Scientia Horticulturae 126 (2010) 338–344 Contents lists available at ScienceDirect Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti CO2 and air circulation effects on photosynthesis and transpiration of tomato seedlings P. Thongbai a,∗, T. Kozai b, K. Ohyama a,b a Graduate School of Horticulture, Chiba University, 648 Matsudo, Chiba 271-8510, Japan b Center for Environment, Health and Field Sciences, Chiba University, Kashiwa-no-ha 6-2-1, Kashiwa, Chiba 277-0882, Japan article info abstract −1 Article history: In the daytime, a CO2 depletion of 10–15% and air circulation of less than 0.5 m s often occur in a Received 29 December 2009 naturally ventilated greenhouse during a sunny day with high wind speed (3–5 m s−1). We, therefore, Received in revised form 13 July 2010 investigated the effects of moderate increase of the CO2 concentration above the atmospheric level Accepted 14 July 2010 (500–600 ␮mol mol−1) and air circulation up to 1.0 m s−1 in a growth chamber on the net photosyn- thetic and transpiration rates of tomato seedlings as the first step. The average net photosynthetic Keywords: rates were 2.1, 1.8, and 1.6 times higher in the growth chambers with increased CO2 concentra- Air current speed −1 −1 tion (500–600 ␮mol mol ) and air circulation (1.0 m s ), increased CO2 concentration, and increased CO2 supply Diffusion resistance air circulation, respectively, compared with those in the control (no increase in CO2 concentration (200–300 ␮mol mol−1) or air circulation (0.3 m s−1). The transpiration rate increased with increased air Net CO2 assimilation Null balance CO2 enrichment circulation, while it decreased with increased CO2 concentration regardless of air circulation. From the results, we consider that increasing the CO2 concentration and/or air circulation in ventilated greenhouses up to the outside concentration (350–450 ␮mol mol−1) and 1.0 m s−1, respectively, can significantly increase the net photosynthetic rate of greenhouse plants. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. 1. Introduction is not practical, because a considerable amount of CO2 would be released to the outside, causing both a high CO2 cost and emis- In general, net photosynthetic rate increases with increasing sion of CO2, a global-warming gas. Thus, CO2 is usually enriched −1 CO2 concentration in a range between 0 and 1000 ␮mol mol only when no ventilation is conducted in the early morning and (Allen and Amthor, 1995). Thus, the CO2 concentration in late afternoon (except for many greenhouses using a co-generating greenhouses is often increased in the daytime up to about system where much CO2 is exhausted to the outside during heat- −1 1000 ␮mol mol (‘CO2 enrichment’) to promote photosynthesis ing and/or power generation) (Hand, 1984; Sanchez-Guerrero et al., and plant growth in greenhouses with the vents closed (Hand et al., 2005). On the other hand, CO2 concentration during the midday in 1993; Nederhoff and Vegter, 1994; Aikman, 1996; Ceulemans et al., a ventilated greenhouse with fully-grown plants is approximately 1997). On the other hand, roof and/or side vents need to be opened 50–60 ␮mol mol−1 lower than that outside (Sanchez-Guerrero et (‘natural ventilation’) or fans need to be turned on (‘forced venti- al., 2005), even though CO2 gas from the outside flows into the lation’) in the daytime to keep the air temperature or water vapor inside through the greenhouse vents. It indicates that the deple- pressure deficit (VPD) at optimal values in the greenhouse when tion of CO2 in ventilated greenhouses limits the net photosynthetic solar radiation and/or air temperature inside are high. Recently, rate of the plants. Thus, CO2 gas should be supplied into the ven- however, there have been some reports on controlling the air tem- tilated greenhouse and maintained at the similar concentration −1 perature and VPD in the greenhouse by descending fog technology, as that outside (350–450 ␮mol mol ) (‘zero or null balance CO2 which are useful to reduce the need for natural ventilation also, enrichment’). This approach is practical when ventilation in the thus allowing for higher CO2 concentrations to be maintained in greenhouse is needed. the greenhouse (Ohyama et al., 2008; Stanghellini and Kempkes, The net photosynthetic rate also increases with increasing air 2008). circulation over leaves in a range between 0 and 0.8 m s−1 when However, when the greenhouse is ventilated, CO2 enrichment the stomata are kept open (facing no water stress) (Kitaya et al., for keeping the CO2 concentration inside higher than that outside 2004; Yabuki, 2004). Also, the transpiration rate increases with increasing air circulation within the range of 0–1.0 m s−1 (Kitaya et al., 2003). This is because the air circulation reduces the leaf ∗ Corresponding author. Tel.: +81 47 137 8114; fax: +81 47 137 8114. boundary layer resistance of CO2 and H2O (water vapor) fluxes. E-mail address: [email protected] (P. Thongbai). The net photosynthetic and transpiration rates can increase with 0304-4238/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2010.07.018 P. Thongbai et al. / Scientia Horticulturae 126 (2010) 338–344 339 Table 1 centration (200–300 ␮mol mol−1) and air circulation (0.3 m s−1) CO2 concentration and air circulation in each treatment. were designed to simulate those conditions in a greenhouse. −1 ␮ −1 Treatment code CO2 concentration Air circulation (m s ) High CO2 concentration (500–600 mol mol ) and air circula- − (␮mol mol 1)a tion (1.0 m s−1) were designed to simulate those conditions in a during photoperiod greenhouse with increased CO2 concentration and increased air cir- LLb (control) Low (200–300) Low (0.3) culation by air mixing fans. For treatment code abbreviation, high LH Low (200–300) High (1.0) and low CO2 concentrations were abbreviated to H and L, respec- HL High (500–600) Low (0.3) tively on the first letter, whereas high and low air circulations were HH High (500–600) High (1.0) abbreviated to H and L, respectively on the second letter. a For the CO2 concentration in each treatment during the photoperiod, see During the photoperiod, CO gas was supplied into the growth Fig. 1(a). 2 −1 −2 −1 b chamber at a flow rate of 2.8 ml min (14.0 ␮mol CO m s )in For treatment code, L and H on the left represent low and high CO2 concentration, 2 respectively; L and H on the right represent low and high air circulation, respectively. treatments HL and HH. Fans (DC 12 V 1.56 W, PWM fan CFY-90P, AINEX) were installed to increase the air circulation in the growth chamber in treatments LH and HH. increasing air circulation until reaching the optimum level (Shibuya Seedlings, each with four true leaves (fresh weight: 2.25 ± 0.14 g, and Kozai, 1998; Kitaya et al., 2003). The optimum air circulation dry weight: 0.23 ± 0.014 g, height: 10 ± 0.2 cm, LAI (leaf area index): for the net photosynthetic and transpiration rates depends on the 2.8 ± 0.1), were selected 16 DAS and kept for 3 days in the growth plant species, structure of plant community, plant canopy depth, chambers (MIR-153, Sanyo Electric Biomedical Co., Ltd., Japan). and wind direction with plant position in the greenhouses, etc. The air movement in growth chambers with and without fans is (Wadsworth, 1959; Morse and Evans, 1962; Shibuya and Kozai, moved in the horizontal direction. Each growth chamber holding 1998; Kitaya et al., 2000; Sase, 2006). On the other hand, insuf- one tray with 72 seedlings was maintained at 25 ◦C air temper- ficient air circulation above the plant canopy causes limited gas ature, 300 ␮mol m−2 s−1 PPF measured at the tray surface, and exchange because of increased leaf boundary layer resistance (Kim 16hd−1 photoperiod. The air temperature of the laboratory room, et al., 1996; Kitaya et al., 1998). where the growth chambers were placed, was set at 25 ◦C. The Despite the fact that the main purpose of vent opening is to opti- CO concentration in the laboratory room ranged between 400 and mize the air temperature and relative humidity, ventilation still 2 500 ␮mol mol−1. A commercial nutrient solution (N:P:K = 6:10:5) increases air circulation in a greenhouse. Many greenhouses with was supplied at a fixed volume to each tray prior to the photoperiod. roof and/or side vents are naturally ventilated driven by pressure differences created at the vent openings either by the wind or by 2.3. Measurements temperature differences (Mistriotis et al., 1997). Thus, the air circu- lation in naturally ventilated greenhouses is related to the degree Air circulation in the growth chamber was measured by using of air exchange between the interior air of the greenhouse and its a hot-wire anemometer (Climomaster 6522, Kanomax Japan Inc., external environment due to the wind and temperature effects Japan) and expressed as an average of 10 measured points. Air (Wang et al., 1999). Moreover, the air circulation in the green- temperature and relative humidity inside and outside the growth houses declines because of its reduction with canopy depth and chambers were measured with thermo recorders (RS-12, Espec distance from the vents (Sase, 2006). Thus, in order to increase the Mic Corp. Aichi, Japan). CO concentrations inside and outside air circulation uniformly in the greenhouses, air mixing fans are 2 the growth chambers were measured with infra-red gas analyz- needed.