Response of canola to different heat stresses

Canola (Brassica napus) is a valuable oilseed crop in Australia and many ot her countries. Environmental variability during seed development can presen t 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 degreesC being i mplemented in several minutes, whereas in normal field conditions, atmosphe ric temperatures usually increase over a number of days. Growth cabinet exp eriments 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 degreesC days and 16 degreesC nights. O ne treatment comprised a rapid increase to 40 degreesC, which was then main tained for 4 h on 5 successive days, giving a total of 15 degree-days of st ress (15 DD/40 degreesC). The second treatment comprised 5 days of stepwise increase in daily maximum temperature to 40 degreesC, giving a total of 45 degree-days above control conditions (45 DD/stepped). The 15 DD/40 degreesC heat treatment accelerated plant maturity and resulte d in drastically decreased seed weight, and therefore yield, compared with the control, whereas the 45 DD/stepped treatment had much less effect on th e 3 lines, particularly Oscar. Oil concentration of the seed was reduced fo llowing the 15 DD/40 degreesC treatment by a half in Range, a quarter in Mo nty, and a fifth in Oscar, in contrast to reductions of a third, a tenth, a nd nil following the 45 DD/stepped treatment. The 15 DD/40 degreesC stress was also associated with much greater increases in protein and glucosinolat e concentrations over control values than the 45 DD/stepped stress. The 15 DD/40 degreesC 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 treatm ent had no significant effect on fatty-acid composition. These results show that although the gradual temperature increase as implem ented in this experiment resulted in a greater degree-days heat load, it wa s not as detrimental to quality as a sudden increase. The cultivars varied in their responses to the heat load.