Response of three Brassica species to high temperature stress during reproductive growth

The effect of short periods of high temperature stress on the reproductive development and yield of three Brassica species were studied in a growth ch amber experiment conducted for 2 yr. Two genotypes from Brassica juncea L. and one each from B. napus L. and B. rapa L. were grown under day/night tem peratures of 20/15 degreesC till early flowering or early pod development, subjected to high temperature stress of 28/15 degreesC or 35/15 degreesC fo r 7 d and then allowed to recover at 20/15 degreesC. Species differed in op timum temperatures, with B, juncea and B, rapa having higher optimum temper ature than B, napus. Dry matter was unaffected by moderate temperature stre ss, while it was reduced by high temperature stress. The 35/15 degreesC tre atment was injurious to reproductive organs at different developmental stag es of all three species. High temperatures at flowering affected yield form ation more than high temperature at pod development. On the main stem, mean seed yield reduction due to heat stress was 89%, but partial compensation by pods on the branches reduced mean per-plant seed yield decrease to 52%. Reduction in fertile pods (not total pod number), thousand seed weight and seeds per pod were responsible for the reduced seed yield. Brassica rapa wa s more sensitive to heat stress than B. napus and B. juncea. Although obser vation did not indicate the exact developmental phase when the reproductive organs were susceptible to heat stress, pods that passed a critical thresh old developmental phase tolerated heat stress, which explained the smaller effect of high temperature stress at pod development. A direct temperature effect on reproductive organs appeared to be responsible for the reduction in yield. All genotypes began to recover from the stress by continuing flow ering after returning to 20/15 degreesC. Brassica napus was least able to r ecover from severe stress at flowering, as evidenced by the formation of ma ny abnormal pods during recovery. Per-plant yield response of canola-qualit y B. juncea line J90-4316 was similar to oriental mustard Cutlass. Thus, he at stress effect depends on the growth stage of canola and mustard and Bras sica species differ in heat stress response.