Some characteristics of reduced leaf photosynthesis at midday in maize growing in the field

Maize was grown in the high-radiation arid summer environment of Davis, Cal ifornia, and its leaf photosynthetic rate was measured over diurnal courses on cloudless days with the leaf held perpendicular to the sunlight. On day s of high atmospheric vapor pressure deficit (VPD), leaf photosynthesis rea ched a maximum in the late morning and then decreased gradually as the day progressed, though the soil was well irrigated. When CO2 concentration in t he measurement chamber was raised to about 1000 mu mol mol(-1), photosynthe sis was enhanced, but more in the afternoon than in the morning. As a resul t, rates measured at high CO2 in the morning and afternoon were essentially the same. There was also no difference in the curves of photosynthetic rat e (A) versus intercellular CO2 concentration (C-i) for the morning and afte rnoon. Hence, photosynthetic capacity was similar for the two periods and t here was no evidence of photoinhibition by the high photosynthetic photon f lux density at noon. Further, C-i and photosynthetic rates A measured over a range of photon flux density were lower in the afternoon than in the morn ing. These results indicate that A at noon and early afternoon was more lim ited than in the morning by epidermal conductance (mostly stomatal). On a d ay of low VPD, however, midday depression in A and epidermal conductance we re not evident for the well-irrigated plants. Without irrigation and with l eaves at a lower midday water potential, midday reduction in conductance an d A was much more marked, beginning late in the morning. Epidermal conducta nce of maize grown in the field in Davis is are not sensitive to VPD. There fore, the midday reduction in conductance and A was more likely the result of low leaf water potential caused by high transpiration rates. (C) 1999 Pu blished by Elsevier Science B.V. All rights reserved.