CASE 6 AGRICULTURE: FEEDING A GROWING POPULATION

In natural ecosystems, competition for sunlight creates a strong selective advantage for growing tall. Compare the slender vine in Fig. 31.1, which grows in a dense forest, with the compact barrel cactus that grows where there is little danger of being shaded by a neighboring plant. Whereas the ancestors of today’s crops competed for sunlight by growing taller, a key element of the high-yielding rice and wheat varieties developed during the Green Revolution is their shorter stems. These shorter-stemmed plants can support the much-larger seed mass induced by high rates of fertilizer application (Chapter 29). Moreover, by investing less in stems, these plants can allocate a greater percentage of their resources to the production of seeds. What happens inside plants of these high-yielding crop varieties to make the stems shorter?

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Gibberellic acid was first identified as a chemical produced by fungal pathogens that caused the stems of young rice plants to grow to many times their normal length. We now know that it is produced naturally in plants, particularly in growing regions such as developing leaves and elongating stems. Gibberellic acid increases internode elongation by reducing the force needed to cause cell walls to expand. This raises the possibility that the high-yielding crop varieties with shorter stems have reduced production of or sensitivity to gibberellic acid.

At the time that the semidwarf varieties were developed, the underlying basis for their reduced stem growth was not known. The genes responsible for reduced stem growth have now been identified in both wheat and rice. The semidwarf wheats in fact do have reduced sensitivity to gibberellic acid, and the shorter rice varieties are deficient in the biosynthetic enzymes needed to produce this hormone. In both cases, the result is shorter but sturdier plants.