Introduction

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Life-giving water molecules, fundamental to our biosphere, are as remarkable as they are abundant. Hydrogen bonds, enhanced by dipole forces, confer extraordinary physical properties on liquid water that would not be expected from atomic structure alone. Water has the strongest surface tension, biggest specific heat, largest latent heat of vaporisation and, with the exception of mercury, the best thermal conductivity of any known natural liquid. A high specific grav-ity is linked to a high specific heat, and very few natural substances require 1 calorie to increase the temperature of 1 gram by 1ºC. Similarly, a high heat of vaporisation means that 500 calories are required to convert 1 gram of water from liquid to vapour at 100ºC. This huge energy requirement (latent heat of vaporisation, Section 14.5) ties up much heat so that massive bodies of water contribute to climatic stability, while tiny bodies of water are significant for heat budgets of organisms. Latent heat of fusion (Section 14.6) is a further property of water with profound implications for biology. Substantial heat is released as liquid water turns to ice, and this thermostatic effect is used to advantage for frost protection in horticulure.

Although water is abundant on earth, fresh water is scarce, and plant-available soil water represents a major environ-mental limitation and thus a continuing selection pressure for terrestrial ecosystems. Accordingly, plants growing in arid and semi-arid environments exhibit a wide range of mechanisms that enable them to grow and reproduce despite water short-age. For example, deserts are transformed into spontaneous gardens by substantial rain as ephemerals including species of everlasting daisy, Helichrysum spp., Helipterum spp. and Schoenia spp., germinate, grow, produce a brilliant carpet of flowers and then set seed before soil moisture is exhausted. Along with many desert ephemerals, they survive drought not as plants but as seeds!

Other species shed their leaves and survive the period of water deprivation as non-transpiring stems or underground plant parts. A well-recognised example in northern Australia is the baobab tree (Adansoia gregoria), which persists through a six-month dry season as a trunk plus leafless branches, rather like a giant inverted root system.

Some species maintain their foliage, but survive periods without rainfall by developing large root systems that seek water at depth or by reducing transpiration and growth during dry seasons via leaf shed. Jarrah (Eucalyptus marginata) is known to have sinker roots that can penetrate to 20 m to access groundwater (Section 3.1). Jarrah also reduces water use and photosynthesis in late summer by stomatal closure (Section 15.2). Such combined attributes ensure survival during hot dry summers in native environments (winter rainfall zone of Western Australia).

Desert succulents, such as pigface (Carpobrotus edulis), which occur along edges of seasonally dry salt pans and near shorelines in southern Australia, have both morphological and biochemical mechanisms that enable them to survive long periods without water. Many desert succulents have very reduced leaves, but enlarged photosynthetic stems. These stems provide a reservoir of water for use during prolonged drought. They also have a specialised carbon pathway (CAM photosynthesis, Section 2.1) that enables them to take in CO2 at night-time and store carbon as organic acids. Those weak acids are converted to sugars, starch and amino acids behind closed stomata during the daytime.

Finally, in one extreme form of drought resistance, some species tolerate complete desiccation to air dryness, but quickly recover physiological activity once water is restored. These remarkable ‘resurrection’ plants are described in Feature essay 15.1.

Much of the earth’s land surface is either too hot or too dry for sustainable agriculture. Nevertheless, species in nature have adapted to many arid situations unsuited to agriculture via survival devices that already find some expression in crop plants. An appreciation of those mechanisms responsible for drought resistance often reveals analogies between domesticated plants and wild progenitors. Further improvement of crop plants will come from enhanced expression of desirable wild traits, as well as from introduction of new properties from unrelated species via molecular methods. In either case, key traits have to be identified, and heritability established. Some of the processes underlying drought resistance, together with features that contribute to genetic differences in water use efficiency, are covered here within a context of soil–plant–atmosphere water relations.

 

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