BIOPHYSICS OF THE INHIBITION OF THE GROWTH OF MAIZE ROOTS BY LOWERED TEMPERATURE

Roots of hydroponically grown maize (Zea mays cv LG11) have a greatly reduced growth rate at 5°C (0.02 millimeters per hour) compared with those at 20°C (1.2 millimeters per hour). Various physical parameters of roots growing at each temperature were compared. Turgor pressure of cells in the elongation zone increased from 0.59 ± 0.05 megapascal at 20°C to 0.82 ± 0.04 megapascal after 70 hours at 5°C; thus, growth rate was not limited by decreased pressure. On cooling, tissue plasticity (measured by Instron/tensiometer) decreased slowly over 70 hours. On rewarming to 20°C from 5°C, growth rate, turgor pressure, and tissue plasticity all returned concertedly to their original values over a period of days. However, immediately following cooling growth rate dropped rapidly from 1.8 to 0.12 millimeters per hour in 30 minutes but turgor pressure and tissue instron plasticity remained unchanged. A plot of turgor pressure against growth rate indicated that, following cooling from 30 to 15°C, the in vivo wall extensibility of the tissue was reduced by 75%. Yield threshold was unchanged. Cells whose expansion was arrested in the long-term cold treatment do not resume growth. Root growth recovers by the expansion of cells newly produced by the meristem. Cessation of extension growth is an effect on the individual expanding cell. Growth recovery is not a reverse of this effect but requires the generation of fresh cells.