Diurnal changes in nitrogen and potassium absorption rates of plants grown in a greenhouse

In order to increase the efficiency of nutrient utilization in solution culture systems, it is indispensable to understand how plants absorb nutrients. A series of experiments were conducted using hydroponic culture system in the first to third experiments and soil culture system in the fourth experiment in a greenhouse in Okinawa, Japan. In the first experiment, changes in NO₃-N absorption were measured every three hours during a 24-hour period on 40 days after transplanting (DAT) in cucumber. In the second experiment, daytime and nighttime NO₃-N absorption were measured from 45 to 48 DAT in five crops including cucumber. In the third experiment, NO₃-N, NH₄-N, and K absorption in daytime, nighttime and shaded conditions were determined from 30 to 34 DAT in bitter gourd, a substitute crop for cucumber, and water spinach. In the fourth experiment, daytime and nighttime (NH₄)₂SO₄-¹⁵N absorption rates were measured from 9 to 12 DAT in cucumber. The results showed that daily NO₃-N absorption in cucumber peaked twice, just before midday and again just after the nightfall. The corresponding fluctuation pattern was observed in all other tested crops, with 20–41% of the total absorption occurring at night. Absorption rate of ¹⁵N in cucumber cultivated in soil also increased in daytime and decreased at night, with the nighttime rate accounting for 39–40% of the daily total absorption. The average nighttime NO₃-N, NH₄-N, and K absorption percentages of the daily total before shading the plant were 34–35%, 49– 51%, and 33–49%, respectively. During the shaded period, these nutrient absorption rates dramatically decreased, except for NH₄-N which was not affected by shading. Across all experiments, 20–51% of total absorption took place at night, the majority of which may concentrate around the early nighttime. Furthermore, the absorption rates may be influenced by photosynthetic products.