Determination of Relative Auxin Content in Hermaphrodite and Andromonoecious Cucumis sativus L.

Abstract

A cau.sal relationship between growth substances and sex expression probably exists in Cucumis sativus L. (cucumber). This.assumption is based on a number of publications which documented a shift towards femaleness in monoe,cious cucumbers treated with auxins (8), or with growth retardants (16). An even more remarkable shift, but towards maleness, was caused by gibbe,rellins (24). The experimental modification of sex expression of C. sativus and other plant species by growth substances was reviewed by Heslop-Harrison (11). According to this review and more recent information (13, 18, 23), the cucumber may be regarded as an example, rather than an exception, in respect to the, relationship between growth substances and sex expression. Nevertheless, the effect of applied auxin does not necessarily prove that this substance is the natural regulator of sex differentiation in plants. Only 2 studies of other authors, who tested the relationship between sex expression and natural auxin content, came to our attention (12, 4). The Heslop-Harrisons (12) found that CO treatment induced a shift to femaleness in male Cannabis plants and tested auxin content of CO treated and nontreated plants. The results obtained indicated the probability that CO has increased the free auxin, though, due to variability of the results, the authors did not accept them as conclusive. A ve,ry large difference in auxin content between male and female Cannabis plants was found by Conrad and Mothes (4). Female plants contained 30 times more auxin than male plants. In Cannabis, as in Cucumis, auxin application to the plants caused a shift toward femaleness (10). In a p,r,evious study (5) the amount of growth substances in stem tips and leaves of monoecious (M/M, St+/st+ ) and heterozygous-gynoecious (M/M, st+/st) cucumber plants was tested. But due, probably, to the low sensitivity of the bioassay used, only differences in inhibitors, but not in auxin, were found. In that study, leaves of equa.l size of the 2 sex types were compared. Later studies showed that such a comparison is inaccurate. I.t was found that in st/st plants floral buds develop in axils of smaller leaves than in the st+/st+ plants (7). It was also found that cucumber floral buds of all 3 sex types pass thirough a common bisexual stage, after which the further development of the anthers, the ovary, o'r neither, are repressed, leading to pistillate, staminate or hermaphrodite fl.owers, respectively (1). When floral buds were isolated at their bisexual stage, auxin directly effected their future sexual differentiation pattern (8). Thus, comparison must be made between auxin contents in the vicinity of equally developed floral buds of different sex types. After more than 3 decades of auxin determination in plants there is still no generally accepted method that had been proven to give reliable results. The choice of solvent us,ed fo!r extraction was recently discussed by Bentley (2). The information seems to indicate that short cold diethylether extracition of lyophilized material is acceptable, altho,ugh not ye.t critically tested. There are still claims (14) that even under such condition's, the conversion of tryptophan to IAA may take place. It was, therefore, decided 'to test the existence of ithis conversion prior to the use of the ether extraction for auxin determination. Anoither consideration was the efficiency 'of recovery of simall amounts of auxin from plant material. Vli,to's and Meudt (22) achieved fair recovery of mg quantities of IAA added to plant material, but Hamilton et al. (9) recovered less. Using IAA-2-C14 the latter showed a recovery of 15 % when roughly 2 ,ug IAA were added at the 'start of ithe extraction o'f the plant material. As we estimated even smaller amounts of auxin in our samples, the -efficiency of recovery was rega'rded as an inseparable part of the present investigation. In the present work the auxin content of 3 regions along [the main shoot of andromonoecious and hermaphro'dite cucumber plants was studied. This wa.s done in order t.o tes,t the hypothesis that sex determination in the embryonal floral bud is controlled by the endogenous auxin level, and that auxins promote ovary development.