DAILY CHANGES IN THE RELATIVE WATER-CONTENT OF VELVETLEAF (ABUTILON-THEOPHRASTI MEDIC) MAY EXPLAIN ITS RHYTHMIC SENSITIVITY TO BENTAZON

In order to explain the rhythmic sensitivity of velvetleaf (Abutilon t heophrasti Medic.) to bentazon, daily changes in plant physiological p arameters were studied. Total Chl content, Chla/Chlb ratio and leaf re lative water content (RWC) oscillated in a circadian fashion. Cosinor analysis of the data indicated an acrophase for leaf Chl content in th e middle of the light span, which was synchronous with the minimum Chl a/Chlb ratio and leaf RWC. The amplitude of leaf RWC oscillations was greater in lower leaves than in upper ones. Decreases in RWC due to wa ter stress were first observed in lower (i.e., older) leaves. Monitori ng photosynthesis after spraying the plants with bentazon indicated a protective effect of brief water stress. A correlation was found betwe en leaf RWC and the rhythmic changes in sensitivity. The acrophase of the RWC oscillations corresponded to that of bentazon sensitivity. The results suggest that the naturally occurring periodic water-stress in the middle of the light span could be responsible for the variable se nsitivity to the herbicide, possibly via lower uptake of the herbicide at the cuticular level.