Acclimation of Rubisco specificity factor to drought in tobacco:: discrepancies between in vitro and in vivo estimations

In studies about the photosynthesis response to environmental stresses, such as drought, the Rubisco specificity factor (tau) is assumed to be constant or derived indirectly from gas exchange measurements. However, an analysis of the acclimation of tau to drought using in vitro determinations is lacking. The aim of the present work was to analyse the acclimation of tau to different drought intensities in tobacco (Nicotiana tabacum L.). Potted tobacco plants were subjected to three different water regimes (100%, 40%, and 15% of field capacity) and new leaves were allowed to develop. When acclimated leaves were fully developed, they were sampled for gas exchange and chlorophyll fluorescence measurements, as well as for the in vitro analysis of Rubisco kinetic properties. Relative water content and gas exchange decreased with increasing water shortage. The apparent Rubisco specificity factor as estimated in vivo by gas exchange decreased with water stress. However, in vitro estimates of tau were identical among treatments, as were Rubisco specific initial activity and activation state. The reasons for the observed discrepancy between in vitro and in vivo estimates are profusely discussed. It is suggested that the Rubisco specificity factor does not acclimate to water stress in the short term (weeks or months) in tobacco, and the validity of the so-called Laisk gas exchange method to estimate tau under drought is questioned.