Nitrate reductase activity during desiccation and rehydration of the desiccation tolerant moss Tortula ruralis and the leafy liverwort Porella platyphylla

Abstract

Nitrate reductase (NR) activity in vivo was detected in eight of nine bryophytes investigated, at levels generally lower than those reported for nitrate-using higher plants. In three species tested, pre-treatment with 5 mol m ⁻³ KNO₃ increased NR activity by a factor of three in Tortula ruraliformis and by a factor of six in Dicranum majus and Hookeria lucens. Water stress imposed osmotically by PEG or sorbitol solutions depressed NR activity to 20% of its full-turgor value at −4.5 MPa. No significant differences in the proportional change were found between the desiccation-tolerant and non-desiccation tolerant species. The original hydration level of the tissue influenced the pattern of response during subsequent water stress. Measurements on field-collected material of Tortula ruralis indicate that continuous exposure to NO₃⁻ can result in the loss of NR inducibility. In NO₃⁻ free conditions there was some indication of reduced NR activity in bryophytes that were water stressed by air drying, but the results were variable and not statistically significant. During the first hour of rehydration of fully-desiccated material (air dry for 30 d), NR activity in Tortula ruralis declined markedly in both dark and light (more steeply in the light) while in Porella platyphylla it remained at a relatively constant low level in the light and increased in the dark. After four days of dark starvation Porella platyphylla showed reduced but measurable NR activity, which was increased by supply of exogenous sugars. The possible physiological basis of these findings is discussed.