No abstract

An open, continuous flow system was used to investigate ethylene production during degreening of maturing seed of mustard (Brassica juncea cv Cutlass and cv Lethbridge 22A) and canola (Brassica napus cv Westar and cv Alto). lsolated mustard seed evolved higher amounts of ethylene than those of canola, and this was particularly evident both early in embryogeny and later during the desiccation phase of seed maturation. The silique walls produced negligible amounts of ethylene in both species. The concentrations of ethylene surrounding seed as they matured within siliques were significantly higher in mustard than in canola, and this interspecies difference was greatest during the seed desiccation phase. In mustard, a 4-fold increase in silique interna1 ethylene levels was apparent during desiccation. In comparison, only a moderate increase in silique-derived ethylene occurred in canola.

Chlorophyll (Chl) retention by mature seed of canola as the result of an early frost or other environmental factors (the “green seed problem”) causes serious economic losses. The relationship of seed degreening to rate of moisture loss by seed and silique and the role of ABA in this process were investigated as a function of developmental age. During the normal predesiccation stage (28–45 days after pollination), seed of Brassica napus (cv. Westar) loses Chl rapidly but seed moisture slowly. After a mild freezing stress, there is a rapid loss of moisture from silique walls, followed by accelerated loss of seed moisture. Chl degradation ceases at 35–45% seed moisture. ABA levels in silique walls of frozen plants (determined by enzyme‐linked immunosorbant assay) increased after freezing, apparently in response to moisture loss. In contrast, ABA levels in the seed increased dramatically 1 day after freezing, then decreased to control levels. The influence of the rate of seed moisture loss on Chl degradation was investigated by fast and slow drying of isolated seed under controlled humidity conditions. Seed dried rapidly at 22% RH retained most of its Chl, whereas seed dried slowly at 86% RH lost Chl as fast or faster than seed on control (unfrozen) plants. In all treatments, Chl loss stopped at about 40% seed moisture.

Canola is seeded when ambient temperatures are below the optimum. Therefore, we wished to determine whether certified seedlots displayed variation in low temperature germination potential and, thereafter, to study the effects of low temperature during germination and early seedling development. Canola (Brassiea napus L. cv. Westar) seed was tested for germination potential at 22, 10, 6, and 2 °C. Two seedlots, designated L and H, which represented the extremes in germination potential at 10 °C, were chosen for further study. Under the optimal conditions of 22 °C the two seedlots exhibited similar germination rates and early seedling growth. At 10 °C, L reached 95% germination within 8 d and displayed rapid seedling growth. This was associated with high isocitrate lyase activities and rapid mobilization of total lipid and protein reserves. Seedlot H, at 10 °C, did not reach a high percentage of germination until Day 12, and seedling growth was slow relative to that of L. Seedlot H had more gradual increases in isocitrate lyase activity, mobidization of storage lipid, and protein mobilization at 10 °C. Both seedlots germinated poorly at 6 and 2 °C. At 6 °C, L displayed higher isocitrate lyase activity and higher rates of lipid and protein mobilization than H. From these studies we have determined that low temperature has a deleterious effect on the germination of canola and this may be reflected in a loss or a delay of coordination in the mobilization of reserves. In addition, differences within the Westar cultivar exist with respect to germination under low temperature.