Comparative water relations and drought tolerance among alfalfa cultivars

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Bonner, Donald M.
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This study examined several divergent alfalfa cultivars over a wide range of soil moisture conditions in controlled watering and natural (field) environments with the objective of characterizing potential differences in plant water relations, productivity and water usage. In addition, the effect of pre-stress conditioning on water relations during a subsequent drought was explored in terms of general impact and possible cultivar differences. A third objective was to evaluate relationships between parameters and yield a better understanding of general water relations in alfalfa. Several water relations parameters including relative water content, and total water, osmotic and turgor potentials were measured or calculated using both field-grown and container-grown alfalfa plants in 1991 and 1992. Aerial dry matter and root production, water usage and water use efficiency were also determined. Water application was controlled in the container-grown experiments to provide well-watered and droughted plant material for comparative measurements. Differences in water relations, productivity and water use between the alfalfa cultivars were revealed. Overall, there appeared to be a tradeoff between productivity and drought tolerance. Benefits of pre-stress conditioning were realized. Osmotic adjustment was determined to be a benefit of pre-stress conditioning, but this benefit was short-lived. Osmotic adjustment of over 0.4 MPa was shown to occur in droughted versus well-watered alfalfa and is likely an important drought tolerance characteristic. Relationships between water relations variables indicated that turgor pressure was generally lost below a leaf relative water content of 72% and a stem water potential of $-$1.76 MPa in moderately to severely drought stressed alfalfa. (Abstract shortened by UMI.)