Steady-State and Oscillating Photosynthesis by a Starchless Mutant of Nicotiana sylvestris

The photosynthetic characteristics of wild type Nicotiana sylvestris (Speg. et Comes) were compared with those of a `starch-less' mutant NS458 that contains a defective plastid phosphoglucomutase (EC 2.1.5.1) (KR Hanson, NA McHale [1988] Plant Physiol 88: 838-844). The steady-state rate of net CO₂ assimilation (A) was studied as a function of [CO₂], [O₂], irradiance, and temperature. At 30°C with saturating light and [CO₂] and low [O₂], A for the mutant was half that for the wild type, whereas in normal air it was 90%. The irradiance and [CO₂] at low [O₂] required for saturation were lower than the values for the wild type. At 2000 microbars CO₂, 30°C, and saturating irradiance A for both the mutant and wild type was stimulated on going from 4 to 25% O₂ by at least 13%. Slow oscillations in A were readily induced with the mutant but not the wild type, provided irradiance and [CO₂] were saturating and [O₂] was low. The period, which was about 5 minutes at 30°C and decreased by about 0.67 minutes per degree, was an order of magnitude slower than periods reported for other plants at corresponding temperatures. To achieve the full oscillation amplitude both irradiance and [CO₂] had to exceed the minimal levels for steady-state saturation. The slowness and duration of the oscillations and the metabolic simplification introduced by deleting starch synthesis makes the mutant especially suitable for investigating the regulatory processes that generate such oscillations.