Adverse effects of abiotic stresses on medicinal and aromatic plants and their alleviation by calcium

Abstract

Plants, because of their sessile nature, are the foremost organisms which always face several environmental stresses such as extreme temperatures, drought, water logging, salinity, and heavy metals, which severely affect crop production. Growth, yield, and quality of medicinal and aromatic plants (MAPs) have been reported to be influenced by these environmental constraints. Salinity is one of the major causes of decrease in agricultural productivity worldwide. An excess of soluble salts in the soil causes osmotic stress, specific ion toxicity, and ionic imbalances that lead to plant death or considerable yield losses both in the MAPs and other crop plants. Soil salinity has a remarkable negative impact on economy, particularly in the case of fundamentally agrarian nation. Salinity has been reported to affect growth, mineral nutrition, and the yield and composition of essential oil of marjoram (Origanum majorana), coriander (Coriandrum sativum), and peppermint (Mentha piperata). Further, increasing salinity and sodicity stresses caused a reduction, both in shoots and root yield of citronella (Cymbopogon nardus), lemongrass (Cymbopogon flexuosus), and vetiver (Vetiveria zizanoides) plants. Drought is a common and serious problem to plants in arid or semi-arid areas. Plants have developed different morphological, physiological, and biochemical mechanisms (photosynthesis, transpiration, transport of water, nutrients and photosynthates, etc.) to withstand drought stress. Evidences from different lines of research suggest that drought stress, by generating AOS (active oxygen species), can induce oxidative stress in plants. Under a drought environment, an imbalance between the generation and elimination of AOS in plants is created, causing an accumulation of AOS. These accumulated AOS destroy the cell membrane system and cause turbulence for a series of metabolic responses. Plants have the capacity to regulate the balance between the generation and elimination of AOS within the limit of the plant-tolerance range to adverse environments. A good deal of scientific literature regarding effects of water stress on the MAPs is, however, still lacking. Adverse effects of water stress have been observed on the growth, yield, and quality of various MAPs including mint (Mentha arvensis), yarrow (Achillea millefolium), chicory (Cichorium intybus), calendula (Calendula officinalis), balm (Melissa officinalis), thyme (Thymus vulgaris), etc. For example, the water deficit has been found to reduce the plant height, leaf length, leaf area, fresh and dry weight, moisture content, and the percentage of essential oil content of lemongrasses (Cymbopogon nardus). Water deficit may also cause the alterations in the yield and composition of essential oils of various MAPs. Among the mineral nutrients, nitrogen, phosphorus, potassium and calcium (Ca) are considered to be of prime importance as they are required by plants in large quantities. Out of the macro nutrient elements mentioned above, structural and physiological roles of calcium are specific in plants. Calcium also plays the role of second messenger, controlling the growth and differentiation of cells and tissues in plants. Further, calcium is highly required by medicinal legumes during nitrogen fixation processes. It is involved in the regulation of plant responses to various abiotic stresses by contributing either directly or indirectly in plant defense mechanisms. Other targets of Ca2+ ameliorative effects on salt-stress damages are intracellular processes. The SOS (salt overly sensitive) stress-signaling pathway is a pivotal regulator of plant ion homeostasis under salinity. This pathway also emphasizes the significance of Ca2+ signals in resuming cellular ion homeostasis. SOS3, a Ca2+ sensor, transduces the signal downstream after interacting with the SOS2 protein kinase. This SOS3/SOS2 complex activates the Na+/H+ antiporter activity of SOS1, thereby reestablishing cellular ion homeostasis. Calcium stimulation of the SOS3/SOS2 pathway also appears to enhance vacuolar Na+ sequestration by the vacuolar Na+/H+ antiporters. Exogenously applied Ca alleviates salt, heat, drought, high temperature, and cold stresses by regulation of antioxidant activities. In several plant cell-elicitor systems, some evidences have been obtained indicating that the activation of defense responses depends on the presence of extracellular Ca. Thus, the growth, yield, and quality of the MAPs could be improved under abiotic stress by supplying the plants with sufficient calcium nutrient.