Proposed relationship between demand for carbon for growth and the rate of starch accumulation. A, At the start of the day in 12:12 conditions, demand for Suc for growth (green arrow) is relatively high. This stimulates flux of carbon from the Calvin-Benson cycle into the cytosol via the triose-P-Pi transporter (purple), creating conditions of relatively high Pi and low 3-PGA in the chloroplast and thus inhibiting AGPase (red line) and causing a relatively low rate of starch synthesis. B, At the start of the day in 6:18 conditions or following an unexpected extension of the night in 12:12 conditions, demand for Suc for growth is low. Suc and intermediates of the pathway build up in the cytosol, resulting in a relatively low flux of triose phosphates from the Calvin-Benson cycle into the cytosol, relatively high 3-PGA and low Pi in the chloroplast, and thus activation of AGPase and a relatively high rate of starch synthesis.