XEROPHYTISM IN MEDICAGO (MESOPHYTE, ALFALFA, STRESS, ANATOMY)

Morphology and anatomy of xeric Medicago prostrata Jacq. were studied to determine its survival strategy. Mesic M. sativa L. served as a control. The F(,1) hybrid of the two species was included to determine the ease of transferring xerophytic characters to a more vigorous species. The plants were grown under irrigated and dryland conditions in the field. Light, scanning and transmission electron microscopy were used to study plant structure.;The cortex/radius ratio in stems of M. prostrata was high and starch granules accumulated in the perimedullary zone when plants were stressed.;Characteristics of the F(,1) hybrid were generally intermediate between those of the parents. Although trends were noted in most of the characters, differences irrigated and nonirrigated treatments were nonsignificant.;Response of M. prostrata to drought was tested in a growth chamber using a permeable membrane technique. PEG 10,000 was used as an osmoticum to create different levels of moisture stress. Stem and root cross sections were viewed under light microscopy to observe starch accumulation and xylem vessel size, respectively.;Many xerophytic characters were identified in M. prostrata. It leaves had thick cuticles and cell walls (especially at the abaxial surface) with heavy wax plates on the adaxial surface and amorphous wax on the abaxial side. It leaves were small and thick with compact veins and isolateral mesophyll. Glandular hairs were distributed generally over the plant and were especially dense on the abaxial side of leaves. Under moisture stress wax production increased for each entry. In addition, many rotund struma developed on the adaxial surface of M. prostrata.;M. prostrata had higher stomatal conductance under high stress treatment, and higher shoot water potential at low stress conditions, than M. sativa. Plant height of M. sativa was reduced more than that of M. prostrata under moisture stress.;Stem cross sections of M. prostrata showed more starch granules in stems than did M. sativa under high moisture stress. Root cross sections revealed wider and better developed secondary xylem vessels in M. prostrata than in M. sativa under high stress conditions. The F(,1) hybrid was closer to M. prostrata than M. sativa in stomatal conductance, shoot water potential, accumulation of starch, and xylem vessel size. It exceeded both parents in plant height and dry weight of shoot and root.