Аннотация:Hypoxia in germinating legume seeds
Development of primary imbibition injury and hypoxia under seed coat were investigated in aged pea seeds of different quality. Measurement of room temperature phosphorescence of individual air-dry pea seeds was used to subdivide a seed lot with 56% germinability into three fractions (sub-lots): viable seeds of fraction I (producing normal seedlings), viable seeds of fraction II (either producing abnormal seedlings with disturbed growth due to impaired cell division, or the seeds did not germinate at all because radicle of embryo axis could not penetrate through the seed coat) and dead seeds of fraction III. Hypoxia beneath the seed coat was detected by porphyrin phosphorescence method during seed imbibition. Higher level of porphyrin phosphorescence means higher oxygen deficiency. The seeds of fraction I showed either weak porphyrin phosphorescence or its absence. Phosphorescence of dead (fraction III) seeds did not rise during first four days of imbibition.
In seeds of fraction II, phosphorescence appeared after 12-14-h imbibition, when seed MC exceeded 45-50%, and subsequently increased with time. The rate of water uptake by seeds was higher in fraction II then in fraction I seeds and oxygen uptake by fraction II intact seeds was approximately twice as that in fraction I at equal imbibition time (20 h). A high respiration rate and oxygen supply limited by seed coat of fraction II induced hypoxia in the seed embryo. If porphyrin phosphorescence of some seeds of fraction II increased during 24-48 h of imbibition to a high level, such alive seeds did not germinate at all (perished from “suffocation” caused by the fermentation products). If porphyrin phosphorescence level was not so high, these seeds germinated but predominantly gave rise to the abnormal seedlings defective in cell division.
As DNA replication occurred within this time period, hypoxia did not influence the processes involved in preparation of meristematic cells for division. Since chemiluminescence measurements indicated an increase in reactive oxygen species (ROS) generated by embryo axes in post-hypoxic time period, it was proposed that post-hypoxic oxidative stress was the cause for the damage of cell division and appearance of abnormal seedlings from fraction II seeds.
The lethal primary imbibition injury occurs at contact of dry seed with liquid water. These damages can be prevented when seeds were allowed to imbibe without direct contact with water (for example, between two layers of filter paper). But in seeds of fraction II, hypoxia develops in this case as well. To reduce the rate of seed hydration, PEG 6000 solution and 4-(chloromercuri)benzoic acid were used. Mercury ions are known to block special water canals of cell membranes – aquaporins. Delayed seed water uptake prevented the development of primary lethal imbibition damage and delayed the hypoxia appearing. In these cases, some seeds of fraction II produced the normal seedlings, and seeds perished from “suffocation” declined in number.
The phenomenon of hypoxia under seed coat during imbibition was observed in other legume seeds, namely soybean, bean and haricot beans, by porphyrin phosphorescence as well.