Effect of heat sums and of heat applied separately to shoots and roots on flowering in potted Piceaglauca grafts
Two greenhouse experiments (1985, 1987) investigated the relationship between flowering and root activity in gibberellin A4/7 treated, potted Piceaglauca (Moench) Voss grafts subjected to heat (30 vs. 20 °C in 1985, 25 vs. 18 °C in 1987) applied separately to shoots and roots for 3 or 4 weeks during the period of cone-bud differentiation. Heat applied to roots significantly reduced the percentage of white-tipped roots during this period but had no effect on seed or pollen cones produced. Heat applied to shoots, which had no consistent effect on root activity, promoted flowering independent of root temperature. These results suggest that the flowering response to heat in P. glauca is mediated through the shoot, and not by retarding root activity as has been proposed. A third experiment (1988), in which entire grafts were exposed to heat treatments of 20, 25, or 30 °C for 5 or 10 h per day, disclosed a higher temperature requirement for maximizing pollen cones (10 h at 30 °C) than for maximizing seed cones (5 h at 20 °C). Male flowering was strongly correlated with the daily sum of degrees per hour above 10 °C.