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Abstract

For crop production in dryland agriculture, it is useful to identify traits that have the potential to improve crop performance. Once recognized, the physiological basis of such traits should be investigated and their potential to improve crop performance must be assessed. Eventually, such traits may be used to breed new genotypes. Alternatively, the effects of beneficial traits may be promoted, or negative traits may be minimized, by appropriate agronomic management techniques. One such trait concerns the communication between root and shoot. In recent years, so called ’root signals' have been recognized as a transmitter of adverse soil conditions, such as soil dehydration and high soil strength, from the root to the shoot. They are likely to be chemicals generated in the roots in response to soil dehydration or increased soil strength. Root signals are transported to the shoot in the transpiration stream, where they affect physiological processes such as growth and leaf conductance. The presence of root signals was recognized, because many researchers observed a depression of growth and leaf conductance before or without a change in leaf water potential and its components, which traditionally have been regarded as the sole transmitter of water deficits from the root to the shoot. The plant growth regulator abscisic acid has been identified as a potential root signal. The objective of this work was to assess the importance of root signals, resulting from soil strength and soil dehydration, on the growth and yield of selected tropical crop plants. Experiments were conducted in the glasshouse and in controlled environments using large pots of soil. The main criterion for the presence and strength of root signals in this study was the observation and the degree of reduction of physiological processes, such as leaf and stem growth, leaf conductance and transpiration, without a change in plant water status, measured as water potential, osmotic potential and relative water content. The relative effect of soil strength and soil dehydration on growth, water relations and leaf conductance of sorghum (Sorghum bicolor, cv Texas 610) was studied in the glasshouse. Differences in soil strength were created by growing plants in large pots of undisturbed or disturbed soil. Different degrees of dehydration were achieved by three watering regimes.