NITRATE REDUCTASE-ACTIVITY DURING DESICCATION AND REHYDRATION OF THE DESICCATION-TOLERANT MOSS TORTULA-RURALIS

Water stress reduces the activity of many plant enzymes. When the stre ss is alleviated there is generally a delay in the recovery of metabol ism to unstressed levels. The enzyme nitrate reductase (EC 1.6.6.1) de clines in plants experiencing water deficits and upon the alleviation of water stress its activity recovers to control levels in 1-7 days. I n desiccation-tolerant plants, metabolic recovery can occur within hou rs following rehydration. It has been proposed that metabolic characte ristics of desiccation-tolerant plants are a source of information tha t can be used to improve the performance of crop plants under water de ficits. In this study, the effect of desiccation and hydration on nitr ate reductase activity in the desiccation-tolerant moss Tortula rurali s has been investigated. It was expected that the activity of nitrate reductase would decline during desiccation and recover rapidly followi ng rehydration. Inclusion of nitrate in the hydration medium increased the activity in the tissue. Nitrate reductase activity increased with nitrate concentration in the bathing medium up to levels of 200 mM. T he highest activity measured in rehydrated moss samples was 5.23 nkat g(-1) dry weight (dw). The activity declined rapidly during dehydratio n and was not detectable in dried moss samples after 24 h of dehydrati on. The recovery of activity following rehydration was dependant on th e rate of the preceding dehydration. In slowly dried moss, the activit y recovered to control levels in less than 8 h while rapidly dried sam ples required 24 h for full recovery. Nitrite accumulated during slow dehydration but did not accumulate when desiccation was rapid. Followi ng rehydration of slowly dried moss, the amount of nitrite declined an d reached a control level within 1 h. It was proposed that nitrite acc umulation might provide a source of nitrogen for metabolism during the time required for nitrate reduction to resume following rehydration. Attempts to measure immunologically reactive nitrate reductase protein levels or-nitrate reductase mRNA levels using heterologous DNA probes were unsuccessful, suggesting that the Tortilla nitrate reductase may be significantly different from that found in algae and higher plants . (C) 1998 Published by Elsevier Science B.V. All rights reserved.