Developmental responses of individual Onobrychis plants to spatial heterogeneity

An experimental study was undertaken to assess the extent to which better-situated branches correlatively inhibit other branches on the same plant which are growing in relatively inferior conditions. The experiments were carried out on naturally-grown Onobrychis squarrosa L. (Papilionaceae), a dominant annual plant of the Mediterranean region of Israel. Treatments were carried out (i) on young plants that only had a seminal shoot, and (ii) on plants after lateral branches had started growing. Differential shade conditions were achieved by the repeated removal of neighbours on one side of the plants. The plants were either left intact or damaged by removing different parts of their branch system. In intact plants, growth responses to the differential light conditions which were expressed by the number of branches, leaves and fruits showed sonic evidence for support of the shaded branches by the exposed branches on the same plant. In contrast, heterogeneous treatments of both shade and branch damaging led to development that reflected marked correlative inhibition of the shaded parts of the plant. The results show that a latent potential for correlative inhibition between branches exists throughout plant development and that in Onobrychis it can be readily realised when plants are damaged. This could depend on known traits of auxin acting as a correlative signal of growing branches. The observed responses of Onobrychis can be understood as an adaptive strategy which reflects the low reliability of environmental signals and the high cost of changing the course of development, especially in short lived annual plants.