Reversible Leaflet Movements in Relation to Drought Adaptation of Cowpeas, Vigna unguiculata (L.) Walp

Abstract

Two aspects of the heliotropic responses of leaflets of V. unguiculata cv. California Blackeye No. 5 were studied: the environmental conditions responsible for the responses, and the significance of the leaflet orientation responses for adaptation.

In the field under drought conditions, leaflets were observed to sun-track (diaheliotropism) in the morning and sun-avoid (paraheliotropism) by becoming more vertical in the afternoon. Overnight irrigation of droughted plants in the field resulted in leaflet movements on the day after irrigation similar to those of the well watered plants. Leaflets of well watered plants were diaheliotropic during the morning and evening, and essentially horizontal during the late morning and afternoon.

Controlled-environment studies confirmed field observations of drought effects on leaflet movement and showed that leaflet orientation responses are reversible within 3 h after irrigating droughted plants. Temperature variations similar to those experienced in the field had only small effects on leaflet orientation.

Vertical leaflet movement affected canopy architecture and significantly reduced percentage cover at midday. Manually moving the vertical leaflets of droughted plants caused substantial changes in leaflet temperatures, indicating that leaflet movement could substantially reduce heat load, water use, and water deficits in cowpeas.