Rubisco activity decreases under
water stress, for reasons as yet unclear. Here, the covariation of stomatal conductance (gs) and relative water content (RWC), often observed during
water stress, was impaired to assess the separate effects of these factors on
Rubisco activity. Three different treatments were applied to soybean (Glycine max) and tobacco (Nicotiana tabacum): leaf desiccation (LD), in which stomatal closure was accompanied by large decreases of RWC;
water stress (WS), in which minor decreases of RWC were observed along with stomatal closure; and exogenous application of
abscisic acid (ABA), which triggered stomatal closure without changing RWC. Decreased RWC did not induce decreased initial
Rubisco activity, which was impaired only in soybean by 40% when the gs dropped below 50 mmol m(-2) s(-1), regardless of the treatment. The mechanism for decreased activity differed among treatments, owing to decreased activation in LD and to total activity and
protein content in WS and ABA. Despite the occurrence of
Rubisco regulation, CO2 availability in the chloroplast, not impairment of
Rubisco activity, limits photosynthesis during WS.