Leaf photosynthesis of Haberlea rhodopensis before and during drought

Haberlea rhodopensis is a homoiochlorophyllous resurrection plant that shows a low rate of leaf net CO2 uptake (4–6 μmol m−2 s−1) under saturating photosynthetic photon flux densities in air (21% O2 and about 390 ppm CO2). However, leaf net CO2 uptake reaches values of 17–18 μmol m−2 s−1 under saturating CO2 and light. H. rhodopensis leaves have a very low mesophyll CO2 conductance that can partly explain the low rate of leaf net CO2 uptake in normal air. Experimental evidences suggest that mesophyll conductance is not sensitive to temperature in the 20–35 °C range. In addition, it is shown that the (1) transpiration rate of H. rhodopensis is nearly linearly related to the vapour pressure difference between the leaf and the ambient air within the interval from 0.5 kPa to 2.5 kPa at a leaf temperature of 25 °C and (2) leaf net CO2 uptake in normal air under saturating light does not change much with leaf temperature (between 20 °C and 30 °C). At a leaf relative water content of between 90% and 30%, the decrease of leaf net CO2 assimilation during drought can be explained by a decrease of leaf CO2 diffusional conductance. Accordingly the non-photochemical chlorophyll fluorescence quenching decreases only at relative water contents lower than 20%, indicating that photosynthetic activity maintains a trans-thylakoidal proton gradient over a wide range of leaf water contents. Moreover, PSII photochemistry (as estimated by the Fv/Fm ratio and the thermoluminescence B band intensity) is only affected at leaf relative water contents lower than about 20%, thus confirming that primary photosynthetic reactions are resistant to drought. Interestingly, the effect of leaf desiccation on photosynthetic capacity, measured at very high ambient CO2 molar ratios under saturating PPFD, is identical to that observed for three non-resurrection C3 mesophytes. This demonstrates that the photosynthetic apparatus of H. rhodopensis is not more resistant to desiccation when compared to other C3 plants. Since the leaf area decreases by more than 50% when the leaf relative water content is reduced to about 40% during drought it is supposed, following Farrant et al. [Farrant, J.M., Vander, W.C., Lofell, D.A., Bartsch, S., Whittaker, A., 2003. An investigation into the role of light during desiccation of three angiosperms resurrection plants. Plant Cell Environ. 26, 1275–1286], that H. rhodopensis leaf cells avoid mechanical stress.