Mechanical Conditioning for Controlling Excessive Elongation in Tomato Transplants: Sensitivity to Dose, Frequency, and Timing of Brushing

During production of plug transplants, the high plant density results in rapid stem elongation as plants compete for light. The resulting tall, weak-stemmed plants are difficult to transplant and are easily damaged. One technique that can prevent excessive elongation is mechanical stimulation by brushing. Wide adoption of brushing is limited by a lack of information on how plants respond to variations in applying the technique. Our investigation examined how tomato (Lycopersicon esculentum cv. Oh8245) seedling growth responded to varying doses of mechanical stimulation, varying intervals between brush strokes during stimulation, time of day that stimulation was applied, and growth stage at which application started. Seedlings were grown in 288-cell flats at 2100 plants/m. Daily doses from 0 to 40 brush strokes were applied from canopy closure until the nontreated plants reached a canopy height of 15 cm. The final height was reduced by ≈20% for all brushed treatments, with little further effect with >10 strokes/d. Intervals between strokes as long as 10 minutes resulted in the same reduction in the rate of stem elongation as the same daily dose applied in one continuous treatment. Treatments were similarly effective whether applied in the morning or late afternoon. Treatments begun at a canopy height of 6 (canopy closure), 8, or 10 cm gave similar reductions in the rate of stem elongation. Plants grew 6 mm·d–1 when they were not treated and 3 mm·d when treated. Therefore, the final height was directly related to the number of treatment days. Stimulation appears to be sensed and integrated over at least half an hour and the reduction in the rate of stem elongation expressed over the subsequent daily cycle of growth. All results indicate that there is substantial flexibility in applying brushing for controlling elongation in tomato transplants. Production of vegetable transplants in Canada and the northern United States is commonly done at high plant densities because the major production costs (greenhouse construction and heating) are related to the production area (Marr and Jirak, 1990). Commercial processing tomatoes are commonly grown in 288-cell plug trays at 2100 plants/m (Garton 1990). Close spacing results in a number of morphological changes collectively referred to as the shade avoidance response. Plants that shade each other distribute a larger proportion of dry matter to stem elongation to compete for the limited amount of light in the plant canopy (Smith 1994). Shade avoidance is characterized by an increase in internode and petiole elongation and a reduction in leaf area, lamina thickness, and specific stem weight. There are several drawbacks to excessive elongation: tall transplants with weak stems are difficult to handle and often lodge after transplanting, thereby increasing the risk of plant damage and disease. In addition, mechanical transplanters are designed to accommodate plants of a specified size. Taller plants catch in the mechanism, resulting in damaged or skipped plants in the field. For these machines to function properly, the plants must be “...uniform, short, and sturdy...” (Shaw 1993). However, unfavorable weather conditions can accelerate growth in the greenhouse or delay preparation of the field for transplanting. Height control, therefore, is an economic necessity for growers. for publication on 27 Dec. 1995. Accepted for publication 26 Apr. 1996. rch was supported by the New York State Tomato Research Association, nt NYG632506 and a Cornell Graduate Research Assistantship to L.C.G. de names in this publication does not imply endorsement of the named r criticism of similar ones not mentioned. The cost of publishing this defrayed in part by the payment of page charges. Under postal regula­ paper therefore must be hereby marked advertisement solely to indicate Many methods are used to control transplant height, including plant growth regulators, withholding water or nutrients, temperature control, and clipping the shoots. These methods require high levels of management, often have long-term effects on plant growth, and may delay early yields (Adler and Wilcox, 1987; Hickman et al., 1989; Heins and Erwin, 1990; Jaworski et al., 1970; Latimer, 1992; ). Vegetable transplant growers need a method for controlling transplant height that does not have these drawbacks (Price and Zandstra 1988). Such a method, which should be easy and inexpensive to apply on a large scale, currently is unavailable to most growers. The simplest method of mechanical stimulation (brushing) can be applied by using a relatively non-abrasive material, such as bond typing paper (Biddington and Dearman, 1985), cardboard (Latimer, 1990), polyvinyl chloride pipe (Latimer and Thomas, 1991), or a wooden dowel (Baden and Latimer, 1991). Many plants are stimulated at once as the material is moved across the plant canopy contacting the leaves and bending the stems. Mechanical stimulation is, in principle, an excellent means of limiting undesirable stem elongation, and it also can increase stem strength and specific chlorophyll content (Latimer, 1991). Mechanical stimulation may avoid the detrimental effects of stressbased treatments and be more easily adapted to commercial use than temperature-based treatments. Furthermore, these changes in plant growth and morphology occur without long-term inhibition of plant growth. However, certain aspects of the physiological response to stimulation need to be better understood to develop a greenhouse treatment that can be adapted for growers’ differing needs. The key questions that need to be answered for each growing condition are how much, how long, and when to treat. These questions were answered for tomato seedlings by investigat­ ing the growth response to varying doses of mechanical stimula­ tion, the ability of the plant to sum the stimuli, the interaction with J. AMER. SOC. HORT. SCI. 121(5):894–900. 1996. the diurnal growth cycle, and the sensitivity to mechanical stimu­ lation at different growth stages. Materials and Methods Plant culture. Experiments were conducted with the processing tomato, ‘Ohio 8245’ (Sunseeds, Hollister, Calif.). Seeds were planted in “No. 288 square deep” plastic plug trays (Landmark Plastics Corp., Akron, Ohio) with an individual cell volume of 6.5 ml and 2100 plants/m. This tray size is recommended for the commercial production of processing tomato transplants (Garton Fig. 1. Applying brushing treatment to plug-grown tomato transplants. The trans­ plants were stimulated mechanically by brushing with a piece of polystyrene foam with enough pressure to bend the stems slightly. Table 1. The effect of the number of brush strokes per day on the height, ste of processing tomato transplants. The repetitions of the experiment are i