terized by higher rates of CO2 fixation, sucrose formation and assimilate export compared with C3 plants (2). Carbon assimilation by the C4 pathway is not completely light saturated at full sunlight and therefore the rate of CO2 uptake is subject to large changes with natural fluctuations in light intensity during the day. In contrast, carbon assimilation in C3 plants is usually saturated at a lower irradiance and therefore does not fluctuate with changes in natural illumination to the same extent as C4 plants.Recently, photosynthesis by the C3 plant soybean was characterized diurnally. In this study (10), the activity of SPS4 was shown to oscillate with an endogenous rhythm. SPS activity was correlated positively with assimilate export rate during the day and maximum SPS activity was similar to the measured assimilate export rate. Maize (11) and barley (12) leafSPS have recently been shown to be light activated; however, it is not known whether SPS from these sources may be controlled by an endogenous rhythm in activity as well as by short-term light modulation.Our goal in conducting the present study was to characterize diurnal changes in maize leaf carbon assimilation, partitioning and export, in relation to changes in the activities of SPS and key enzymes of the C4 pathway. To determine whether diurnal fluctuations in enzyme activities were caused by an endogenous rhythm, enzyme activities were also measured in plants after reciprocal light/dark transfers at different times during the diurnal cycle.

Maize (Zea mays L. cv. Pioneer 3184) leaf elongation rate was measured diurnally and was related to diurnal changes in the activities of sucrose metabolizing enzymes and carbohydrate content in the elonpting portion of the leaf. The rate of leaf elongation was greatest at midday (1300 hours) and was coincident with the maximum assimilate export rte from the distal portion of the leaf. Leaf elongation during the light period accounted for 70% of the total observed increase in leaf length per 24 hour period. Pronounced diurnal fluctuations were observed in the activities of acid and neutral invertase and sucrose phosphate synthase. Maximum activities of sucrose phosphate synthase and acid invertase were observed at 0900 hours, after which activity declined rapidly. The activity of sucrose phosphate synthase was substantially lower than that observed in maize leaf source tissue. Neutral invertase activity was greatest at midday (1200 hours) and was correlated positively with diurnal changes in leaf elongation rate. There was no significant change in the activity of sucrose synthase over the light/dark cycle. Sucrose accumulation rate increased during a period when leaf elongation rate was maximal and beginning to decline. Maximum sucrose concentration was observed at 1500 hours, when the activities of sucrose metabolizing enzymes were low. At no time was there a significant accumulation of hexose sugars. The rate of starch accumulation increased after the maximum sucrose concentration was observed, continuing until the end of the light period. There was no delay in the onset of starch mobilization at the beginning of the dark period, and essentially all of the starch was depleted by the end of the night. Mobilization of starch in the elongating tissue at night could account for a significant proportion of the calculated increase in the tissue dry weight due to growth. Collectively, the results suggested that leaf growth may be controlled by the activities of certain sucrose metabolizing enzymes and may be coordinated with assimilate export from the distal, source portion of the leaf. Results are discussed with reference to diurnal photoassimilation and export in the distal, source portion of the leaf.Regulation of leaf growth and development is a critical factor in crop productivitiy because these processes ultimately determine the total photosynthetic area available for light interception and carbon assimilation. The growth and development of maize leaves occurs in a manner unique to monocots; leaf growth