Ethylene biosynthesis in oilseed rape pods in relation to pod shatter

Abstract

ethylene to trigger the process when IAA levels are low. Ethylene production was studied during the develop- ment and senescence of seeds and pericarp tissues Key words: Oilseed rape, pod shatter, ethylene metabolism, of oilseed rape (Brassica napus L.) pods (siliquae). In aminoethoxyvinylglycine (AVG), parthenocarpy. the course of the rise to a pre-senescence climacteric, little change in 1-aminocyclopropane-1-carboxylic acid Introduction (ACC) was recorded in the seeds, indicating a rapid conversion to ethylene. In contrast, very small Seed loss from fully mature pods (pod shatter) is a amounts of ethylene were produced by the pod wall problem with monocultures of dicotyledonous crop (PW) tissues, which included the dehiscence zone (DZ), species with an extended flowering period and many- while levels of free and conjugated ACC in the PW seeded, dry fruits, such as Brassica spp., soybean (Glycine increased consistently. As climacteric ethylene pro- max L.), and sesame (Sesame indicum L.). Pod shattering duction by the seeds declined, biosynthesis of ethy- is a particular problem in oilseed rape (Brassica napus L.) lene by the PW increased. Effects of reducing ethylene because of a marked tendency of fully mature pods to production by various means were examined in relation open. Average annual losses of about 20%, represent a to cell separation in the dehiscence zone. Amino- production eYciency that is far less than in any other ethoxyvinylglycine (AVG) applied during the pre- major arable crop. There is little variation in the suscepti- senescence climacteric reduced ACC levels and bility of current cultivars to pod shatter (Downey and ethylene production by the seeds, but did not affect Robbelen, 1989), and conventional breeding has so far subsequent values in the PW. The production of been unable to produce new cultivars which are more b-1,4-glucanase and the separation of the cells of the resistant to opening. DZ were delayed for 3-4 d by AVG, but the force The oilseed rape pod (silique or siliqua) consists of two required to open fully mature pods was unaltered. In carpels whose walls are separated by a replum containing parthenocarpic (seedless) pods, ethylene was pro- the main vascular tissue of the pod. A layer of 2-3 duced during senescence. Cell separation in the DZ parenchymatous cells between the carpel edges and the took place as in seeded pods, although it was also replum forms the separation or dehiscence zone (DZ ). delayed by 3-4 d. The results are related to changes Separation of the cells in the DZ takes place about 7 in indole-3-acetic acid (IAA) levels in oilseed rape pods weeks after anthesis, just before moisture loss is complete. which decline in PW and DZ tissues during senes- The pods open as a result of external force supplied by cence. It is concluded that separation in the cells of contact with other pods, racemes or harvesting machinery, which severs the vascular connections. the dehiscence zone requires only small amounts of