THE EXPERIMENTAL MODIFICATION OF SEX EXPRESSION IN FLOWERING PLANTS (1957)

SUMMARY 1. This article summarizes the evidence concerning the modification of sex expression in the sporophytes of flowering plants by external agencies, excluding those which act through genetical paths. 2. Three of the primary variables of the plant environment, mineral nutrition, light régime and temperature régime, directly influence sex expression in hermaphrodite, monoecious and dioecious species; their effects, moreover, are reasonably consistent. In general: ( a ) High levels of nitrogenous nutrition promote femaleness and depress maleness. ( b ) In short‐day plants and some day‐neutral plants whose development is accelerated by short days, short‐day treatment promotes female and depresses male sex expression. Low levels of light intensity during growth may also inhibit staminal development. In one long‐day plant, spinach, it seems probable that short days lead to the production of male flowers on plants which in normal photoperiod would be phenotypically female. (c) Low temperatures, particularly when experienced through the dark period of a daily photoperiodic cycle, promote female sex expression and depress male. 3. Sporophyte sex expression may be modified through certain chemical agencies. In one experiment with Melandrium dioicum , the animal sex hormones have been shown to affect sexuality, oestrone, oestradiol and oestradiolbenzoate promoting the development of pistillate structures in the flower, and testosterone and testosterone propionate of staminate. In monoecious cucurbits, applied auxins simultaneously suppress early flowering, accelerate the appearance of female flowers and increase their over‐all proportion; a‐naphthaleneacetic acid is the most effective auxin in this role. Similar effects are produced in cucumber by exposure to atmospheres containing low percentages of carbon monoxide. Under some circumstances, the general growth‐suppressor maleic hydrazide may differentially inhibit micro‐ and megasporogenesis in hermaphrodite and monoecious species, so altering the functional sex balance. 4. Certain forms of mutilation affect sex expression. The reduction of leaf surface and of volume of storage tissue militates against female sex expression in one genus of monoecious aroids. The removal of fruits and female flowers in certain monoecious genera increases the over‐all proportion of female flowers formed. Severe mutilation involving the regeneration of new leading shoots during recovery may lead to later anomalies in sex expression in a number of monoecious and dioecious species. 5. In grafting experiments with dioecious species, no authentic cases of the influence of a stock of one sex upon the sexuality of a scion genetically of the other have been recorded. 6. The interrelationships of the hypothetical flowering hormone (florigen), auxin and plant sexuality are discussed. In general, photoperiodic treatment which causes early and heavy flowering also promotes female sex expression as against male; also, factors which raise the auxin levels available at the differentiating apex promote femaleness and suppress maleness. These facts appear paradoxical, since, at least in short‐day plants, high auxin levels inhibit the transition from the vegetative to the flowering state. It seems probable, however, that while low auxin levels may be necessary for the initiation of flowering, auxin is necessary for flower development after the formation of the primordia. It may be that in those plants which are accelerated throughout the entire course of flowering by short‐day treatment, such treatment actually increases auxin availability at the apex after the first period of flower initiation. 7. A rationalization of two apparently contradictory responses along these lines would permit the formulation of an hypothesis that floral organogenesis is regulated, although not determined, by auxin levels at the apex. Much of the existing evidence would be explained by the assumption that the growth of stamen and pistil primordia is governed by auxin in the characteristic manner, the response following an optimum curve. It seems that the concentration causing maximal stamen growth is lower than that promoting maximal pistil growth, so that auxin level at the differentiating apex determines the sex balance of the flowers produced. In some plants it is probable that this level is susceptible to control in the apex by influences such as temperature, and through the auxin economy of the whole plant, by such factors as nutrition and photoperiodism. It is through these agencies that the sex balance of the flowers may be modified.