Chemical Induction of Male Sterility in Cucurbits.

Male sterility in crop plants is receiving increased attention for its usefulness in the production of hybrid seed. A chemical method of inducing male sterility would have great merit in certain cucurbits where daily hand removal of staminate flowers on a large scale is economically prohibitive and the maintenance of naturally occurring sterile lines is extremely difficult. This primarily monoecious family presents a special challenge in that flower sex expression has been reportedly altered by chemical treatment (1-3). The chemical suppression of all male flowers in cucurbits would be a means of producing purely female plants. Preliminary studies were conducted in the greenhouse with cucumbers (Cucumis sativus, vars., National Pickling and Burpee Hybrid) and squash (Cucurbita pepo, var. Table Queen). By spraying young cucumber plants after 2 to 3 true leaves had formed with 100 ppm of a-naphthaleneacetic acid (NA) or 25 ppm of 2,3,5-triiodobenzoic acid, the ratios of staminate to pistillate flowers were reduced from approximately 23: 1 to 8: 1 in National Pickling and from 14: 1 to 2: 1 in Burpee Hybrid. Results were comparable to those reported by Laibach and Kribben (1), in which the decreases in ratios were accompanied by both an increase in the number of pistillate flowers and a decrease in the number of staminate flowers. NA (100 ppm) applied to Table Queen squash when the first true leaf had fully expanded resulted in an average decrease in staminate-pistillate flower ratio from 1.47: 1 to 0.4: 1, with occasional plants producing no staminate flowers. Subsequent field studies (1953) with Table Queen squash revealed that NA (100 ppm) applied as a spray when the seedlings had 1 to 2 true leaves and the treatment repeated 10 days later (4 to 5 true leaves) delayed the appearance of staminate flowers, and treated plants produced exclusively pistillate flowers for 8 days prior to the opening of any staminate flowers. During the fall, winter, and spring months (195354), additional chemicals, including maleic hydrazide (4), were evaluated. Table Queen squash (Stock No. D0421, Ferry-Morse Seed Co.) was used as the test plant (2) and was grown in the greenhouse at 68°F night temperature and a photoperiod of 16 hr. Such an environment normally favored the early production of staminate flowers followed by the presence of both staminate and pistilate types, as described by Nitsch et al. (2). A remarkable suppression of staminate flower buds was obtained with maleic hydrazide (MH); the selective inhibition was comparable to that reported by Moore (5) and Naylor (6) for maize. Several spray patterns successfully induced male sterility during the 3to 4-wk interval that each crop was allowed to flower in the greenhouse. Dipping or spraying the plants in a solution containing 250 or 350 ppm of MH when the first true leaf was expanding and followed by a second treatment when 4 to 5 true leaves had developed resulted in plants that produced the usual number (8 to 10) of pistillate flowers in normal spatial arrangement, with no staminate flowers. Characteristic flowering patterns following treatment showed only rudimentary staminate flower buds or bud initials at the nodes in which they normally occurred. In other instances nonfunctional staminate male flower buds developed in which the sepals were abnormally enlarged (Fig. 1B) and some flowers eventually opened, but the androecia failed to develop (Fig. 1D), having aborted in a manner similar to that of the male-sterile mutant in winter squash described by Scott and Riner (7). It was further found that a single application of MH (250, 350, or 500 ppm), applied when 1 to 2 true leaves had developed, resulted in a high percentage of plants that produced only pistillate flowers.