How do plants survive drought and which avenues of exploration should be pursued on this topic?

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Water is an essential ingredient for life. Gymnosperm and Angiosperm plants, whether monocotyledons or dicotyledons, require water to photosynthesise and produce sugars; this facilitates respiration and growth. Unfortunately, water is not always an abundant resource in the environment. The modern world has ever increasing demands for constant supplies of water in agricultural, industrial and domestic contexts.

Drought stress in plants occurs when the available water in the soil reduces to a point where it has negative effects on the growth of a plant. Lack of necessary water can be defined as an abiotic stress in plants. Our current climate seems to be changing, with more frequent drought events predicted over the coming years. Given that plants are the basis of the human food chain coupled with the fact that they produce the oxygen that we breathe, plant drought survival, its understanding and avenues of investigation are very important for continued human existence.

Water in plants is multifunctional; it is moved via two main transport systems. Short-distance transport occurs when water moves between cells close to each other. Long-distance transport occurs when water moves between cells further away from each other (Mauseth 2009). In taller plants there is a constant stream of water running from the roots to the leaves. This is called the transpiration stream. Around 95-97% of water drawn out of the soil by a plant is evaporated through the leaves; the remainder is used by the plant. This constant flow helps to transport nutrients throughout the plant.

Moreover the process helps to keep the plant cool when evaporation occurs from the leaves. The bio-chemical reactions that take place in a plant happen in solution. The movement of water through a plant is a passive process, in that there is no energy expended by the plant to move water to its desired place. This is due to a number of principles. At the extremities of a plant diffusion occurs; water vaporises and enters the atmosphere as a gas. The movement of water up the vessels (xylem) occurs in accordance with the cohesion and adhesion theory. Water moving from the soil, through the roots and into the xylem occurs due to osmosis. Plants are very adept at only letting water and minerals escape when required.

The Casparian strip, endodermis and cortex are parts of the root which do not allow water and minerals that have been accumulated to leave the plant. However, these parts allow essential water and minerals to enter (Campbell et al 2008). The process of allowing water and minerals into the xylem cells at the roots – but not out, creates a positive pressure in the vascular system at the base of the plant. This helps water and nutrients travel up the vessels of the plant, although it is not present in some plants so not considered a significant mechanism (Lack and Evans 2005). The most widely recognised mechanism of Long-distance transport in plants is transpiration in accordance with the cohesion and adhesion theory. This process requires a number of factors in order to function. Firstly, there must be evaporation occurring at the leaves through the stomata, caused by the sun heating the leaves’ surface. This process pulls water through the rest of the system because of the negative pressure created by evaporation in the leaves. Due to the properties of water and its molecules being charged and attracted to one another, the water moves up the plant in one long column. Also, the plant must have relatively rigid vascular walls for the water to travel up. If the walls were elastic, consequently they would collapse due to the negative pressure being exerted throughout the system.

Due to plants’ inability to relocate themselves to an infinite source of water, they have evolved various strategies to minimise the loss of water; these are called xerophytes. Also, adaptions and alterations in physiology occur to deal with the problem of water deficits (Watson 2006). Not only does water availability in the soil fluctuate, but the amount of water used by the plant can vary. For example, high temperatures, high wind and high altitude can all greatly increase the evaporation rate from leaves. Certain plants have waxy cuticles (surface of the leaf) which reduce water loss through the surface cells. Furthermore, the waxy cuticles reflect light and excessive heat away from the leaf. Another adaption are leaf hairs. These maintain a higher humidity around the surface of the leaf, which reduces the transpiration rate of the plant (Watson 2006).

Most desert plants will complete their whole life cycle during the rainy season when there is adequate water to survive (Campbell et al 2008). The Stoma in leaves of dicots and monocot plants are pores located at the leaf’s surface, by which water escapes as vapour and gases enter the system. These have sausage shaped cells when turgid (full of water) are open. However, when the plant has a water or CO2 deficit, Potassium ions are sent out of the cells and water leaves. Next, the guard cells become flaccid and close the aperture. This restricts water vapour loss. However, it additionally reduces photosynthesis, as the gaseous exchange is also restricted. Therefore, this is not an ideal state for the plant to be in during daylight hours (Lack and Evans 2005).

Another adaption in certain plants is the presence of crassulacean acid metabolism (CAM). These types of plants have a fixed rhythm to open and close their stomata to reduce water loss. These plants also have a unique ability to fix CO2 at night, enabling the stomata to stay closed during the day when evaporation would be at its highest (Lack and Evans 2005). The signaling mechanisms of plants to react to water deficit is not fully understood at this time. We know that a plant can react to a lack of water in the soil. We also know that the hormone abscisic acid (ABA) is present. However, how this signal is triggered and how the leaves receive this signal within minutes of changes to environmental circumstance has not been fully established (Elizabeth A. Bray. 1997).

In 1996, Hans J. Bohnert et al. suggested that the reason biomechanical engineering has been slow in the area of developing drought resistance in plants is due to the many different ways in which plants cope with this problem and the alternative strategies used, depending of the age and developmental stage of each plant. Alexander Gallé et al. studied the recovery of induced drought stress on young Pubescent Oaks (Quercus pubescens) in 2004 and 2005. They found that this particular species reacted to the lack of available water by reversibly reducing its photosynthetic output, whilst the cell pigmentation and antioxidants remained unaltered. When rewetted, the plants recovered very quickly. They concluded that this species was capable of surviving extreme drought events.McCann et al. studied the effects of applying ABA and trinexapac-ethyl (synthesised growth inhibitors) to two species of turfgrass before a period of induced drought stress. The team found that by applying these chemicals, the grasses had a lesser initial growth rate. However, over the period of the experiment, the grasses had a higher overall total growth and therefore had better water efficiency throughout the induced drought period.

In 2005 Basia Vinocur and Arie Altman discussed the ‘Recent advances in engineering plant tolerance to abiotic stress: achievements and limitations’. Their view on the subject was that advances can be made with both cross breeding of plants and genetic engineering to give plants a better resistance to drought stress and other abiotic stresses plants are subject to. Xu et al. carried out some very interesting research in 2008 by observing the drought resistant properties of Beet, Pepper, Watermelon, Cucumber, Tomato Zucchini, Tabacco and Rice plants. When inoculated with viruses, these plants exhibited greater tolerance to drought induced conditions. A higher level of ABA was noted in these plants during the experiment. However, it is not clear whether this was a response to the virus or general stress to the plant caused by another related factor.

The research carried out in this area of study has shown that understanding of a plant’s response to drought stress is a complicated problem. There have been new responses and new interpretations of previously observed responses discovered. However, there are many more questions that must be looked into in order to obtain a better view of plant mechanisms in response to water deficit: How does a plant perceive stress, and how does it lead to the various chemical signals being sent? How is ABA recognized by the receiving plant cells? Which water deficit-induced genes are required for a greater tolerance to water deficit? Which attributes of tolerant plants can be incorporated successfully into crops? These are questions that should be made to individual families of plants in the environments that they grow or will grow in, and to ensure continued successful plant production even under adverse conditions.

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