Sex Expression of Flowers of the Proximal Nodes on the Lateral Shoots in Cucumber Plants: Influence of Apical Dominance

Abstract

In most cucumber cultivars, the proximal nodes (especially the first node) of the lateral shoots stably bear pistillate flowers and this is of great practical importance in the“pinching cultivation of cucurbits”where, to achieve yield of pistillate flowers from the proximal nodes, the primary lateral shoots are pinched, leaving only two or three nodes to enable the secondary lateral shoots to outgrow. The present investigation was carried out to clarify the mechanism of sex determination on the proximal nodes of the lateral shoots. The results, summarized below, indicate that the rigid sex expression on the proximal nodes of the lateral shoots appears under the influence of apical dominance.
1. The first node of the lateral shoots, which developed on up to the fifteenth node on the main shoots, stably produced pistillate flowers in all four cultivars of different genotypes, but the formation of pistillate flowers on the second to fifth node of the lateral shoots declined dramatically in three monoecious cultivars, except for a gynoecious cultivar, ‘Higan-fushinari’. However, a number of the lateral shoots which developed on the proximal location, the second to third node of the main shoots, produced staminate flowers even on the first node in monoecious cultivars.
2. When the lateral shoots of a monoecious cultivar, ‘Otone No. 1’were released from apical dominance as early as possible by pinching the main shoots under a stereoscopic microscope, the first node of the uppermost lateral shoots formed no pistillate flowers, although pistillate flowers are usually borne on those nodes in nonpinched plants. The production of pistillate flowers on the first node of the lateral shoots increased as the location of the node of the lateral shoots became farther from the pinching site on the main shoot.
3. When the main shoots of a monoecious cultivar, ‘Natsusairaku No. 3’were pinched just above the fifth, tenth or fifteenth node as early as possible, a number of the uppermost lateral shoots produced staminate flowers on the first node, whereas those of nonpinched plants regularly produced pistillate flowers. When pinching just above the fifth node was delayed until immediately after the fifth, tenth or fifteenth node became visibly detectable, there was a less pronounced effect on the production of staminate flowers on the first node of the uppermost lateral shoots.
4. A foliar spray of GA₄₊₇ and AgNO₃ to‘Otone No. 1’ at the 3-leaf stage caused the production of staminate flowers on the first node of the lateral shoots which had developed on the sixth to twelveth node of the main shoots, whereas pistillate flowers were produced on all those nodes in control plants. Silver ions had a greater effect on the modification of sex expression and also caused a substantial production of bisexual flowers on the first node of the lateral shoots.
5. Simultaneous treatment of GA₃ or CEPA with pinching strongly modified the pinching effect on sex expression. GA₃ promoted the pinching effect, increasing the production of staminate flowers on the first node of the uppermost lateral shoots, while CEPA increased the production of pistillate flowers, nullifying the pinching effect.