Effects of Intermittent Desiccation on Nutrient Economy and Growth of Two Ecologically Contrasted Mosses

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Abstract
The mossesBrachythecium rutabulum (Hedw.) B., S. & G. andPseudoscleropodium purum (Hedw.) Fleisch. were cultivated for more than 50 d in a growth cabinet with or without weekly drying interludes of 24 h. Some plants also received applications of a dilute NPK nutrient solution at weekly intervals. The continuously hydrated plants showed appreciably more biomass production than those receiving intermittent desiccation. Desiccation led to some bleaching of the green tissues inB. rutabulum but not inP. purum which appeared more desiccation-tolerant. NPK addition caused a further significant growth stimulation in continuously hydrated plants, but not in intermittently desiccatedB. rutabulum. Pseudoscleropodium purum showed NPK-induced growth stimulation even when intermittently desiccated. Net uptake of N was similar in desiccated and hydrated plants in both species. Considerable net uptake of P and K+occurred in continuously wetB. rutabulum , but uptake was much reduced in intermittently desiccated plants. Net uptake of P and K+byP. purum was similar in desiccated and hydrated samples. Intracellular K+, leaked from the cells during the desiccation treatment, was retained by cation exchange on the negatively charged cell walls in both species. Levels of intracellular K+and Mg2+in the new growth were maintained at the expense of the pool of exchangeable cations. The growth stimulation and the net uptake of nutrients under intermittent desiccation was greatest when the NPK application was made at the start of rehydration, possibly because of accentuated uptake in the early stages of recovery. The results support the hypothesis thatP. purum has a lower nutrient requirement thanB. rutabulum and highlight the importance of continuous hydration for the latter's more productive plant life strategy. The data also show that considerable new growth of bryophyte tissues is possible without additional nutrient absorption.

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