Functional Ecology 2000 14, 215 – 225 © 2000 British Ecological Society 215 Blackwell Science, Ltd Effects of herbivory on internal ethylene production and sex expression in Cucurbita texana G. A. KRUPNICK,* G. AVILA, K. M. BROWN and A. G. STEPHENSON Department of Biology, 208 Mueller Laboratory, The Pennsylvania State University, University Park, PA 16802, USA Summary 1. Field-grown Cucurbita texana was used in a study to determine if stress ethylene from damaged leaves promotes changes in sex expression. 2. Internal ethylene concentrations were experimentally enhanced by inserting an ethylene-filled syringe into the hollow chamber of an internode seven nodes from the growing tip of a branch. Branches enhanced with ethylene produced significantly more pistillate buds than control branches and experienced greater bud abortion later in floral development. 3. The timing and magnitude of Diabrotica beetle leaf damage was simulated using a paper-hole puncher (15% removed from all leaves). Simulated herbivory was applied to new growth every 3 days throughout the growing season. 4. Endogenous ethylene concentrations recovered from the internode adjacent to the growing tip were significantly greater in damaged plants than undamaged plants 1 day after damage to new growth during the second and third week of the experiment, and 2 days after damage during the fourth and fifth week. By the sixth week, no significant differences were evident. 5. Damaged plants produced fewer pistillate buds than undamaged plants, indicat- ing that stress ethylene from simulated herbivory does not induce femaleness. A second messenger that suppresses pistillate bud production may be produced during damage. Key-words: Insect damage, monoecious, phytohormones, reproductive biology, sink–source interactions Functional Ecology (2000) 14, 215 –225 Introduction Like other phytohormones, the gaseous hormone ethylene has multiple functions in plant growth and development (Abeles, Morgan & Saltveit 1992). For instance, ethylene is involved in the regulation of fruit ripening and sex expression, and as a response to her- bivore damage (Abeles et al. 1992). In a previous study, we found a link between two of these functions in Cucurbita texana (Scheele) A. Gray (Cucurbitaceae) – internal endogenous ethylene concentrations climb dur- ing fruit maturation and ethylene produced from mature fruits correlates positively with pistillate flower produc- tion at the tip of growing branches (Krupnick, Brown & Stephenson 1999). In short, the final stage of fruit development corresponds with the differentiation of additional pistillate floral buds by way of a hormonal signal. While the timing between fruit ripening and pistillate flower bud production may be an appro- priate way to separate competing resource sinks (i.e. fruit) adequately, stress-induced ethylene may be an inappropriate regulator for pistillate flower production because leaf herbivory may reduce the acquisition of resources and the amount of stored resources needed for successful fruit development and, in some circum- stances, it may divert resources away from reproduc- tion and towards inducible defenses (Baldwin, Sims & Kean 1990; Karban & Baldwin 1997). Nevertheless, if herbivory increases ethylene concentrations, stress- produced ethylene may also regulate the differentiation of additional floral buds. In this study, we examine whether the sex expression of C. texana responds to ethylene induced by herbivory in the same way that it responds to endogenously produced ethylene. Several studies have shown that ethylene regulates sex expression in many monoecious species including members of the Cucurbitaceae (Rudich 1990; for review). Studies have used exogenous growth regulators (e.g. ethephon, as an ethylene-releasing agent), ethylene biosynthesis inhibitors (e.g. aminoethoxyvinyl glycine) and ethylene action inhibitors (e.g. silver nitrate and silver thiosulphate) to show that ethylene promotes *Present address: Department of Botany, NMNH, Smithso- nian Institution, Washington, DC 20560 –0166, USA.