Figure 1.

Response of CEF to changes in ATP/NADPH demand under subsaturating PPFD. CEF was measured from the relationship of LEF (μmol e⁻ m⁻² s⁻¹) to the rate of v_H+ in relative units. The subsaturating light intensities ranged from 34 to 377 μmol m⁻² s⁻¹ PPFD. v_H+ was determined spectroscopically from the dark relaxation of the ECS and LEF was determined using chlorophyll fluorescence. CEF was measured under various partial pressures of CO₂ and oxygen (14 Pa of CO₂ and 37 kPa of oxygen [circles], 37 Pa of CO₂ and 20 kPa of oxygen [triangles], and 205 Pa of CO₂ and 2 kPa of oxygen [squares]) and hydroponically supplied nitrogen (NH₄⁺ [white symbols] or NO₃⁻ [black symbols]). All treatments (A) and treatments of the largest ATP/NADPH demand differences (B) are shown with modeled lines representing expected slopes assuming 24% CEF (solid line) and 39% CEF (dashed line), as presented in Table II. Values shown are averages of measurements from five separate plants ± se.