Effect of climate on the salt tolerance of two Phragmites australis populations. : II. Diurnal CO 2 exchange and transpiration

Abstract

This study examined the effects of salinity and climate on instantaneous CO 2 exchange rates and daily carbon balance for two populations of Phragmites australis. Plants propagated from seeds collected in Denmark (=Danish population) and Spain (=Spanish population) were grown at salinities of 0, 5 and 10‰ at outdoor experimental plots situated at a nemoral growth site (Denmark, 56°N) and at a mediterranean growth site (Spain, 41°N). In situ measurements of shoot CO 2 exchange were made under different meteorological conditions. Maximum CO 2 uptake rates were 24 and 19 μmol m −2 s −1 in Spain and Denmark, respectively, and were only reached under conditions of full sunlight. On sunny days, CO 2 uptake was reduced less by salinity at the Spanish site than at the Danish site. Photosynthetic photon flux densities saturating shoot photosynthesis were much lower in Spain (ca. 600 μmol m −2 s −1) as compared to those in Denmark (ca. 1300 μmol m −2 s −1). This affected the period of maximum CO 2 uptake rates which was 2–3 h longer at the Spanish growth site, despite shorter day length at this site (15 h) than at the Danish site (17 h). Dark respiration rates of shoots were higher at the Spanish growth site (2–8 μmol CO 2 m −2 s −1) as compared to the Danish site (less than 2 μmol CO 2 m −2 s −1). Salinity was observed to accelerate rates of dark respiration only at the Spanish site. Integrals of shoot CO 2 exchange rates over a 24 h period indicated lower total daily carbon gain at the Danish site, which under clear sky conditions was 65–80% of that at the Spanish site. Cloud cover was found to reduce net photosynthetic CO 2 uptake considerably and could even cause a net loss of carbon during periods, which in absence of clouds could confer maximum uptake rates. The salt tolerance and productivity of P. australis is therefore strongly related to the ratio of realised to potential sun hours. This ratio may override the importance of other environmental factors such as temperature. This study also evaluated the effects of moderate salinity stress (0–15‰ salinity) on in situ P. australis transpiration. Daily transpiration rates were 30–75% higher at the Spanish site (0.38–0.56 g H 2O cm −2 day −1) as compared to the Danish site (0.22–0.38 g H 2O cm −2 day −1). Rates were equally affected by salinity at both the growth sites. Since water loss is an important factor behind P. australis salt tolerance it is speculated that salt tolerance may increase under conditions favouring high transpiration, contrary to most other halophytes.