Short Communication Environ. Control Biol., 51 (4), 215220, 2013 DOI: 10.2525/ecb.51.215 Use of Air Circulation to Reduce Wet Leaves under High Humidity Conditions 1 1 2 1 Takeshi KUROYANAGI , Hisashi YOSHIKOSHI , Takafumi KINOSHITA and Hiroki KAWASHIMA 1 NARO Western Region Agricultural Research Center, Zentsuji, Kagawa 7650053, Japan 2 NARO Tohoku Agricultural Research Center, Morioka, Iwate 0200123, Japan (Received August 16, 2013; Accepted November 21, 2013) The wetting of plants due to guttation (i.e., drops of xylem sap that exude onto the leaves) represents a potential risk for incidence and outbreak of pathogens. Here, we investigated the effect of air circulation on guttation of tomato leaves under dark and high humidity conditions. The tomato plants were grown in a container and pinched above the second truss and were then separately placed in a darkened and constantly humidified growth cabinet that was exposed to three levels of air circulation intensity (air velocities of 0.05 m s1, 0.16 m s1, and 0.29 m s1). The evaporation rate increased in direct proportion to air velocity around the plants regardless of differences in leaf area. The guttation rate varied with leaf area; specially, tomato plants with small leaf areas secreted larger amounts of guttation water than those with large leaf areas. However, guttation was completely suppressed in both large and small leaves under well-circulated conditions (i.e., a veloc- ity of 0.3 m s1). This study indicates that air circulation reduces the wetting of plants by guttation under dark and high humidity conditions, which is likely to suppress the secondary spread of pathogens. Keywords : circulator, greenhouse, guttation, tomato, transpiration, wetting commercial-like greenhouse conditions, the secondary INTRODUCTION spread of C. michiganensis subsp. michiganensis is caused by workers touching the guttation droplets exuded from in- The wetting of plants is regarded as an undesirable oculated source plants. In comparison, once the guttation condition in greenhouses because of an increased risk of droplets have dried, spread does not occur by touching in- fungal and bacterial-incited diseases (Csizinszky et al., oculated plants (Sharabani et al., 2013). Tomato mosaic 2005). Droplets form on plants as a result of 3 factors as- virus (ToMV) and pepper mild mottle virus (PMMV) have sociated with the high humidity of greenhouses: (1) con- also been identified in the guttation water of infected to- densation falling from greenhouse covers; (2) condensation mato and green pepper plants, with the concentrations of on the leaf or fruit surface; and (3) guttation, which is the the virus particles being sufficient to lead to the infection of exudation of drops of xylem sap due to root pressure. The healthy plants (French et al., 1993). Since hydathodes presence of water on plants is often unavoidable in green- serve as efficient infection routes via guttation, the imple- houses. mentation of certain greenhouse air conditions that inhibit Among growers, guttation is widely believed to be a guttation might prevent the secondary spread of critical sign of plants having good root spread. Depending on plant pathogens. species and weather conditions, guttation on a plant may be Water droplets on leaf margins due to guttation are comparable to condensation on the leaves (Hughes and brought through the intercellular spaces of the leaf, called Brimblecombe, 1994), with this phenomenon being fre- the epithem, which results in these droplets being in con- quently observed under greenhouse conditions tinuous contact with the water in the vascular system (Joachimsmeier et al., 2011). Since guttation water is de- (Wilkinson, 1979). This channel through the leaf becomes rived from xylem sap through hydathodes (the structure active in darkness, when almost all the stomata close. through which water exudation occurs), it has a similar Guttation might be effectively suppressed by dehumidify- composition to the exudates that flow from the root to the ing greenhouse air and increasing transpiration rates. shoot in healthy plants (such as tomato and cucumber), and However, dehumidification is not unavailable for more than contains various minerals, such as P, K, Ca, and Mg half of the greenhouses in Japan, which are not equipped (Masuda, 1989). However, the appearance of droplet with dehumidifiers or heaters. Therefore, circulating air through hydathodes is regarded as a major invasion route of around the leaves, which decreases the thickness of leaf pathogens into host plants (Huang, 1986). For example, boundary layer (Yabuki and Harazono, 1978), might pro- Clavibacter michiganensis subsp. michiganensis, which vide an alternative means of increasing transpiration. causes bacterial canker in tomato plants, is transported into The present study aimed to examine methods to sup- the leaves via guttation droplets containing bacteria, and press the appearance of droplets on leaves by circulating air causes marginal necrosis (Carlton et al., 1998). Under the around plants. Guttation was provoked by placing individ- Corresponding author : Takeshi Kuroyanagi, fax: 81877621130, e-mail : [email protected] Vol. 51, No. 4 (2013) T. KUROYANAGI ET AL. ual tomato plants in a closed growth cabinet under constant and the chamber was taken under conditions being passive. dark and high humidity conditions. A stable temperature The fluorescent lamps in the chamber were not used during was produced in the cabinet and root zone, with three dif- the experiments. ferent air circulation intensities being created in the cabinet Measurement by adjusting the number of active fans attached to the floor Eight tomato plants were used in the experiments, of and the ceiling. The transpiration rate was monitored auto- which 4 were grown in 2011 and the other 4 were grown in matically, whereas the amount of droplets on the leaf mar- 2012. The experiments were conducted from November 8 gins was manually collected after subjecting plants to each to 18 in 2011, and from December 4, 2012 to January 11, air circulation level. Based on the findings of this study, 2013. The leaf area and fresh weight of the fruits of all the mechanism of transpiration under dark and high humid- plants were measured destructively after the experiments. ity conditions, along with the relationship between plant Individual plants were exposed to the three-level air characteristics and guttation, were considered towards im- circulation intensity: “No” (no fan running), “Low” (1 fan proving greenhouse management protocol. running on the ceiling), and “High” (all 8 fans running). Plants were subjected to each air circulation intensity for 1, MATERIALS AND METHODS 3, 6, and 15 h, to examine the effect of the exposure time on the magnitude of guttation and transpiration. One plant Plant and experimental system was subjected to 3 to 11 treatments, consisting of a combi- Non-grafted tomato (Solanum lycopersicum) cultivars nation of the air circulation intensity and the exposure time. (Reiyo, Sakata Seed Corporation, Kanagawa, Japan), which Between treatments, plants were rested for an interval of 1 was at the fruit developmental stage after being pinched h and more, during which time the cabinet and the chamber above second fruit truss, were used for the experiments. were not saturated with water vapor. Water with the same The individual seedlings were transplanted on September temperature to the room was supplied to the substrate be- 20, 2011, and October 5, 2012, into plastic containers with fore each treatment until drainage was observed at the bot- bars to support the stems and a rockwool substrate (200 tom of the container. Evaporation from the substrate was mm200 mm75 mm). The tomato plants were grown in prevented by the presence of a plastic board and a thin plas- an unheated greenhouse located at Kagawa, Japan (34.1°N, tic film. The droplets that formed on leaves as a result of 133.5°E), until use in the experiments. A mixture of guttation were collected by a researcher using about 15 Otsuka House No. 1 and No. 2 nutrient solutions (Otsuka pieces of cotton (66 mm50 mm2 mm) per treatment. Chemical Co., Ltd., Osaka, Japan) adjusted to 0.6 to 1.6 dS Droplets on the floor of the upper compartment were also m1 was supplied to the plants. 4-Chlorophenoxy acetate collected, as they were also regarded as guttation droplets. (Tomato Tone, ISK Biosciences K. K., Tokyo, Japan) was Droplets were collected from the chamber within 1 h, while sprayed onto young flowers to promote fruit set. The humidification lasted. The amount of the droplets on each leaves of the plants were not pruned; however, the fruit was leaf was derived from the difference of weight on each pruned to retain less than 5 fruits per truss. Apical and lat- piece of cotton before and after collection, using an electric eral buds were removed at least 1 week before the onset of balance (Adventurer Pro AV4102CU, Ohaus Corporation, each experiment. NJ, USA; repeatability 0.01 g). Moisture absorption of the A growth cabinet (680 mm680 mm1990 mm) in a cotton was negligible, as the cotton did not increase weight thermostatic chamber (MBCR-C5040, Sanyo Electric Co., after being left for 1 h in the humidified chamber. Ltd.; 2600 mm3250 mm2200 mm) was used to pro- The arrangement of measurement devices in the cabi- vide the three-level air circulation conditions under dark net is presented as a schematic in Fig. 1. The transpiration and high humidity conditions. The cabinet was composed rate of individual plants was measured using an electric of transparent PVC rigid plates which were sealed with balance (XP8002S, Mettler-Toledo International Inc., transparent adhesive tape. A perforated PVC rigid plate Greifensee, Switzerland; repeatability 0.008 g) placed in separated the cabinet into an upper and lower compartment. the upper compartment of the cabinet. The weight of the For air circulation, the upper compartment was equipped plant and container was recorded every minute on a PC via with 8 axial fans (ASEN 60511, Panasonic Corporation; software (Balance Link, Mettler-Toledo International Inc., 0.26 m3 min1); 4 of the fans were attached to the ceiling to Greifensee, Switzerland).