Mechanical Conditioning of Tomato Seedlings Improves Transplant Quality without Deleterious Effects on Field Performance

Excessive stem elongation reduces plant survival in the field and hinders mechanical transplanting. Mechanical conditioning is an effective method for reducing stem elongation during transplant production. This investigation examined the conse­ quences of mechanical conditioning, using brushing and impedance, on subsequent field performance of tomatoes (Lycopersicon esculentum Mill.). Mechanically conditioned transplants of processing tomatoes resumed growth after transplant shock as quickly as did untreated plants, and subsequent canopy development was also equal. In 4 years of field trials, yield was not reduced by mechanical conditioning. Transplants for freshmarket tomatoes may be more sensitive to injury than those for processing tomatoes because they flower sooner after the conditioning treatments. Nevertheless, neither earliness nor defects in the fruits of the first cluster were affected by mechanical conditioning. Early and total yields were equal in both years that fresh-market crops were tested. Thus, there were no adverse effects on field performance of either processing or fresh-market tomatoes as a result of reducing stem elongation by mechanical conditioning before transplanting. Improved wind tolerance was tested both in a wind tunnel and in the field. In wind-tunnel tests, brushed and impeded plants resisted stem bending at wind speeds 4 to 12 km·h higher than did untreated plants. A 70 km·h wind after transplant­ ing killed 12% of untreated plants but only 2% of treated plants. Mechanical conditioning with brushing and impedance produced transplants with desirable qualities without adverse effects on field performance. Mechanical conditioning, either by brushplants are easier to handle and less susceptible ing or by impedance, produces stocky, unito damage during manual transplanting form, high-quality tomato transplants (Garner (Johijima and Latimer, 1992; Latimer and and Björkman, 1996; Latimer and Thomas, Mitchell, 1988; Latimer et al., 1991). Also, 1991). Plants must be within a narrow height mechanical conditioning results in stronger, range (12–16 cm) to go through a mechanical more elastic tomato transplant stems (Heuchert transplanter consistently and without damage et al., 1983) so that newly set transplants (Shaw, 1993). Mechanical conditioning reshould resist wind breakage, and tilt less onto sults in several morphological and physiologihot plastic mulch or disease-bearing soil. Fi­ cal changes that should help to increase the nally, mechanical conditioning reduces the survival rate of tomato seedlings soon after need for hardening by making transplants of transplanting to the field. Conditioned transmany species less susceptible to water and temperature stress (Biddington, 1986; Jaffe and Biro, 1979). Received for publication on 11 Nov. 1998. AcHowever, any growth-inhibiting treatment cepted for publication 2 Mar. 1999. This research may continue to inhibit growth after transwas supported by the New York State Tomato planting. Chemical growth inhibitors such as Research Association; by donations of seed from daminozide, formerly used on tomatoes, have Furman Foods, Northumberland, Pa., and flats from this drawback (Jaworski et al., 1970). FurtherLandmark Plastics, Akron, Ohio; by Hatch Grant more, mechanical conditioning can delay flow­ NYG632506; and by a Cornell Graduate Research Assistantship to L.C.G. Use of trade names in this ering (Akers and Mitchell, 1985), reduce nupublication does not imply endorsement of the named trient uptake (Adler and Wilcox, 1987), and products, nor criticism of similar ones not meninjure the apical meristem (Latimer, 1994). tioned. The cost of publishing this paper was deWe investigated whether any of these negative frayed in part by the payment of page charges. Under consequences to field production of tomatoes postal regulations, this paper therefore must be occurred when mechanical conditioning was hereby marked advertisement solely to indicate this used for height control during transplant profact. duction. Current address: Dept. of Botany and Plant SciChanges in transplant growth due to me­ ences, Univ. of California Riverside, Riverside, CA chanical conditioning could ultimately reduce To whom reprint requests should be addressed. Efield performance of tomato plants. Treatmail: [email protected] ments could have long-term effects on the growth rate of the plant, and size differences at transplanting may not be overcome in the field. Furthermore, early flowers and fruit may be delayed or damaged because the last mechanical treatments are being applied dur­ ing the development of the meristem that forms the first flower cluster. These problems could ultimately result in yield reduction. Mechani­ cally conditioned tomato transplants show no adverse after-effects for greenhouse tomato production (Johijima and Latimer, 1992). The objective of our experiments was to determine if mechanical conditioning had an adverse effect on the field performance of processing and fresh market tomatoes. Materials and Methods Culture and treatment: processing toma­ toes. ‘Ohio 8245’ processing tomatoes were seeded in 288-cell flats (Landmark Plastic Corp., Akron, Ohio) on 9 Apr. in 1992 through 1995. Each cell is 20 mm square, 44 mm deep, and has a volume of 9 mL. The plants were grown in a greenhouse at 20 °C day/15 °C night. They were fertilized twice weekly at watering with Peters Professional 20–20–20 fertilizer (Grace-Sierra Horticultural Products Co., Milpitas, Calif.; 20N–8.7P–16.6K) at an N concentration of 100 g·m. Mechanical conditioning treatments were begun when the leaf canopy closed, at which time the seedlings were 6 cm tall and 17 d old. The brushing treatment was applied with an unpainted, 25­ mm-diameter hardwood dowel pulled gently 20 times, back and forth, across the canopy at 8:30 AM each day for ≈15 d until the plants were moved outside. The impedance treat­ ment was applied by suspending an acrylic sheet (4 mm thick) just below canopy height overnight (Samimy, 1993). The characteris­ tics of the processing seedlings at transplant­ ing were reported in Garner and Björkman (1997). Briefly, the treated plants were 3 to 4 cm shorter than the controls and the stems of impeded plants were ≈20% thicker. After 4–5 d hardening in an outdoor cold frame, the seedlings were transplanted to the field at the Fruit and Vegetable Research Farm (Lima silt loam) in Geneva, N.Y., on 20 May in 1992 and 1993, and on 23 May in 1994 and 1995. A different field was used in each year. Plants were transplanted with a mechanical transplanter (Mechanical Transplanter Co., Holland, Mich.) in rows 1.25 m apart, with plants spaced 0.5 m apart in the row. Each plot was 12 m long. Treatments were laid out in a randomized complete block with three (1992– 94) or five (1995) replications. Culture and treatment: fresh-market to­ matoes. ‘Sunrise’ fresh-market tomatoes were seeded on 5 Apr. 1994 and 1995, in 50-cell flats (Landmark) with cells 44 mm square and 55 mm deep, holding 66 mL. The greenhouse conditions and fertilizer were the same as for processing transplants. Brushing treatments were begun when the leaf canopy closed, when seedlings were 28 d old. Twenty gentle strokes were applied daily for 20 d with an unpainted broomstick. At transplanting on 23 May, seedlings were 25 cm (control) and 20 HORTSCIENCE, VOL. 34(5), AUGUST 1999 848 92521 cm (brushed) tall in 1994, and 42 cm (control) and 31 cm (brushed) tall in 1995. The differ­ ence between treatments was significant at P < 0.001. Plants were grown at the Fruit and Veg­ etable Research Farm in Geneva, N.Y. In 1994, plants were transplanted by hand onto bare ground in rows 1 m apart with 0.5 m between plants. In 1995, plants were trans­ planted with a water-wheel transplanter through black polyethylene mulch into raised beds 1.5 m apart with 0.3 m between plants; plots were 3 m long. The treatments were arranged as five paired plots. Suckers more than three nodes below the first flower cluster were removed before short-stake trellising (Peirce, 1987). Data collection: processing tomatoes. For measuring recovery from transplant shock in processing transplants, the stem length of all plants from the soil level to the growing point was measured every 3 to 4 d, until lateral growth became pronounced and stem length was no longer a valid estimate of the rate of plant growth. The stem diameter 2 cm above the cotyledons was measured 30 d after trans­ planting. The canopy area of young plants was estimated during the field season by measur­ ing the diameter of the canopy of nine plants per plot. The time to flowering (50% of the plants with open flowers) was determined from observations every 1 to 2 d. The crop was harvested for yield when the fruit were full size and ≈50% were at the red ripe stage. The single hand harvest was on 17 Sept. 1992, 9 Sept. 1993, 1 Sept. 1994, and 25 Aug. 1995. Differences in yield and time to flowering were tested by one-way analysis of variance (Schaefer and Farber, 1992). Data collection: fresh-market tomatoes. The time to flowering (50% of the plants with open flowers) was determined from observa­ tions every 1 to 2 d. Fruit were harvested weekly for 5 weeks beginning when ≈10% of the fruit were at the breaker stage (27 July 1994 and 12 July 1995). The early yield was the combined yield of the first 2 weeks. These harvests included all of the fruit produced at the first flower cluster of each plant. Harvest data were analyzed by paired t test each year. Wind tunnel. The effect of high wind on stem strain was studied in the Upson LowNoise Wind Tunnel (Mechanical and Aero­ space Engineering Dept., Cornell Univ., Ithaca, N.Y.). Brushed and impeded plants of ‘Ohio 8245’ were compared to controls in separate tests of 72 plants each on the day that the plants would otherwise have been moved outside. Randomly selected plants were arranged in a 288-cell flat, taking care to minimize physical interaction among the plants. The flat was placed at the tunnel exit, in the region of uniform nonturbulent flow. stems. The data were analyzed as a logistic response to estimate the critical wind speed to bend half the untreated plants (vc), and the difference in critical wind speed due to condi­ tioning (vd). The model Wind speed = vc + vd + α logit (proportion bent) was fit to give estimates and standard devia­ tions for each of the parameters vc, vd, and α. Separate models were fit for each type of conditioning. Results and Discussion Growth of processing tomatoes. Tomato transplants that had been mechanically condi­ tioned using brushing or impedance, while initially shorter, suffered no long-term growth effects after transplanting to the field. Neither treatment delayed recovery from transplant shock: stem elongation resumed on the same day in all treatments (Fig. 1). Rapid elongation began after 15 d in 1994 and 17 d in 1995. In 1995, a few cold days 15 to 17 d after trans­ planting slowed growth. There is some con­ solidation of the soil immediately after trans­ planting that results in an artifactually nega­ tive apparent growth rate. The close corre­ spondence in elongation rate among the treat­ ments, even as the rate fluctuated, emphasizes that there is no lingering growth inhibition. The three treatments began to flower within a day of each other (Table 1). Four weeks after transplanting, there were no significant differ­ ences between treatments in stem diameter or canopy area (Table 2). Thus, the amount of mechanical stimulus that was sufficient for effective height control of tomato transplants did not have significant long-term effects on the growth rate of the plants after transplanting. This result is consis­ tent with the finding that many plant species quickly resume growth after mechanical stimu­ lation. The rate of stem elongation is the same as, or higher than, controls within 3 to 4 d after the discontinuation of mechanical stimulation (Jaffe, 1973). Reproductive development of fresh-market tomatoes. There are additional concerns about the effects of conditioning on subsequent deFig 1. Effect of mechanical conditioning on stem elongation following transplant shock in ‘Ohio 8245’ processing tomato transplants. The stem elongation rate is the mean for the period since the previous measurement. The vertical bars are the standard error, when it exceeds the size of the symbol. Table 1. Effect of mechanical conditioning on time to flowering in process­ ing and fresh-market tomatoes. No variations among treatments were statistically significant at P < 0.05. The susceptibility of plants to wind injury was measured as the amount of stem bending. The number of plants with the basal 1 cm of the stem bent >45° from vertical was recorded while the wind velocity was increased from 35 to 94 km·h. Wind velocity was increased once per minute in increments of ≈3 km·h . The wind-tunnel treatment did not break the Conditioning