It will be evident by now that many of the wild Brassica spp. and their close allies inhabit dry coastal, arid rocky or desert habitats. These wild plants have very thick leaves containing less chlorophyll and many more cell wall components compared with cultivated plants. Typically, they have well-developed xylem vessels and small leaf areas; these characteristics increase the efficiency of water conservation in plants. The foliage of wild xerophyllous
plants has evolved high photosynthetic rates per unit leaf area (or per quantum of light received) even in dry air conditions.
Conversely, cultivated brassicas have broadly expanded, thin leaves well supplied with chlorophyll. These characteristics are advantageous for receiv ing, absorbing and utilizing solar radiation when there are ample supplies of water and nutrients available. Typically these are mesophyllous environments found in fertile, cultivated fields where growers strive to provide optimal nutrition and water.
Similar contrasts between the ecology of wild progenitors of crops and cultivated plants are found between the wild allies of wheat and artificial cultivars. Wild forms possess small, thick leaves, whereas wheat cultivars have large, thin leaves. Both wild Brassica and Hordeum spp. evolved strategies for successful growth under arid conditions involving the restriction of transpiration, intensification of water movement to sites of photosynthesis, restriction of light absorption and efficient fixation of the absorbed solar radiation. Such traits became redundant in cultivation and consequently were removed by many generations of on-farm selection and, since the start of the 20th century, breeding based on increasing knowledge of the biological components of plant productivity.
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