The effect of soaking injury in seeds of Phaseolus vulgaris L. on germination, respiration and adenylate energy charge

Submerging Phaseolus vulgaris cv. Top Crop seed in air-saturated water for 16 h markedly depresses subsequent germination. This is termed soaking injury. The postulate by Norton (1986) that soak-injured seeds merely run out of available energy was investigated. Soaking in air-saturated water reduced the total respiration (V(T); in terms of O2 uptake) of excised axes and cotyledons resulting in lower ATP levels. The contribution of both the mitochondrial cytochrome respiratory pathway (V(CYT)) and the alternative respiratory pathway (V(ALT)) to the total respiration (V(T)) was significantly lower in these axes. However, the adenylate energy charge (AEC) values for both axes and cotyledons excised from soak-injured seeds did not drop below 0.6, the threshold value indicating death of plant material. Differences between the ultrastructure of mitochondria in the radicle tips of axes from unsoaked and water-soaked seeds were observed. However, the capacity to operate the Krebs cycle was similar in all axes and soak-injured seeds were still capable of germinating when the testas were removed or the seeds dried. This means that axes contained a sufficient amount of carbohydrates and potential energy to germinate after soaking. Moreover, soak-injured seeds produced ethanol at a rate which was five times higher than that of unsoaked control seeds, both during the soaking and post-soaking periods. Together with the low rate of oxygen consumption by soak-injured seeds during the same soaking and incubation time, it appears that soak-injured seeds lack the potential to switch from an anaerobic to an aerobic respiration pattern causing low ATP levels in these axes. It is speculated that the latter seems to be the result of limited pyruvate supply, probably due to accelerated and prolonged fermentation or futile combustion through other pathways or both.