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 (δ¹⁸O_TW), and the CO₂ and H₂O fluxes, were analysed. After drying, carbon (δ¹³C_C) and oxygen (δ¹⁸O_C) cellulose compositions were determined. During desiccation, assimilation and ¹³CO₂ discrimination increased to a maximum and then declined; δ¹⁸O_TW increased progressively by 8‰, 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 δ¹⁸O_TW were consistent with instantaneous measurements. δ¹³C_C values suggest that net assimilation occurs at 25% of maximum relative water content, while δ¹⁸O_C 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.