INTERACTING EFFECTS OF PHOTOPERIOD AND PHOTOSYNTHETIC PHOTON FLUX ON NET CARBON ASSIMILATION AND STARCH ACCUMULATION IN POTATO LEAVES

The effect of photoperiod (PP) on net carbon assimilation rate (A(net) ) and starch accumulation in newly mature canopy leaves of 'Norland) p otato (Solanum tuberosum L.) was determined under high (412 proportion al to mol .(m-)2 . s(-1)) and low (263 proportional to mol . m(-2). s( -1)) photosynthetic photon flux (PPF) conditions. The A(net) decreased from 13.9 to 11.6 and 9.3 mu mol . m(-2). s(-1), and leaf starch incr eased from 70 to 129 and 118 mg . g(-1) drymass (DM) as photoperiod (P P) was increased from 12/12 to 18/6, and 24/0, respectively. Longer PP had a greater effect with high PPF conditions than with low PPF treat ments, with high PPF showing greater decline in A(net). Photoperiod di d not affect either the CO2 compensation point (50 mu mol . mol(-1)) o r CO2 saturation point (1100-1200 mu mol . mol(-1)) for A(net). These results show an apparent limit to the amount of starch that can be sto red (approximate to 15% DM) in potato leaves. An apparent feedback mec hanism exists for regulating A(net) under high PPF, high CO2 and long PP, but there was no correlation between A(net) and starch concentrati on in individual leaves. This suggests that maximum A(net) cannot be s ustained with elevated CO2 conditions under long PP (greater than or e qual to 12 hours) and high PPF conditions. If a physiological limit ex ists for the fixation and transport of carbon, then increasing photope riod and light intensity under high CO2 conditions Is not the most app ropriate means to maximize the yield of potatoes.