Agriculture depends to a large extent on the success of plant reproduction. Not only is it necessary for the next crop, but reproductive products like grain, fruit, many vegetables, and nuts are the bulk of the world food supply. In the USA, more than 75% of the harvested acreage is devoted to such crops (Agricultural Statistics, 1999). Consequently, when world economies give attention every year to the success of crop production, they are focusing mostly on plant reproduction.
The success of reproduction is determined to a large extent by the environmental conditions prevailing during the growing season. Boyer (1982) surveyed the yields of eight crops in the US, six of which had valuable reproductive structures (maize [Zea mays L.], sorghum [Sorghum spp., probably bicolor or halepense L.], wheat [Triticum aestivum L.], barley [Hordeum vulgare L.], oats [Avena sativa L.], and soybean [Glycine max L.]). The six reproductive crops had average yields during agricultural production that were only about 18% of the record yield. Assuming that record yields measure the maximum biological potential for reproduction in agricultural conditions, a large fraction of the potential was not realized.
These losses vary somewhat from year to year in different locations around the world. In the US, the availability of water was a major contributor to the variation (Boyer, 1982). Drought affected crop productivity nearly as much as all the other environmental factors combined. Irrigation has been one solution but is becoming less so as global water demand increases. In a detailed review of hundreds of studies, Salter and Goode (1967) point out that the largest response to irrigation is during early reproductive development for most crops. The biggest return per unit of water can be obtained by irrigating at this time. But despite the susceptibility, Salter and Goode (1967) could find few reasons for it. Research on mechanisms of reproductive failure was rare.
Since the analysis of Salter and Goode (1967), increased attention has been paid to reproductive failure during water deficits, focusing mostly on grain crops. Saini and Westgate (2000) reviewed this literature and highlight the accumulating evidence for signals emanating from the parent plant, especially carbohydrate availability and metabolism, as well as hormones. In wheat, barley, and rice (Oryza sativa L.), abscisic acid (ABA) was implicated as a cause of pollen sterility, and in maize decreases in the sugar stream from losses in photosynthesis appeared important for development of the female inflorescence. Since the Saini and Westgate (2000) review, however, most new work has involved maize, which will be the focus of this review. For the other small grains, the reader is directed toward the Saini and Westgate (2000) review.