Studies On Ambrosia. Iv. Effects Of Short Photoperiod And Temperature On Sex Expression

AMBROSIA ELATIOR was one of the species studied by Garner and Allard (1920) in the first work on photoperiodism. They found that plants grown under a 7-hour photoperiod for a month produced pistillate flowers almost exclusively when returned to natural conditions. A genetical interpretation of sex in A. elatior was given by the writer (1936a) which was followed (1936b) by evidence of the photoperiod altering some strains. Mann (1942) reported that A. trifida was modified in sex by short photoperiods. This species, unlike A. elatior, seldom shows sexual variation in nature. Mann states that "In A. trifida, on the other hand, length of photoperiods and number of photoinductive cycles have a pronounced effect on sex expression. Lacking experimental information, the uniformity in field plants would suggest that genetic factors are of less importance in A. trifida than in A. elatior." The writer (1943) pointed out that "The uniformity of A. triflda in the field may be as genetical in its causes as is the variability of A. elatior. From the environmental angle, the experimental work has not been comparable, but there is evidence that A. elatior of certain strains is alterable in sex." The obj ect of the present investigation was to determine (1) the relative effects of a short photoperiod on the sex expression in A. trifida and in known genetical strains of 4. elatior, and (2) the effect of temperature during the dark period on A. trifida. MATERIAL AND METHODS.-Achenes of A. trifida and of two strains of A. elatior, namely, the truebreeding, monoecious A. elatior "N", and the pistillate A. elatior "P" were after-ripened at 5 ?C. (Davis, 1930), germinated at 300C., and sown in the greenhouse, June 14, 1943. Two-branched plants were obtained by removing the plumules above the cotyledons. The most uniform of these were selected and one branch was arranged so that it could be darkened independently of the other branch (fig. 1 and 2). One branch received 20 short days of 6 hours each, and the other the natural photoperiod which was at the beginning of the experiment on July 20, 14 hours and 53 minutes from sunrise to sunset. The branches were covered from 3:00 or 4:00 p.m. to 9:00 or 10:00 a.m. the following day. Treatments were started July 20 for twenty-nine plants of A. triflda 36 days old, and July 26 for twenty-seven A. elatior "N" and thirtynine A. elatior "P", each 42 days old. Fifty-three 1 Received for publication February 10, 1947. Paper from the Department of Botany, University of Michigan, Number 734. I am indebted to Dr. Josephine Burkette for the use of much of the photoperiod equipment which she had employed in other studies. I wish also to thank Professor R. H. Roberts for suggesting that the factor of temperature be checked in the experiments. entire two-branched plants of A. trifida and fiftyone of A. elatior "N" were exposed to 20 short days of 6 hours each beginning July 20. During the dark period the plants were placed in a dark room where the temperature varied from 210C. to 260C. In 1945, whole, unbranched plants of A. trifida, 10 days old, were grown under 20 short days of 6 hours each beginning May 30. Four lots of 100 plants each received a different temperature during the dark period, viz., 50, 150?, 200-250, and 280C. The plants at 50 and 15? C. were placed in refrigerator rooms and those at 280C. in a culture room. These rooms have only double doors as outlets. The room at 200-250C. adjoins the greenhouse and the air is kept circulating between the two by a fan. During the period of light the plants were in the greenhouse at a temperature of 200-250C. The treatments ended June 18 when the plants were returned to natural light and temperature conditions. In 1946, whole, two-branched and unbranched plants of A. triflda, 36 days old, were grown under 20 short days of 6 hours each beginning July 20. Four lots of about fifty plants each received a different temperature during the dark period, viz., 5?, 15? 21o-250, and 350C. in the rooms used in 1945. When exposed to light they were in the greenhouse at 20 0-30 C. The treatments ended August 10 when the plants were returned to the greenhouse. RESULTS.-1943.-The plants were 36 to 42 days old at the beginning of the experiment. They were vegetative, macroscopically, but undoubtedly possessed floral primordia because floral buds appeared as early as 4 days on control branches. The results thus concerned the effect of the short photoperiod on floral development, not initiation. Table 1 gives the degree of floral development at the end of the 20 six-hour days, before the plants were returned to natural light conditions. Entire, treated plants were dwarfed and unbranched. They ordinarily had mature pistillate heads in the axils of the leaves, but terminal floral buds were barely visible. Two plants of A. elatior "N" formed pistillate terminal racemes and hence were sex reversals (fig. 4). Where only one branch received short days, the plants were strikingly dimorphic (fig. 5). The treated branch was about one-half the height of the other and showed a marked maturing of lateral pistillate heads and a suppression of terminal staminate structures. The control branch usually had a terminal staminate bud but no lateral pistillate heads. Some plants of A. elatior "P" normally bear terminal pistillate inflorescences. These were usually accelerated in their growth by the short-days.