The Effect of CO2 Concentration on the CO2 Exchange Rate in a Small Plant Stand of Cucumber during Different Periods of the Day

The effect of CO2 concentration on the carbon dioxide exchange rate (CER) was studied in small plant stands of cucumber for 12 hours a day(-1) in three experiments (Exp.) conducted in summer. A photon flux density (PFD) of 300 mu mol m(-2) s(-1) of supplementary light was provided in addition to daylight, except for during the middle of the day (12.00-17.00). The photoperiod was 20 hours per day(-1) (02.00-22.00). In Exp. 1, an increase in CO2 concentration from 380 to 800 mu mol mol(-1) from 08.00 until 20.00 increased CER by about 30 % during the period, irrespective of the PFD. In Exp. 2, a decrease in CO2 concentration from 380 to 210 mu mol mol(-1) during the same period reduced the CER by 20-36 %, with an increasingly negative effect towards the end of the period. Measured at the same CO2 concentration during the first hours of the photoperiod (02.00-08.00) and at end of the photoperiod (20.00-22.00), the CER was the same in both plant stands in both experiments. In Exp. 3, an increase in the CO2 concentration from 800 to 1200 mu mol mol(-1) during the same 12-hour period increased CER much more early in the period (28 A) than late in the period (7 %). Measured at the same CO2 concentration, the CER of the 1200 mu mol mol(-1) exposed plants was 12% lower at end of the photoperiod (20.00-22.00) compared with the control plants, but the same during the first part of the photoperiod (02.00-08.00). CER increased with increasing PFD in all three experiments. In a fourth experiment, CER was measured at three different temperatures throughout the day in three different plant stands. Supplementary light (300 mu mol m(-2) s(-1) PFD) was provided for 20 hours per day(-1). When 800 mu mol mol(-1) CO2 was combined with moderate PFD levels (about 300 mu mol m(-2) s(-1)), the CER was the same at 25 and 30 degrees C, decreasing at 35 degrees C. At a high PFD level (about 750 mu mol m(-2) s(-1)) and intermediate CO2 level (500 mu mol mol(-1)), the CER was the same irrespective of temperature. When the CO2 concentration decreased to 300 mu mol mol(-1), the CER decreased with increasing temperature.