A CHRONICALLY WILTED MUTANT OF MAIZE

AMONG THE highly inbred strains of maize is a wide range of heritable differences with regard to drought tolerance. Unfortunately for our understanding of the physiological mechanisms and anatomical features contributing to such tolerance, these relative differences appear to be conditioned by multigenic systems (Heyne and Brunson, 1940). Therefore, it has not been possible to isolate and investigate major components which differentiate drought-tolerant from drought-susceptible types. In contrast to such quantitatively determinied differences is a new, monofactorial recessive mutant wilted (wi) which is severely wilted throughout most of its growing period. Plants, which are wi/wi, begin to show signs of water deficiency at the tips of the youngest leaves approximately 3 weeks after emergence. As growth continues, all leaves are wilted during most of the daylight hours. Characteristically, the youngest leaves show the most pronounced symptoms of water deficiency. The wilting in wi/wi plants is clearly expressed in plants growing in soils which are nearly saturated with water and even in plants growing in water culture. In soils with adequate water, the leaves of wilted plants recover turgidity during the night hours but wilt early the next morning as increased transpiration imposes new demands on the water economy of the plants. As the plants mature, it is apparent that they make a partial recovery as shown in fig. 1. Over a period of time, the leaves recover successively beginning with the lowest leaf and moving up the plant. Before this recovery can progress beyond the ear-bearing node, the plants are fully mature. The chronic water deficiency throughout most of the life cycle of wilted plants has a marked dwarfing effect. Such plants may be only one-third to one-half as tall as normal plants in the same progeny. Wilted plants usually shed pollen, but exceptionally favorable environmental circumstances (high rainfall and cool, cloudy days) must prevail in order that seed may be obtained. Early observations showed that stomata number, size, and action were comparable in the wilted plants to normal. Further studies with wilted and normal plants, sealed into containers with plastic covers in such a way that the only water loss from the containers was through transpiration, showed that per gram of leaf tissue (dry wt.) the normal plants had a somewhat higher water loss than the wilted plants. Thus, the rapid wilting tendency of the mutant strain could not be accounted for by excessive water loss. Attention was then turned to water uptake and transport in mutant plaTnts. Stems of wilted and normal plants, which were 1 Received for publication March 18, 1957. Journal Paper Number 1083. 74 days old, were cut under the surface of a dilute solution of eosin. The plants were allowed to remain in the solution for 30 min. The dye moved much more rapidly through the normal plants and at the end of the 30 min. was visible at the tips of all the leaves. In contrast to this, only the lowest leaves of the wilted plants showed the presence of eosin throughout their length. The next few leaves showed evidence of eosin in the areas closest to the stem, while the youngest leaves did not have any eosin. These observations were paralleled by low power microscopic studies of freehand sections from the internodes of the test plants. In normal plants all the vascular bundles in the first 5 internodes showed traces of eosin. In the sixth internode a few bundles did not conduct eosin. In each successive internode up the plant more bundles were found which did not have eosin until, by the eleventh internode, only one-third of the vascular bundles present showed traces of the dye. In wilted plants, however, only in the first three internodes did all the bundles have eosin, and by the third internode, the amount of eosin present was noticeably less than in normal. In the fourth internode many bundles were devoid of eosin and by the tenth internode no vascular bundle gave evidence of eosin transport. With regard to the wilted plants, it was obvious even under a cursory scrutiny, that many of the vascular bundles were abnormal since metaxylem elements were either immature or lacking altogether. Further, the presence of these abnormal bundles appeared to be governed in large degree by the stage of development since in the basal internodes many of the bundles were normal, while in the youngest intemodes few or none of the bundles were normal. Because of these definite indications that the wilted phenotype had a morphological basis, more critical examinations of the organization of the vascular system of normal and wilted plants were undertaken. MORPHOLOGY.-Samples of leaves and stems were collected from normal and wilted plants from a backcross progeny at various stages of development. A second set of collections was made of pigmy wilted (py wi) and pigmy normal (py Wi) plants from a F, progeny. Pigmy is a semi-dwarf recessive mutant (Emerson et al. 1935). Material was killed and fixed in FAA, dehydrated in the tertiary butyl alcohol series, sectioned at 15,u, and stained in safranin and fast green. Normal and wilted plants were compared for variations in ontogeny of the vascular tissue. The more valuable comparison was between the pigmy wilted and pigmy normal plants since both