seaweed extract (SE)] on the fruit yield and quality, leaf mineral contents, and some critical physio-chemical characteristics of grapevine (Vitis vinifera L.) cv. ‘Yaghouti’ were investigated under well-watered (WW) and drought-stressed (DS) conditions.

Drought stress caused a remarkable reduction in the weight of 20 berries and fruit yield, and meanwhile a marked increase in the titratable acidity (TA) and total soluble solid (TSS) content of fruits. Application of biostimulants, especially SE, enhanced the weight of 20 berries, fruit yield, and TSS content, and decreased TA in fruits of DS vines. Although drought stress had a negative effect on the chlorophyll content of grapevine, this effect was alleviated by the application of biostimulants, especially SE. Moreover, drought stress made the accumulation of abscisic acid (ABA), proline, total phenol, and soluble carbohydrates, the level of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), as well as the activity of guaiacol peroxidase (GPX) and catalase (CAT) enzymes increased in leaves. Application of biostimulants, especially SE, further increased the accumulation of ABA, proline, total phenol, and soluble carbohydrates and the activity of the antioxidant enzymes, but reduced the level of MDA and H₂O₂ in DS vines. Under drought stress conditions, concentrations of N, P, and K increased, and concentrations of Fe and Zn decreased; however, DS grapevines treated with biostimulants and especially SE accumulated a higher level of these mineral nutrients than CON vines.

In sum, as evidenced by the study results, biostimulants have a high potential for promoting fruit yield and quality of grapevine in drought-prone regions.

Boron (B) is an essential micronutrient and necessary to plants for their proper growth. The concentration of B in soil and plant is greatly critical for the growth and productivity. Even at the low amount available in soils, B can be extremely toxic to plants especially in semi-arid and arid environments. In the present study, the effects of high B concentrations (2.5, 5, 10, 20 and 30 mg/kg) were elucidated on two important essential oil-bearing plants;Mentha arvensisandCymbopogon flexuosuswhich are aromatic and antimicrobial herbs having well-known medicinal values.

Application of different concentrations of B showed growth inhibitory effects on plant as evident by shoot and root lengths, fresh and dry weights of shoot in the studied plants. Treatments of B also reduced the total chlorophyll and carotenoid content, chlorophyll fluorescence and reduced the activities of carbonic anhydrase and nitrate reductase enzymes. Moreover, B stress considerably increased the proline content and lipid peroxide content as compared to control. The activities of antioxidant enzymes like catalase, peroxidase and superoxide dismutase were also significantly increased under B stress. The content of essential oil ofM. arvensisandC. flexuosusincreased at 2.5 mg/kg of B and decreased with further increase in concentrations of B.

The findings of present work suggest that increasing concentrations of B inhibited growth and photosynthetic pigments, increased oxidative damage and activities of antioxidant enzymes; however, a mild stress of B increased essential oil production inM. arvensisandC. flexuosusplants.

In the present study, different concentrations of lead (factor A; 0, 15, and 30 mg L ⁻¹) and cadmium (factor B; 0 and 5 mg L ⁻¹) were applied via irrigation water during 6 months to evaluate their effects on growth of European hackberry (Celtis australis L.) plants. The experiment was arranged in factorial with completely randomized design and four replications under greenhouse conditions.

Application of 5 mg L ⁻¹ Cd or the lead levels (15 and 30 mg L ⁻¹) significantly reduced new shoot growth, plant leaf area, SPAD value, leaf water conductance and leaf photosynthesis, whereas significant increase in number of chlorotic and necrotic leaves, leaf transpiration rate, leaf proline and soluble sugars was observed. Higher reduction in new shoot growth and leaf water conductance and higher increase in leaf soluble sugars and proline was observed over the interaction of 5 mg L ⁻¹ Cd and 30 mg L ⁻¹ Pb. The highest soluble sugars and proline were in Pb30Cd5 (a3b2) treatment. Leaf Pb or Cd concentration was increased following their treatment. Application of cadmium significantly reduced leaf Pb, and similarly leaf Cd was significantly reduced by application of lead at both levels compared to untreated plants.

The results showed that the hackberry growth was influenced by positive and negative interactions of Pb and Cd applied in irrigation water. The extent of growth reduction indicates that hackberry represents a relatively tolerant ornamental tree to high Pb and Cd levels.

The reliance on chemical inputs to support high yields is the Achilles’ heel of modern crop production. The soil organic matter management is as old as agriculture itself. Recently, the use of soluble humic substances as plant growth promoters has been brought to attention due to their effects on nutrient uptake and water use efficiency. Humic substances applied directly at low concentrations can trigger different molecular, biochemical, and physiological processes in plants. However, how humic substances exert this plethoric regulatory action remains unclear. The objective of this study was to evaluate changes in the transcription level of genes coding cell receptors, phosphatases, synthesis, and function of different plant hormones and transcription factors.

After seven days of humic acid treatment, we used RNAseq in maize root seedlings. The level of gene transcription was compared with control plants.

Plant kinase receptors and different phosphatases were regulated by humic acids. Likewise, genes related to plant hormones (auxin, gibberellin, ethylene, cytokinin, abscisic acid, brassinosteroids, jasmonic and salicylic acids) were transcript in differential levels in maize root seedlings as well as the expression of a hundred of transcription factors modifying the signal transduction pathway via alterations of the subsequent gene response.

We showed a general mechanism for simultaneously regulating the activity of several hormones where humic acids act as a key regulatory hub in plant responses integrating hormonal signalling and response pathways.