Effect of Rht Alleles on the Tolerance of Wheat Grain Set to High Temperature and Drought Stress During Booting and Anthesis

Factorial pot experiments were conducted to compare the responses of GA-sensitive and GA-insensitive reduced height (Rht) alleles in wheat for susceptibility to heat and drought stress during booting and anthesis. Grain set (grains/spikelet) of near-isogenic lines (NILs) was assessed following three day transfers to controlled environments imposing day temperatures (t) from 20 to 40 degrees C. Transfers were during booting and/or anthesis and pots maintained at field capacity (FC) or had water withheld. Logistic responses (y=c/1+e(-b(t -m))) described declining grain set with increasing t, and t(5) was that fitted to give a 5% reduction in grain set. Averaged over NIL, t(5) for anthesis at FC was 31.7 +/- 0.47 degrees C (S.E.M., 26 d.f.). Drought at anthesis reduced t(5) by <2 degrees C. Maintaining FC at booting conferred considerable resistance to high temperatures (t(5)=33.9 degrees C) but booting was particularly heat susceptible without water (t(5)=26.5 degrees C). In one background (cv. Mercia), for NILs varying at the Rht-D1 locus, there was progressive reduction in t(5) with dwarfing and reduced gibberellic acid (GA) sensitivity (Rht-D1a, tall, 32.7 +/- 0.72; Rht-D1b, semi-dwarf, 29.5 +/- 0.85; Rht-D1c, severe dwarf, 24.2 +/- 0.72). This trend was not evident for the Rht-B1 locus or for Rht-D1b in an alternative background (Maris Widgeon). The GA-sensitive severe dwarf Rht12 was more heat tolerant (t(5)=29.4 +/- 0.72) than the similarly statured GA-insensitive Rht-D1c. The GA-sensitive, semidwarfing Rht8 conferred greater drought tolerance in one experiment. Despite the effects of Rht-D1 alleles in Mercia on stress tolerance, the inconsistency of the effects over background and locus led to the conclusion that semidwarfing with GA-insensitivity did not necessarily increase sensitivity to stress at booting and flowering. In comparison with effects of semidwarfing alleles, responses to heat stress are much more dramatically affected by water availability and the precise growth stage at which the stress is experienced by the plants.