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Agriculture, a highly water-consuming sector, uses about 70 % of potable water for irrigation, which affects the availability of this scarce and limited resource. For decades, the biostimulant action of aqueous extracts of humic substances from cattle manure vermicompost has been demonstrated in numerous crops, and the protective effect against abiotic stress conditions has also been proven. Lettuce (Lactuca sativa L.) is a waterdemanding vegetable that responds to the stimulating effect of humic extracts. For these reasons, the aim of this work was to evaluate the effect of the application of a humic extract of vermicompost (HEV) on lettuce plants grown under low water supply. Experiments were conducted under controlled conditions, treating the plants with a combination of three dilutions of the obtained HEV and two water conditions (25 and 50 % of the maximum water holding capacity (MC), following a completely randomized experimental design. At 42 days after plant emergence, biological productivity indicators and biochemical-physiological indicators were evaluated. The humic extract of vermicompost presented advantageous physicochemical properties for use as a biostimulant. Plants sprayed with dilutions of the extract and under conditions of low water supply showed similar or higher values than the control with adequate water supply (MC 85 %). Increases in proline and leaf protein content were observed, but not in relative chlorophyll content. It is recommended to perform the total characterization of vermicompost extracts and to study the response of lettuce under field conditions. [ABSTRACT FROM AUTHOR]

Plants are frequently subjected to unfavorable situations for their optimal development and operation caused by alterations in the environment. This set of unfavorable situations is known as environmental stress. Biostimulants are substances that by their action can stimulate the plant growth, improve the absorption of nutrients and increase yields under environmental stress conditions regardless of whether they contain nutrients in their composition. There are several categories of specific biostimulants, among them protein hydrolysates, algae extracts, chitosan, humic and fulvic acids, mycorrhizal fungi and growth promoting bacteria. The objective of this review is to inform the biostimulating effect of the application of humic substances in plants under stress conditions. It also synthesizes aspects related to humic substances such as their structural characteristics and classification. Results are shown using humic substances as biostimulants where their potential to stimulate different metabolic and physiological processes in abiotic stress conditions is demonstrated. It concludes that humic substances present a complex structure, a multiplicity of functional groups and small molecules that interact by weak junctions, which makes them exhibit a great variety of beneficial functions among which is their potential to increase the yields and attenuate the effects of abiotic stresses. Therefore they constitute a viable alternative to avoid the consequences of climate change and use natural and environmentally friendly products. [ABSTRACT FROM AUTHOR]