Temporal separation between CO2 assimilation and growth? Experimental and theoretical evidence from the desiccation-tolerant moss Syntrichia ruralis

The extent of an external water layer around moss tissue influences CO2 assimilation. Experiments on the desiccation-tolerant moss Syntrichia ruralis assessed the real-time dependence of the carbon and oxygen isotopic compositions of CO2 and H2O in terms of moss water status and integrated isotope signals in cellulose. As external (capillary) water, and then mesophyll water, evaporated from moss tissue, assimilation rate, relative water content and the stable isotope composition of tissue water (18OTW), and the CO2 and H2O fluxes, were analysed. After drying, carbon (13CC) and oxygen (18OC) cellulose compositions were determined. During desiccation, assimilation and 13CO2 discrimination increased to a maximum and then declined; 18OTW increased progressively by 8 parts per thousand, indicative of evaporative isotopic enrichment. Experimental and meteorological data were combined to predict tissue hydration dynamics over one growing season. Nonsteady-state model predictions of 18OTW were consistent with instantaneous measurements. 13CC values suggest that net assimilation occurs at 25% of maximum relative water content, while 18OC data suggests that cellulose is synthesized during much higher relative water content conditions. This implies that carbon assimilation and cellulose synthesis (growth) may be temporally separated, with carbon reserves possibly contributing to desiccation tolerance and resumption of metabolism upon rehydration.