Response of canola to different heat stresses

Canola (Brassica napus) is a valuable oilseed crop in Australia and many other countries. Environmental variability during seed development can present important problems for the grower, particularly when that variability is in the form of high temperatures. In conventional experiments, heat stress is implemented suddenly, with a temperature increase of 20°C being implemented in several minutes, whereas in normal field conditions, atmospheric temperatures usually increase over a number of days. Growth cabinet experiments to determine the effects of contrasting heat stresses during seed development were conducted on 3 cultivars of canola, namely, Monty, Range, and Oscar. The control comprised 21°C days and 16°C nights. One treatment comprised a rapid increase to 40°C, which was then maintained for 4 h on 5 successive days, giving a total of 15 degree-days of stress (15 DD/40°C). The second treatment comprised 5 days of stepwise increase in daily maximum temperature to 40°C, giving a total of 45 degree-days above control conditions (45 DD/stepped). The 15 DD/40°C heat treatment accelerated plant maturity and resulted in drastically decreased seed weight, and therefore yield, compared with the control, whereas the 45 DD/stepped treatment had much less effect on the 3 lines, particularly Oscar. Oil concentration of the seed was reduced following the 15 DD/40°C treatment by a half in Range, a quarter in Monty, and a fifth in Oscar, in contrast to reductions of a third, a tenth, and nil following the 45 DD/stepped treatment. The 15 DD/40°C stress was also associated with much greater increases in protein and glucosinolate concentrations over control values than the 45 DD/stepped stress. The 15 DD/40°C treatment had a pronounced effect on fatty-acid composition, increasing saturated fatty-acid content and decreasing oleic acid content, in both Monty and Range but not in Oscar, whereas the 45 DD/stepped treatment had no significant effect on fatty-acid composition These results show that although the gradual temperature increase as implemented in this experiment resulted in a greater degree-days heat load, it was not as detrimental to quality as a sudden increase. The cultivars varied in their responses to the heat load. N. M B bs rdai N. Mu et ect la