The formation of rubber-producing cortical parenchyma cells in guayule ( Parthenium argentatum Gray) by low temperature

Abstract

Exposure of guayule plants to low temperature promotes the development of rubber-producing undifferentiated cortical parenchyma. Electron photomicrographs of cross-sections 1 cm below the stem tip of guayule plants grown at 27–32 °C days and 7 °C nights with a 16 h photoperiod for 6 months showed cortical parenchyma containing micro-vacuoles appressing a wide band of cytosol embedded with rubber particles and mitochondria against a thick 2° cell wall. The undifferentiated cortical parenchyma did not contain a central vacuole, tonoplast or parietal cytosol. Mature cortical parenchyma were developed at 1 cm from the stem tip in plants grown in the greenhouse at 27–32 °C days and 21–24 °C nights with a 16 h photoperiod for 6 months. These results demonstrates that the rubber-producing undifferentiated cortical parenchyma cells were promoted as a result of the exposure of the guayule plants to the low night temperature and not the result of plant age. Electron photomicrographs of cross-sections 3 cm from the stem tips of plants grown through the high temperatures of the summer of the Chihuahuan Desert and sampled in September showed mature cortical parenchyma with a large defined central vacuole, tonoplast and a thin layer of parietal cytosol. The cortical parenchyma in a cross-section of stems 3 cm from the tip from plants exposed to the high temperature of the summer and low temperature of the fall and winter of the desert showed immature cells with a high population of rubber particles in the cytosol that nearly fill the cell. There was a small central space in the cytosol indicating an early stage of the development of a central vacuole, but no tonoplast or parietal cytosol were present. High magnification photomicrographs showed the cytosol was disorganized and digested. Cortical parenchyma in cross-sections at the base of the stems of plants growing through the high summer temperatures of the desert and sampled in September showed a well developed central vacuole with tonoplast and a thin layer of parietal cytosol but only a few rubber particles. In contrast, the cortical parenchyma at the base of the stems of plants exposed to high temperature of the summer and low fall and winter temperatures of the desert, although more mature than parenchyma 3 cm from the tips of the stems, did not contain a central vacuole, tonoplast or parietal cytosol. The developing central vacuole in the center of the cell was filled with rubber particles in a digested cytosol. Measurements of rubber synthesis in the field grown guayule plants from September to March increased 97.4% while growth of the plants from August to March increased 81.1%, demonstrating that rubber synthesis increases as new growth and new rubber-producing cells were produced.