Bioactive Compounds from Leaves and Twigs of Guayule Grown in a Mediterranean Environment

Morphophysiological Characterisation of Guayule (Parthenium argentatum A. Gray) in Response to Increasing NaCl Concentrations: Phytomanagement and Phytodesalinisation in Arid and Semiarid Areas

Water and soil salinity continuously rises due to climate change and irrigation with reused waters. Guayule (Parthenium argentatum A. Gray) is a desert perennial shrub native to northern Mexico and the southwestern United States; it is known worldwide for rubber production and is suitable for cultivation in arid and semiarid regions, such as the Mediterranean. In the present study, we investigated the effects of high and increasing concentrations of sodium chloride (NaCl) on the growth and the morphophysiological and biochemical characteristics of guayule to evaluate its tolerance to salt stress and suitability in phytomanagement and, eventually, the phytodesalinisation of salt-affected areas. Guayule originates from desert areas, but has not been found in salt-affected soils; thus, here, we tested the potential tolerance to salinity of this species, identifying the toxicity threshold and its possible sodium (Na) accumulation capacity. In a hydroponic floating root system, guayule seedlings were subjected to salinity-tolerance tests using increasing NaCl concentrations (from 2.5 to 40 g L-1 and from 43 to 684 mM). The first impairments in leaf morphophysiological traits appeared after adding 15 g L-1 (257 mM) NaCl, but the plants survived up to the hypersaline conditions of 35-40 g L-1 NaCl (about 600 mM). The distribution of major cell cations modulated the high Na content in the leaves, stems and roots; Na bioconcentration and translocation factors were close to one and greater than one, respectively. This is the first study on the morphophysiological and (bio)chemical response of guayule to different high and increasing levels of NaCl, showing the parameters and indices useful for identifying its salt tolerance threshold, adaptative mechanisms and reclamation potential in high-saline environments.

Argentatin Content in Guayule Leaves (Parthenium argentatum A. Gray)

Approximately one-third of the waste biomass from the cultivation of guayule (Parthenium argentatum A. Gray) for natural rubber production is leaf tissue; however, whether it can be valorized is not known. Guayulins and argentatins are potential high-value products that can be recovered from guayule resin during rubber/latex processing. Argentatins are highly abundant in guayule stem resin; however, unlike the guayulins, their occurrence in leaves has not been investigated. The present study determined the content of argentatins and isoargentatins A and B in the leaves of a pure guayule accession (R1040) and two hybrids (CAL-1 and AZ-2) under conditions of irrigation and non-irrigation. The resin content in leaves was ~10%, which provides a suitable starting point for economic exploitation. The highest production of argentatins occurred in plants under irrigation, with yields of 4.2 and 3.6 kg ha-1 for R1040 and AZ-2, respectively. The R1040 accession had the highest percentage of resin and the greatest total argentatin content (24.5 g kg-1 dried leaf), principally due to the abundance of argentatin A. Contrastingly, CAL-1 consistently showed the lowest argentatin content based on dried leaf weight and production (0.6 kg ha-1). The substantial abundance of argentatins in guayule leaves suggests the potential for future exploitation.

Composition of Guayule (Parthenium argentatum Gray) resin

Guayule (Parthenium argentatum Gray) is a semi-arid shrub, native from the Chihuahan desert. This plant produces polyisoprene and resin. Polyisoprene is the main focal point of many researches, from structure to properties. Today, some processes are used to extract polyisoprene under its dry form, using solvent extraction, to produce rubber (used in truck or airplane tires) or as an emulsion, to make latex products by dipping (used in medical gloves, condoms, etc.). This article focuses on guayule resin which has some interesting applications in adhesives, coatings, pharmaceuticals, etc. In order to better know the resin composition and to be able to perform comparisons between varieties or seasons, liquid and gas chromatographic analysis methods have been described, for the groups of molecules composing the resin (polyphenols, guayulins, free fatty acids, di and triacylglycerols, argentatins, alkanes, alkanals, sugars, organic acids). Unlike other articles, this study aims to analyze all components of the same resin; the average composition of a guayule resin is given.

Antioxidant Contents in a Mediterranean Population of Plantago lanceolata L. Exploited for Quarry Reclamation Interventions

Plantago lanceolata L. (plantain) is an interesting multipurpose perennial species whose aerial parts are used in herbal medicine due to its precious phytochemicals and are palatable to animals. Moreover, peculiar traits such as drought tolerance, an extended growth season and a deep root system, make plantain a promising pioneer plant for quarry reclamation based on the use of native species. This study evaluated the effects of different environmental conditions and seasons on the accumulation of the bioactive compounds of its aerial organs. An autochthonous plantain population was grown in three locations in Sardinia (Italy). Leaves, peduncles and inflorescences were collected between October 2020 and July 2021. Phenolic contents and antioxidant capacity were determined. The analysis of the individual phenolic compounds was performed using liquid chromatography. In leaves, the content of total phenolics, antioxidant capacity and total flavonoids were significantly influenced by location and season. Total phenolic content ranged from 65 to 240 g gallic acid equivalent kg^-1, whereas total flavonoids were from 16 to about 89 g catechin equivalent kg^-1. Neochlorogenic, chlorogenic, cryptochlorogenic acids, verbascoside, diosmin and luteolin were identified in the methanolic extracts of leaves, peduncles and inflorescences. Verbascoside was the main antioxidant isolated from plantain extracts. Results evidenced an increasing accumulation pattern of phenolics from vegetative stage to flowering, followed by a decrement towards the seed ripening as well as site-specific differences with amounts of phenolics even 25% higher for same plantain accession.

Effect of Seasonal Decrease in Temperature on the Content and Composition of Guayulins in Stems of Guayule (Parthenium argentatum, Gray)

The guayulins are a family of sesquiterpene compounds that consist of an isoprenoid nucleus substituted either by trans-cinnamic or p-anisic acid, and are present only in the resinous fraction of the rubber plant guayule (Parthenium argentatum, Gray). While the natural role of the guayulins remains enigmatic, they may serve as a defense function against other plants or herbivores by virtue of the accumulation of cinnamic acid. Prior research has suggested seasonal variation in guayulin content, which has been shown to decrease as winter arrives in two different varieties. In the present study, the effect of guayulins has been evaluated in 13 different accessions cultivated under the same conditions during autumn. A general reduction in guayulin content was found in the stems from all varieties between the September and November harvest, which was accompanied by an increase in the resin content. With respect to individual guayulins, while guayulin A was the most prominent member during most of the year, guayulin C had more prominence when temperature started to decrease. In this seasonal period, the production of each member of the guayulin family in the leaves was very balanced.

Guayule (Parthenium argentatum A. Gray), a Renewable Resource for Natural Polyisoprene and Resin: Composition, Processes and Applications

Natural rubber is an essential material, especially for plane and truck tyres but also for medical gloves. Asia ranks first in the production of natural rubber, of which the Hevea tree is currently the sole source. However, it is anticipated that this source alone will not be able to fulfill the growing demand. Guayule, a shrub native to northern Mexico and southern United States, may also contribute. This plant not only contains polyisoprene, but also resin, a mixture of lipids and terpenoids. This review summarizes various aspects of this plant, from the usage history, botanical description, geographical distribution and cultivation practices, down to polyisoprene and resin biosynthesis including their distribution within the plant and molecular composition. Finally, the main processes yielding dry rubber or latex are depicted, as well as the properties of the various extracts along with economic considerations. The aim is to provide a wide picture of current knowledge available about this promising crop, a good feedstock candidate for a multiple-product biorefinery.