Imidacloprid has little effect on growth or drought tolerance of citrus rootstock seedlings without pests

Abstract

Imidacloprid, a widely used systemic chloronicotinyl insecticide, has been credited with being able to increase plant growth and increase resistance to plant stress even in the absence of pest pressures. Greenhouse studies using imidacloprid and drought stress on 6-month-old seedlings of the citrus rootstocks Carrizo citrange (Carr) and Cleopatra mandarin (Cleo) were repeated in two consecutive years in the absence of pests. Imidacloprid was applied at the manufacturer’s recommended rate either as a soil drench (Admire 2F) or as a foliar spray (Provado 1.6F). Half of the plants were drought stressed and the other half of the plants were kept well-irrigated. After 10 to 12 weeks, leaf N, plant water status, and growth were determined along with leaf photosynthesis and leaf water use effi ciency (WUE) by gas exchange. In year 1, drought stress reduced leaf N but higher fertilizer rates in year 2 resulted in higher leaf N in drought stressed than in well-irrigated plants in both species. Imidacloprid had no effect on leaf N (percent dry weight), plant water status or total plant growth of either species in either year. Imidacloprid increased root growth but decreased total leaf area such that there was an increase in leaf thickness and an increase in leaf N when expressed on a leaf area basis. Imidacloprid increased leaf photosynthesis and WUE of both species in both years regardless of plant water status. Thus, imidacloprid changed growth allocation between roots and shoots and increased leaf dryweight per area, leaf photosynthesis, and WUE but these responses did not translate   any changes in drought tolerance or total plant growth.