Effects of Temperature and Water Potential on Germination, Radicle Elongation and Emergence of Mungbean

Controlled environment experiments were performed to determine the effects of temperature and water potential on germination, radicle elongation and emergence of mungbean (Vigna radiata (L.) Wilczek cv. IPB-M79-17-79). The effects of a range of constant temperatures (15–45 °C) and water potentials (0 to −2.2 MPa) on germination and radicle elongation rates were studied using an osmoticum technique, in which seeds were held against a semi-permeable membrane sac containing a polyethylene glycol solution. Linear relationships were established between median germination time (Gt50) and water potential at different temperatures, and between reciprocal Gt50 (germination rate) and temperature at different water potentials. Germination occurred at potentials as low as −2.2 MPa at favourable temperatures (30–40 °C), but was fastest at 40 °C when water was not limiting, with an estimated base temperature (Tb) of about 10 °C. Subsequent radicle elongation, however, was restricted to a slightly narrower temperature range and was fastest at 35 °C. The concept of thermal time was used to develop an equation to model the combined effects of water potential and temperature on germination. Predictions made using this model were compared with the actual germination obtained in a related series of experiments in columns of soil. Some differences observed suggested the additional importance of the seed/soil/water contact zone in influencing seed germination in soil. Seedling emergence appeared to reflect further the radicle elongation results by occurring within a narrower range of temperatures and water potentials than germination. Emergence had an estimated Tb of 12.6 °C and was fastest at 35 °C. A soil matric potential of not less than about −0.5 MPa at sowing was required to obtain 50% or more seedling emergence.