Responses of isotope discrimination and intercellular relative humidity to vapour pressure deficit in wildtype and abscisic acid insensitive populus X canescens
- James Cook University, Cairns, Australia.
- Chapman University, Orange, California, USA.
- Basel University, Basel, Switzerland.
- Paul Scherrer Institute, Villigen, Switzerland.
We investigated responses to vapour pressure deficit (D) in abscisic acid insensitive (abi) and wildtype (WT) Populus x canescens, in order to better understand the crucial role of stomata in modulating leaf gas exchange and stable isotope discrimination, and to test for unsaturation of intercellular relative humidity (hi). Transpiration rate increased linearly with D in abi plants up to D of about 2 kPa, beyond which it declined precipitously; this was followed by leaf death within hours of removing the leaf from the gas exchange cuvette. On the other hand, WT leaves showed a steady or slightly declining transpiration rate up to D of nearly 7 kPa, and fully recovered photosynthetic function when measured the next day. Discrimination against 13CO2 (Δ13C) declined in response to increasing D in WT plants, consistent with declining intercellular CO2 concentrations. For the D range over which abi plants were measured, Δ13C was higher in abi plants than in WT plants by about 6 permil on average, and did not decrease in response to increasing D. Discrimination against C18OO (Δ18O) differed markedly between WT and abi plants. In WT plants, Δ18O increased with increasing D by about 2 permil kPa-1, whereas in abi plants the rate of increase was about 66 permil kPa-1. This reflected progressive stomatal closure in the WT plants with increasing D. In abi plants, in contrast, stomata remained open in response to increasing D allowing CO2 to readily diffuse into the leaf interior, exchange oxygen atoms with the 18O enriched leaf water, and then diffuse back into the air exiting the gas exchange cuvette. The coupled measurements of Δ18O and gas exchange allowed us to estimate hi. In WT leaves we saw no evidence of unsaturation of hi, even at D above 6 kPa. However, in abi leaves, hi decreased at a rate of about -0.25 kPa-1, thus declining to values approaching 0.6 before the precipitous decline in transpiration rate and subsequent leaf death.