Evidence of active NADP(+) phosphatase in dormant seeds of Avena sativa L.

Freshly-harvested seeds of Avena sativa L. do not germinate when imbibed at temperatures higher than 25 degreesC, This high temperature dormancy is du e to the seed coats, and to the low activities of glycolysis and the oxidat ive pentose phosphate pathway (OPP) in the embryo. The analysis by exclusio n chromatography of soluble NADP(+) phosphatase activities of embryos revea led two isoforms: a 37 kDa isoform present in both dormant and after-ripene d caryopses, and a second isoform, with an apparent molecular weight of 160 kDa, five times more active in embryos of dormant seeds than in the after- ripened ones, after 6 h of imbibition at 30 degreesC. Moreover, the activit y of this 160 kDa isoform was three times less in embryos from dormant cary opses when they were grown at 10 degreesC, a permissive temperature for rad icle protrusion. These results suggest a correlation between the activity o f the 160 kDa NADP(+) phosphatase and the dormancy state of the caryopsis. The two isoforms differed in the pH required for optimal activity: pH 5.7 a nd 6.5 for the 37 kDa and the 160 kDa phosphatases, respectively. Furthermo re, the 160 kDa NADP(+) phosphatase displayed a strong specificity for NADP (+), whereas the 37 kDa isoform was able to hydrolyse numerous other phosph orylated compounds.