General Introduction to ‘drought’
Water availability is a limiting factor for the growth and development of plants, ultimately affecting fitness and seed set. When water availability is seriously limited during a drought, the effect can be very harmful to plants: leaf senescence accelerates, photosynthesis becomes limited as chlorophyll degrades, and nutrients are less efficiently used in plants under drought stress.
In the case of crops, water deficiency has serious implications for us humans as we rely on the yield of these plants for food and feed. It has been estimated that three-quarters of the land used to cultivate the major crops worldwide is experiencing drought-induced yield losses.
Considerable effort in recent years has been put in research to the plants’ response to drought and to try and come up with breeding strategies to produce more drought-resistant crops. There are numerous ways to this approach, comparing the physiology of dry and well-watered plants of the same species, looking into ways by which certain species are more resistant to drought than others, and exploring ways to treat seeds, seedlings or fully grown plants to make them less susceptible to drought are but a few examples.
In this special issue of Physiologia Plantarum we have combined an abundance of papers (editorial here) with drought as the common theme. We shall highlight 5 papers in the coming weeks, attempting to capture the variation of the topic.
An exploration of the variability of physiological responses to soil drying in relation with C/N balance across three species of the under-utilized genus Vigna
Plants from the legume family, for example beans, peas, lentils, and alfalfa, form a symbiotic relationship with nitrogen fixing bacteria that allows them to fix atmospheric nitrogen. Because of this, crop species of this family are a vital source of protein for people, mainly in poorer regions around the world. They furthermore play a big role in crop-rotation, reducing the need for artificial fertilizer due to their nitrogen fixing ability. Several species of legume are drought tolerant which makes these crops beneficial, as more than 10% of the world’s population live in areas with a climate that requires the growth of crops adapted to dry conditions.
Legumes are therefore very well fit for these regions although a shortage of available water in the soil affects the efficiency by which certain legumes fix nitrogen. This seems to vary among species and cultivars and in the paper by Guiguitant et al., (2021), the authors investigated three species of the Vigna genus. They show the effect of soil drying on nitrogen fixation and transpiration and what this meant for plant growth and nitrogen accumulation in leaves.
Melatonin improves the seed filling rate and endogenous hormonal mechanism in grains of summer maize
Melatonin is probably best-known for its role in our sleep-wake cycle, though it also functions as an antioxidant and free radical scavenger and is known to interact with our immune system. Less commonly known might be its role in plants. Since discovering its presence in plants in 1995, melatonin has been attributed roles during plant development and abiotic and biotic stress response. The most common way in which melatonin acts to deal with stress in plants is through antioxidant defense activity and scavenging of reactive oxygen species (ROS), improving tolerance to stresses such as heat, drought, salinity, and others.
In this paper by Ahmad et al., (2021) it was shown that applying melatonin to seeds of summer maize can enhance the productivity of this crop under water limiting conditions. One of the ways the hormone can aid productivity is by improving its seed filling rate, by some described as a promising approach to higher yields. The rate and duration at which plants allocate resources to the developing seeds are of great influence on the final yield yet are highly susceptible to drought. Water stress during seed filling accelerates leaf senescence, reducing the amounts of nutrients that are available to the seeds and thereby shortening the seed-filling period. Circumventing this obstacle by the application of melatonin might be a promising way to limit the yield-loss generally associated with drought.
Dew absorption by leaf trichomes in Caragana korshinskii: An alternative water acquisition strategy for withstanding drought in arid environments
In environments with little or no liquid water, such as deserts, plants have adapted several ways to survive. Succulents retain water in thick, waxy leaves, Ephemerals complete their life cycle in a matter of weeks when water is available, some perennials stay dormant during the dry season, Phreatophytes have developed extremely long root systems to draw water deep below the surface. The ‘ultimate’ desert species, Cacti, combine several characteristics such as the absence of leaves, shallow root systems, water storage in their stems, and a waxy skin to avoid water evaporation.
Some plants have another trick up their sleeves, trichomes. Trichomes are hair-like structures on the leaves and stem of a plant that can serve various functions. Some protect plant organs from herbivory by secreting chemicals that produce a stinging sensation or unpleasant taste upon contact. Others might serve as a protective layer between young developing buds or a meristem and the harsh outside world, avoiding mechanical damage, UV irradiation or preventing excessive water loss through leaves or stems. The article by Waseem et al., (2021) showed that leaf trichomes on a pea shrub species adapted to drought (Caragana korshinskii) help the plant take up dew. They show that drought-stress grown C. korshinskii grows significantly more trichomes than well-watered plants of the same species and that the trichomes can absorb dew droplets in a matter of seconds.
Seed inoculation of desert plant with growth-promoting rhizobacteria induces biochemical alterations and develops resistance against water stress in wheat
The human gut microbiome can be considered an organ, it has both a physiology and pathology, and disturbance in the microbiome can make a patient seriously ill. What’s more, in some cases a sick individual can be greatly helped by a ‘stool-transplant’ from a healthy donor. Could the same be true for organisms in the rhizosphere of plants? It has long been known that microbes in the rhizosphere are of great influence on the growth and fitness of plants and they are sometimes referred to as the plants’ ‘other genome’. Best known are perhaps the nitrogen-fixing rhizobacteria, supplying the host with nitrogen in return for nutrients, and mycorrhizal fungi that provide increased water and nutrient uptake while the plant provides the fungus with carbohydrates in exchange.
There are numerous other species however that can be of benefit to several aspects of plant growth and development. In this article by Zia et al., (2021) the authors show that by inoculating wheat seeds with bacteria naturally present in certain species of desert plant, they increase the resistance against water stress in wheat. Inoculation improved plant growth, leaf area and biomass under water limiting conditions.
Physiological and metabolic adjustments in the xero-halophyte Haloxylon salicornicum conferring drought tolerance
Plants can be found in all but the most inhospitable places on the planet, showing a remarkable versatility in dealing with unfavourable conditions. Even in Antarctica, possibly having the harshest climate on the planet, one can find two species of flowering plants. Most of our crops however are not adapted to harsh conditions and when circumstances take a turn for the worse, this often results in a disappointing yield. Could we improve the resilience of crops by learning from plants that are adapted to harsh conditions such as Xerophytes? Adapted to environments with little liquid water, Xerophytes could serve as role models for drought-resistant crops.
In the article by Panda et al., (2021) the authors try to unveil the metabolic and physiological adaptation mechanisms in the drought- and salt-adapted (Xero-Halophyte) plant H. salicornicum subjected to drought stress. The species was found to maintain a relatively high-water content through a decrease stomatal pore area, kept an efficient ion homeostasis, its photosystem was relatively unaffected by drought and the authors postulate that the plant keeps a presence of antioxidants at threshold level to scavenge drought-induced ROS, preventing oxidative damage.
A challenge for future research is to translate these physiological qualities to the underlying genetic and molecular mechanisms in an attempt to help breeders develop drought-tolerant crop varieties.