In situ histochemical localization of alkaloids in leaves and pods of Prosopis ruscifolia

Abstract

The tree Prosopis ruscifolia produces indolizidine alkaloids which function in plant defense as antimicrobials. Here, we propose the use of Drangendorff´s reagent, picric acid and lugol for the in situ histochemical localization of indolizidine alkaloids in the leaf parts (petiole, rachis and leaflet) and pods of P. ruscifolia. The main surface and internal anatomical features of the aerial plant parts were visualized with cresyl violet and astra blue safranin. Extracts from leaflets and pods were separated by TLC and revealed with the Dragendorff´s reagent, picric acid and lugol. Fresh cross sections of leaflets, petioles, rachis and pods were also stained with these chromogenic reagents. Glandular trichomes were absent from the surface of mature leaflets, petioles, rachis and pods. Large idioblasts were distributed in the palisade mesophylls of the leaflets and the cortical and medullar parenchyma of the petiole and the rachis. TLC of the extracts revealed with Dragendorff´s reagent, picric acid and lugol gave orange, yellow and light brown spots for the indolizidine alkaloids, respectively. Spraying with picric acid also revealed some non-alkaloid constituents. The three chromogenic reagents indicated the presence of indolizidine alkaloids in the large idioblasts of the leaflets, petioles, rachis and pods. Dragendorff´s reagent and picric acid also showed the alkaloids in the cuticles of all leaf parts and pods. The first was the most sensitive towards alkaloids and the only one that visualizes them in the phloem of the petioles and pods. The indolizidine alkaloids of P. ruscifolia seemed to be transported from green tissues to the pods, and should exert their protective function after the breakdown of leaves and pods.