Snap Bean Yield and Photosynthesis During Twilight Extended Field Conditions

Snap bean (Phaseolus vulgaris 'Provider') was direct seeded or transplanted after 10 days to high tunnel or field environments during the summer season of 2008. At the Fairbanks research location (latitude 64°49'N), the month of June had normal average temperatures while precipitation was 17.3 mm above normal. The average high temperature in July was 4 °C below normal and precipitation more than twice the normal amount. End of season frost was experienced on Sept. 21. Yield more than doubled for transplanting versus direct seeding. A high tunnel environment also resulted in higher yield than the field. Yield of pods was 3.1 ± 0.03 kg·m^-2 in the field and 3.9 ± 0.11 kg·m^-2 in the high tunnel for planted beans. The seeded plants produced 1.1 ± 0.05 kg·m^-2 in the field and 1.6 ± 0.03 kg·m^-2 in the protection of the high tunnel. A LI-6400 portable photosynthesis system (LI-COR, Lincoln, NE) with a clear top leaf chamber was used to measure net photosynthetic rate (Pn) of single leaves. Measurements were made intermittently in the field environment for 24-h during July 6 and 7, at 400 ppm CO₂. Sunset on July 7 was at 0023 HR with sunrise at 0329 HR Alaska daylight saving time and the sun remained above civil twilight limits throughout the 24 h. The average temperature for the 24 h cycle measured one meter above ground was 18.7 ± 5.75 °C with a high of 28 °C at 1500 HR and a low of 10.5 °C at 0500 HR. Photosynthetic photon flux (PPF, 400 to 700 nm) averaged 433 μmol·m^-2s^-1 for the 24-h cycle. Over the 24 h, Pn generally correlated to available PPF. Recorded peak Pn was 10.6 μmol CO₂·m^-2s^-1 at 1355 HR. In comparison for sweet corn (Zea mays 'Yellow Supersweet'), peak Pn was 28.3 μmol CO₂·m^-2s^-1 at 1335 HR. Pn dropped to zero or less from approximately 2300 to 0600 HR in both beans and corn under ambient PPF levels of 2 to 30 μmol·m^-2s^-1.