Reactivation of phloem export in mature maize leaves after a dark period.

Mature leaves of corn plants (Zea mays L. cv. Prior) which were darkened for 48 h contain neither bundle-sheath starch nor glucose, and their sucrose content is below 5 μM. In such leaves phloem export has ceased. When re-illuminated, photosynthetic sucrose production starts without delay, but the sucrose: glucose ratio is 1.25:1. Obviously, most of the new-formed sugar is utilized locally. Labeling with (14)CO2 has shown that phloen export starts 30 to 40 min after the onset of photosynthesis, when the sucrose: glucose ratio has increased to 13:1. The first newly formed starch can be detected when phloem export is reactivated. Glucose content remains constantly low af about 2 μM for at least 2 h, and it never exceeds 10 μM. Radioactivity in the exporting veins is about five times higher after 2 to 7 h of re-illumination than in the 14-h-day plant. Therefore, phloem export is either intensified during the period of reactivation or exported assimilates are partly unloaded along their way. Comparison of photosynthetic activity of equal-sized leaf strips has shown that both accumulation of photosynthates and radioactivity of exporting veins are about three times higher in the detached strip than in the strip which remained attached to the mother plant.