Climatic Factors Influencing Parthenocarpy and Normal Fruit-Set in Tomatoes

A study has been made of the influence of day and night temperatures, photoperiod, and light intensity upon the effectiveness of 2-naphthoxyacetic acid (2NA) for the setting of parthenocarpic fruit in the tomato, Lycopersicon esculentum, variety Essex Wonder. Results include a quantitative comparison of percentage set, yield, and average weight and size, for fruits from normally fertilized plants and from those sprayed with the growth substance. 2. In general, the conditions under which 2NA was most effective paralleled those under which the highest yields of normally fertilized fruits were obtained. 3. At very high day or night temperatures, especially when light intensity was low, 2NA was either poorly effective or failed completely to induce ovary development, even though small sets of fruits arose by natural fertilization. Under lower temperatures percentage setting was generally somewhat higher in 2NA-treated blossoms, while at very low temperatures there was an indication that 2NA may induce a higher set than would occur naturally. 4. Apparently 2NA was ineffective for inducing ovary development in the sterile, malformed blossoms differentiated under unfavorable conditions. 5. Under all conditions 2NA prevented the abscission of sprayed blossoms, even when fruit formation did not occur. 6. The average weights of spray-induced and normal fruits were not appreciably different. Spray-induced fruits, however, were generally larger at maturity and showed a tendency to hollowness, particularly when grown in low light intensity or short photoperiod. 7. A study of the conditions necessary for induction of natural parthenocarpy in unsprayed emasculated blossoms indicated that low temperatures and high light intensity are determining factors.