Plant Growth Responses to Environmental Stress

Agriculture is one of the world's largest industries, and plant products constitute 93% of the human diet. Yet, our understanding of the detailed biology of plants is far behind that of animals and some microorganisms. For example, we know that roots supply water and minerals to shoots and provide other compounds that influence growth and development. We also know that the above-ground portion of the plant must capture light and energy to fix carbon dioxide into sugars that are transported into roots. Plants must somehow balance the flow of materials between roots and shoots.

When plants are grown under environmental stress, the resource-gathering ability of roots or shoots is limited. This can result in changed growth patterns in the plant. An example of the balance between roots and shoots can be demonstrated when plants are grown in containers with limited space for root growth. When the plant has filled the available rooting volume, the rate at which leaves grow is reduced. This results in smaller leaves and less total leaf area on the plant relative to plants in unrestricted soil volumes. Investigations of this sort may provide an avenue for understanding how roots and shoots integrate carbon, water, and hormonal systems into an early warning signal of unfavorable root-zone conditions.

It is possible that the decline in leaf growth reduces the demand for carbon, resulting in accumulation of carbohydrates in source leaves. Root stresses may, therefore, simply result in fewer growing roots and a lower demand for assimilates (reduced sink strength), which is somehow sensed in leaves as the build-up of assimilates and subsequently slows leaf growth. It is possible that a change in the balance of carbohydrates would act as a "warning system" that results in reduced growth before there is a severe imbalance in the resource-gathering capacity of the plant.

In one of the other research projects in my laboratory, my students and I are studying the role of specific hormones in the integration between roots and shoots. Our strategy is to compare a bean hybrid plant that develops abnormally to its normal parental stocks. A plant hormone (cytokinin) is involved in the abnormal development because normal shoot growth is partially restored by application of cytokinins to the roots.

The long-range goal of this research on root-shoot integration is twofold. First, it will provide plant growers/breeders with an important new understanding of root-shoot relationships, which will lead to increased crop yield. Second, it will give molecular biologists necessary information for studying plant development. The combination of basic physiology with molecular technologies now makes it realistic to anticipate that we will ultimately be able to exploit this system as well as other plants for increased crop yield.