Comparative study on plant latex particles and latex coagulation in Ficus benjamina, Campanula glomerata and three Euphorbia species

Abstract

Among latex-producing plants, mainly the latex of Hevea brasiliensis has been studied in detail so far, while comprehensive comparative studies of latex coagulation mechanisms among the more than 20,000 latex-bearing plant species are lacking. In order to give new insights into the potential variety of coagulation mechanisms, the untreated natural latices of five latex-bearing plants from the families Euphorbiaceae, Moraceae and Campanulaceae were visualised using Cryo-SEM and their particle size compared using the laser diffraction method. Additionally, the laticifers of these plants species were examined in planta via Cryo-SEM. Similar latex particle sizes and shape were found in Ficus benjamina and Hevea brasiliensis. Hence, and due to other similarities, we hypothesize comparable, mainly chemical, coagulation mechanisms in these two species, whereas a physical coagulation mechanism is proposed for the latex of Euphorbia spp. The latter mechanism is based on the huge amount of densely packed particles that after evaporation of water build a large surface area, which accelerates the coagulation procedure.

Figure 1. Laticifer types of plant species used in this study.

Furthermore – as a comparison – the laticifer type of Hevea brasiliensis is included as well. Details of the systematic position of plant species, such as family and subfamily, are partially enclosed , , .

Figure 2. Phylogenetic relationship of the plant species used in this publication.

Species studied belong either to the families of Euphorbiaceae, Moraceae or Campanulaceae. The orders corresponding to the species studied are highlighted in bold. Adapted from .

Figure 3. Particle size distributions of the fresh latex of Ficus benjamina and of the three Euphorbia species.

On the ordinate, the percentage of the volume of all particles within the respective size class related to the total volume (added volume of all particles in all size classes), is given. The latex was diluted in water and ultrasonic treatment was applied to dissolve loose aggregates before measuring. Fresh, uncoagulated latex of Campanula glomerata could not be measured, as it coagulates irreversibly before or while stirring in the dispersion unit. As a comparison, the particle size distribution of purified H. brasiliensis latex (data taken from Cornish & Brichta (2002) [27]) was added. The small inserted box shows the particle size distribution of F. benjamina latex when transformed to the number mode.

Figure 4. Cryo-SEM images of latex from Euphorbia characias and Ficus benjamina.

Latex droplets were used both immediately after collecting (a, d) and as soon as they were coagulated (b including details at higher magnification, e). Pictures were taken from central regions of latex droplets (a, b, d, e). Latex particles of similar sizes can also be found in planta in laticifers of these plants (c including details at higher magnification, f).

Figure 5. Fractures in latex droplets of Ficus benjamina.

The solid latex particles (e.g. black arrow in a) are not destroyed by fractures. Instead, fractures run around the latex particles (a, c). However, the larger and collapsed structures (e.g. white arrow in a) are divided by fractures (b) (Cryo-SEM images).

Figure 6. Laticifers in Campanula glomerata petiole (Cryo-SEM image).

Figure 7. Longitudinal fracture of a laticifer in a shoot axis of Ficus benjamina.

Latex particles are abundant in the laticifer, but are absent in the surrounding cells (Cryo-SEM image).