The effect of a day at low irradiance of a maize crop

During the growth of maize (Zea mays L.cv. INRA F7× F2) under constant climatic conditions, the effects of reductions in irradiance simulating a cloudy day were studied. Hourly and daily measurements made in an assimilation chamber (C2 3A) showed important and lasting effects in root activity. After reduction of photosynthesis, it took approximately 2 hours to start a lowering of the uptake of NO3− and NH4+, three hours for K+ and four hours for phosphate. Root respiration started to fall after 3 hours. The level to which these activities were reduced also varied. Phosphate uptake was reduced by a mean of 27%, nitrate uptake by 47%, and K+ uptake by 55% while the root respiration was reduced by 55%. After return to the initial irradiance, root activities took 3 days to recover their initial rates. Shoot respiration was re‐established after one day, while the effects on photosynthesis and transpiration were immediate. The delay of the effect of a change of photosynthesis on the activities of the root, indicates the existence of considerable metabolic reserves. Over longer periods, root metabolism depends on photosynthetic assimilates, but in the short term it is much more dependent on the level of metabolic reserves than on the direct flow of photosynthetic translocates.