Toward understanding the fast latex coagulation in Campanula spp. (Campanulaceae).

The plant most commonly known for producing latex is the Pará rubber tree, Hevea brasiliensis. There are, however, thousands of latex-bearing plant species, and these species exhibit a diverse array of different types of latex, each type in accordance with its producers' main selective pressure after injury. One key function of latex is to seal, but the most crucial necessities for wound sealing differ by the environment. For species growing in arid climates, for example, minimizing water loss is crucial whereas in tropical ecosystems a strong (chemical) defense against herbivores, parasites, and germs is of more imminent importance. This diversity of ecosystems and species' environments is mirrored by a respective diversity in latices' chemical compositions, material properties, and coagulation times. While some plant species solely rely on evaporation of water for their latex coagulation, the H. brasiliensis latex contains the coagulation assisting protein Hevein, allowing for coagulation in 30 min. With coagulation times of 10 s and below, species of the genus Campanula pose considerable challenge to the measurement of latex characteristics. We here present an overview to the coagulation of latex in the genus Campanula and reveal substantial differences to the latex coagulation of H. brasiliensis. For a collection of 6 different Campanula species, we determined coagulation times under different temperatures, latex dry weights, contact angles of water droplets on latex surfaces and imaged laticifer cross-sections using cryo-SEM. We found Campanula latex to coagulate significantly faster than Hevea latex and no evidence of Hevea-like lutoids in the laticifers. A coagulation test in a pressure chamber further revealed Campanula latex to coagulate at pressures of 8 bar, where latex coagulation in Ficus benjamina, which is described to have similar coagulation mechanism as Hevea, has previously been reported to be impaired. Our findings thus suggest Campanula latex coagulation to follow a different mechanism than the one described in Hevea.