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Water Consumption and Plant Production: Comparison of Plant Types with Differing Water Economy

Life on earth developed in water and despite evolution over many millions of years, today as then, all living processes with their underlying biochemical reactions are only possible in the aqueous milieu (see also Chap. 1.5). Most land plants in their active state do not tolerate any drying out. This is illustrated by the water content: In the active state the protoplasm of most

"Funnel" oases near El Qued, Algeria, northern Sahara: About 10 m of sand is removed in bags on donkeys. The limestone crust of an additional 10 m is broken and removed to reach a layer of loose material in which fresh water is found at a depth of about 4-6 m. On the sandy material typical oasis plants (date palms, pomegranates) and vegetables are grown; water is supplied by wells. The funnel is protected from refilling with sand by a series of concentric circles of dry leaves of date palms which are placed on the perimeter. These trap sand that is blown inwards from the desert. The outer ring is visible on the photograph. The oasis has a total diameter of approx. 1 km. Thus, the donkey moved about 7 million m3 per garden. Despite the rings of date leaves, permanent removal of sand that is blown in by wind is still necessary (Photo E.-D. Schulze)

leaves and fruits contains 0.85-0.90 g H20 g_1 FW (fresh weight). The water content of wood decreases to about 0.50 g H20 g_1 FW because of the high percentage of dead water-conducting tissue in the xylem. Lowest values are reached in ripe seeds: 0.05-0.15 g H20 g"1 FW.

Land plants must keep the water content of cells close to or fully saturated in relatively dry air, and still maintain exchange of C02 with the air in order to photosynthesise. Life outside water brings benefits as well as dangers for plants for the following reasons:

The diffusion coefficient of C02 in air is about 0.14X 10~4 m2 s_1 and decreases in water to 0.16X 10~8 m2 s-1. Thus, C02 diffuses in air 10,000 times faster than in water (Sestak et al. 1971). However, it must be considered that the C02 concentration in the Lower Devonian, the period of first settlement of plants on land, was significantly higher (ca. 4000 ppm) than today (ca. 370 ppm).

During the evolution of plants, no membrane was "invented" which is permeable to C02 but remains impermeable to H20 vapour. Even in the future, there will not be a type of "GoreTex" for C02 because the molecular weight of C02 is larger than that of H20 (44/18). Of course, it was the availability of C02 in the atmosphere that provided the incentive for plants to adapt

Table 2.2.1 Water use and resources acquisition (C02 in plants and 02 in animals) at 20 "C and 50% relative humidity and taking into account the diffusability of gases which depends on their molecular weight. A human with a body temperature of 36.6 °C acts as an example for the animal kingdom

Plants

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