Variation in the reaction of Stylosanthes guianensis lines to radiation frosts in controlled environments

The reaction of six lines of S. guianensis to radiation frosts was studied in controlled environments. At frost temperatures of -6.1" and -4.6"C (measured at 2.5 cm height) plant survival in the three fine-stem stylo (FSS) lines was superior to that in the three robust (R) lines, in agreement with field observations. Leaf damage was severe on all lines in frosts where leaf temperatures were -2.4" or lower. In frosts with leaf temperatures of - 0.8" and - 1.3", leaf damage ranged from 0 to 37 % and the FSS lines showed slightly less leaf damage than the R lines. Exposure to a hardening regime of low temperature and short days did not decrease the leaf damage to either an ~ss or an R line, but substantially improved plant survival in the ~ss line. Three small F2 populations of crosses between ~ss and R lines showed good survival after severe frosts, with values approaching those of the ~ss parents. Plant survival after severe frosts of both parents and F2 populations was negatively related to the height above the soil surface of the lowest buds. Because temperature in radiation frosts increases as the soil surface is approached, development of buds at or below the soil surface appears to represent an effective adaptation for frost avoidance. Direct selection for height of the lowest bud is suggested as an efficient procedure for developing lines able to survive severe frosting in the field.