Responses of soybean to oxygen deficiency and elevated root-zone carbon dioxide concentration.

Root flooding is damaging to the growth of crop plants such as soybean (Glycine max L.). Field flooding for 3 d often results in leaf chlorosis, defoliation, cessation of growth and plant death. These effects have been widely attributed solely to a lack of oxygen in the root-zone. However, an additional damaging factor may be CO(2), which attains levels of 30 % (v/v) of total dissolved gases. Accordingly, the effects of root-zone CO(2) on oxygen-deficient soybean plants were investigated in hydroponic culture. Soybean plants are shown to be very tolerant of excess water and anaerobiosis. No oxygen (100 % N(2) gas) and low oxygen (non-aerated) treatments for 14 d had no effect on soybean survival or leaf greenness, but plants became severely chlorotic and stunted when the roots were exposed to no oxygen together with CO(2) concentrations similar to those in flooded fields (equilibrium concentrations of 30 %). When root-zone CO(2) was increased to 50 %, a quarter of soybean plants died. Those plants that survived showed severe symptoms of chlorosis, necrosis and root death. In contrast, rice (Oryza sativa L.) plants were not affected by the combination of no oxygen and elevated root-zone CO(2.) A concentration of 50 % CO(2) did not affect rice plant survival or leaf colour. These results suggest that the high susceptibility of soybean to soil flooding, compared with that of rice, is an outcome of its greater sensitivity to CO(2).

Molecular characterization of the soybean alcohol dehydrogenase gene family amplified in vitro by the polymerase chain reaction.

Soybean (Glycine max) alcohol dehydrogenase (ADH) cDNAs were amplified in vitro from total RNA by the polymerase chain reaction (PCR). The amplification strategy involved first strand cDNA synthesis from anaerobic cotyledon total RNA using an 18-thymidine primer. The second strand cDNA primer was a conserved sequence near the 5' end of known plant ADH transcripts. The PCR products were ligated into a plasmid vector and unique clones were isolated on the basis of size and restriction pattern. Sequence analysis revealed three distinct classes of soybean ADH cDNAs, all of which showed high homology to Adh genes from maize and peas. RNA blot hybridization analyses showed differential expression patterns for these genes. One gene, expressed constitutively in all seedling organs, was inducible by anaerobiosis, one gene was expressed only in anaerobic organs, and the third gene was expressed predominantly in anaerobic roots.

Postanoxic Injury in Soybean (Glycine max) Seedlings.

The postanoxic injury, also known as reperfusion injury, is associated with the returning of anoxic tissues to normal atmosphere. Using tetrazolium chloride staining, ATP content, and seedling growth rate as indicators, we found that postanoxic injuries in soybean (Glycine max) seedlings were more severe after 1 and 2 hours of anoxia than after longer anoxic durations (3 to 5 hours). Anaerobic incubation of root tips in the presence of 100 mm ascorbate, an antioxidant and free radical-scavenging compound, alleviated the postanoxic injury associated with the short durations of anoxia. Extracts from soybean seedling roots returned to air from 1 hour of anoxia had an elevated capacity to produce superoxide radicals over extracts from postanoxic roots stressed for 3 or 5 hours. Activity of superoxide dismutase in soybean roots returned to air from 1 and 2 hours of anoxia was 30 to 50% lower than activities in roots returned to air from 5 hours of anoxia. Superoxide dismutase-specific transcripts were also lower in postanoxic roots stressed for 1 hour than in roots stressed for longer anoxic durations. The evidence suggested that the postanoxic injury of soybean roots after a short anoxic stress was associated with an increased superoxide radicals production capacity coupled with a reduced superoxide dismutase activity. Periods of anoxia of at least 3 hours were necessary for soybean seedlings to develop the ability to cope with postanoxic stress.

Abscisic Acid induces anaerobiosis tolerance in corn.

Flooding is a frequently occurring environmental stress that can severely affect plant growth. This study shows that treatment of corn (Zea mays L.) seedlings with abscisic acid (ABA) increases their tolerance to anoxia 10-fold over untreated seedlings and twofold over seedlings treated with water. Corn seedlings stressed anoxically for 1 day showed only 8% survival when planted in vermiculite. Pretreatment of root tips with 100 micromolar ABA or water for 24 hours before the 1 day anoxic stress increased the anoxic survivability of seedlings to 87% and 47%, respectively. Cycloheximide (5 milligrams per liter), added together with ABA, reduced the seedling survival rate, indicating that the induction of anoxic tolerance in corn by ABA was partly a result of the synthesis of new proteins. ABA treatment induced a threefold increase in alcohol dehydrogenase enzyme activity in corn roots. However, after 24 h of anoxia, alcohol dehydrogenase enzyme activity between the ABA-pretreated and non-pretreated corn roots was not significantly different. The results indicated that ABA played an important role in inducing anoxic tolerance in corn and that the induced tolerance was probably mediated by an increase in alcohol dehydrogenase enzyme activity before the anoxic stress.