Effects of Submergence on Rice Seedlings
When 4 day old rice seedlings were entirely submerged in water, a substantial reduction in the rate of root growth, with shrinking and curling, became apparent within 24 h. Prolonged exposure to submergence (72 h) suppressed the growth of roots and shoots, but stimulated elongation of coleoptiles. The adenylate kinase activity in shoots of submerged plants was elevated at 72 h compared to that in unsubmerged control plants. The concentration of soluble protein in submerged plants showed a tendency to be decreased compared to controls, and both values slightly declined with incubation. Estimated total adenylate kinase activity in each shoot also indicated enhanced activity in submerged seedlings at 72 h. Adenylate kinase activity in each organ was measured after submergence for 72 h, and relative induction against control plants was calculated. Adenylate kinase specific activity was enhanced in every organ by submergence. The comparison of total activity in each organ indicated that strong enhancement was shown in coleoptile and endosperm.
Effects of Partial Submergence and N2 Gas on Adenylate Kinase Activity
To determine whether the induction of adenylate kinase activity by submergence was due to oxygen deficiency or to some other aspect of submergence, rice seedlings were exposed to partial submergence or to nitrogen gas. The adenylate kinase activity in partially submerged seedlings was similar to that in aerobic seedlings (control). However, nitrogen gas alone stimulated adenylate kinase activity, which reached a maximum 72 h after the start of treatment. The changes in protein concentrations were similar in all seedlings.
Adenylate Kinase mRNA Accumulation in Rice Seedlings by Submergence
To examine the level of adenylate kinase mRNA in response to submergence, Northern blot analysis was carried out using adenylate kinase cDNA as a probe. The relative amount of transcripts under submergence stress reached a peak at 24 h of treatment.
Adenylate Kinase Activity in Submergence Tolerant and Intolerant Rice Varieties
FR13A and IR42 are, respectively, tolerant and intolerant of submergence.12 A substantial reduction in shoot growth was observed when 4 day old IR42 seedlings were entirely submerged. Shoots of the two varieties were used for enzyme assay to determine whether different patterns of adenylate kinase activity could be seen. The changes in adenylate kinase activity were different; the maximum adeny-late kinase activity in FR13A and IR42 occurred at 24 h and 96 h, respectively. The estimated total activity in a shoot also demonstrated similar results; a tremendously high amount of adenylate kinase activity, 14.6 m units/shoot, was shown in submerged FR13A at 24 h. The other samples, control IR42, submerged IR42 and control FR13A, indicated 6.6, 5.6, and 5.2 m units/shoot at 24 h, respectively. The changes in protein concentration between control and submerged seedlings were similar in each variety.
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