Rubia peregrina L.: a stress resistant weed.

Rubia peregrina L. is a weed able to survive under extreme hydric and thermal stress conditions. Morphological and physiological studies were conducted in an attempt to understand the survival strategies used by this plant. Its main morphological characteristic is the presence of underground stems, partially red and partially yellow, which form a closely branching underground network. They can be considered to be stolons because of a lack of a thickened endodermis, a typical structure found in rhizomes. The yellow and red dyes are glycosil-alizarines. Ultrastructural and pigment studies showed that the yellow segments have starch-filled plastids and carotenoids, while the red segments have no starch reserves. The yellow structures have a higher water content and a correspondingly higher PEP-case (phosphoenolpyruvate carboxylase) activity than the other organs. The survival of the plant after hydric and thermal stress is due to several factors: low transpiration with good photosynthesis in the aerial part and the capacity to conserve water and to have large starch reserves in the subterranean portion. The reserves provide energy and substrates and probably also function as gravity receptors (statoliths). The PEP-case activity helps maintain turgor by means of the malate-cation relationship, and the alizarinic pigments perform an antiparasitic role. Under favorable conditions new buds emerge from the underground stems which then cover the infested area with branches and leaves.