Effects of Nitrapyrin and Nitrogen Form on Tomato Growth, Water Relations, and Ion Composition

Nitrapyrin (Nt) at 10 ppm, compared to water control, added to a soil-sand-peat medium decreased shoot growth of tomatoes (Lycopersicon esculentum Mill. cv. Marglobe) grown with NOJ-N nutrition but had no effect on growth with NH.-N nutrition. While plant water stress was decreased by NI compared to the control with NOJ-N nutrition, the toxic effect of increased uncombined NH. concentration in the shoots may have caused the growth reduction. Denitrification suppression by NI with NOJ-N nutrition was evidenced by increased media and shoot NOJ concentrations. Nitrification suppression by Nt with NH.-N nutrition was evidenced by increased media and shoot NH. concentrations. Nitrapyrin, irrespective of N form, decreased shoot Ca concentration, increased shoot K and uncombined NH. concentrations, and had no effect on shoot Mg concentration. Plant water stress was decreased by NI with both N forms at the third and lifth weeks after linal transplanting. While NI decreased the differential effect of N form on transpiration rate and leaf diffusive resistance (Rt ), in NI's absence, NH.-N nutrition decreased tran­ spiration and increased RI. relative to NOJ-N. Decreased shoot growth with NH.·N relative to NOJ·N nutrition, both in the presence and absence of NI, was associated with increases in plant water stress, root hydraulic resistance, and shoot uncombined NH. concentration, but a decrease in shoot Ca concentration. Nitrapyrin (2-chloro-6(trichloromethyl)pyridine), N-Serve (Dow In this study, tomato responses to medium-incorporated NI Chemical Co.), was developed to inhibit Nitrosomonas spp ac­ with NO,-N and NH.-N nutritions were examined. The objec­ tivity (7). By inhibiting nitrification, nitrapyrin (Nl) tends to tives were to determine whether NI incorporation altered the conserve the more stable ammoniacal-N in the growth medium deleterious and beneficial effects of NH4 -N and NO,-N nutri­ (7) and therefore would be expected to accentuate the deleterious tions, respectively, on tomato growth. effects of NH. nutrition on tomato. Toxicity symptoms of NH. nutrition include reduced vegetative growth (10, 17) and leaf Materials and Methods and stem lesion formation (10). These symptoms have been After steam pasteurization (8rC minimum, 30 min) of a I linked to reduced plant Ca and Mg concentrations, increased Matapeake silt loam: I sphagnum peatmoss: I sand (by volume) anion concentration (8, 9), and increased uncombined NH. con­ medium, dolomitic lime and triple super-phosphate were incor­ centration in plants (2, 17). Ammonium nutrition, compared to porated at the rate of 5.5 and 2.8 kg m ", respectively. To one NO] nutrition, increased plant water stress (18), increased root half of the medium, an aqueous emulsion of 24E nitrapyrin (NI) and leaf resistances to water flux ( 17), and decreased plant water was incorporated by atomization at the rate of 10 ppm (weight use efficiency (8). a.i. per medium volume). The remaining half of the growth It has been reported recently that NI increased NO, retention medium received the same volume of distilled water (the control with NO, fertilization in a soil-peat-sand medium (12), a pine treatment). bark-sand medium (13), and a peat-vermiculite medium (16). Two weeks after sowing in vermiculite, seedlings of 'Mar­ Nitrapyrin was hypothesized to exert an inhibitory effect on globe' tomato were transplanted at 3 x 3 cm spacing in ver­ biodenitrification as well as on nitrification (12). This hypothesis miculite. Eighteen days later, 48 seedlings were transplanted was further substantiated when it was found that decreased ni­ singly into 4.5-liter glazed ceramic pots each containing 4 liters trous oxide generation in NI-treated media was due neither to of the growth medium. depletion of substrate NO, nor inadequate carbon source (5). Beginning 7 days after final transplanting, 6 weekly appli­ Since it is estimated that 10-30% of the N applied to soils is cations of 25 meq of NO,-N or NH4-N (0.35 g N) per pot were denitrified, with values often being much higher in excessively made. Nitrate-N was supplied as KNO" and NH4-N as (NH4 )2 wet soils (7), a NImediated reduction in denitrification would S04 with 1.87 g KCI to balance K. The salts were added to the be of great value in conserving medium NO,. medium surface and watered in. The 2 N forms and 2 NI levels in factorial combination were arranged in randomized complete block design with 6 replica­ 'Received for publication Aug. 17, 1981. Published with approval of tions and 2 plants per replication. The staked but unpruned plants the Director of the Delaware Agricultural Experiment Station as Mis­ were grown under natural light (June-August) on wire mesh cellaneous Paper No. 935. Contribution No. 128 of the Plant Science benches in a greenhouse maintained at 22'" 5°C. All treatments Dept. were watered as required with the same frequency and with The cost of publishing this paper was defrayed in part by the payment sufficient volume to cause a small amount of drainage through of page charges. Under postal regulations. this paper therefore must be hereby marked advertisement solely to indicate this fact. the port at the side wall base. 1Assistant Professors. The authors acknowledge financial support from At 3, 5, and 7 weeks from final transplanting, 3 days after the University of Delaware Research Foundation. N fertilizations, two IO-ml medium samples were obtained from J. Amer. Soc. Hort. Sci. 107(3):487-492. 1982. 487 a thoroughly mixed core 1-5 cm below the medium surface in each pot. Medium pH, electrical conductivity (EC), and NH4 concentration were determined on I sample at 1:5, growth me­ dium:distilled water volumetric dilution. Medium NO, concen­ tration was determined potentiometrically on the other sample at a 1:5 volumetric dilution. Potentiometric methods were de­ scribed previously (16). Leaf xylem pressure potential ($p), abaxial leaf diffusive re­ sistance (RL ), and abaxial leaf transpiration rate were determined on all plants during the 3rd, 5th, and 7th week from final trans­ planting, I day after medium pH, EC, and NO, and NH4 con­ centration determinations. Two hours after container capacity had been reached, $p was determined from 1300-1430 hr on leaflets from the plant mid-portion using a pressure chamber ( 18). Leaflet abaxial transpiration rate and RL were determined at 1430-1600 hr with aLi-Cor 1600 steady-state porometer. Shoot NO, and lmcombined NH4 concentrations, and shoot total N, Ca, Mg, and K concentrations were determined by methods described previously (16). After medium preparation and at the end of the study, medium total N concentration and exchangeableNH4 and fixedNH. concentrations were determined by the methods of Orion (15), Bremner and Keeney (4), and Silva and Bremner (19). respec­ tively. Results and Discussion The reduced shoot growth associated with NH.-N nutrition compared to that of NO,-N that occurred in both Nt-treated and control media (Table l) is documented (10, 17). Mills and Po­ korny (13) found NI to reduce tomato growth with NH4 -N nu­ trition in a 100% sand medium but to increase growth when sand was 0-75% of a pine bark:sand mix. They concluded that when pine bark comprised part of the medium, growth was enhanced by Nt incorporation in part by chemical "tie-up" of NH. and in part by increased NO, retention in the medium. In the present study, shoot growth was unaffected by NI incor­ poration in the medium with NH.-N nutrition (Table I) indi­ cating that NI did not intensify the adverse (toxic) effect of NH4 ­ N on growth. Some studies show NI incorporation in media to have little if any effect on crop growth with 100% NO,-N nutrition (12, 13). tn agreement with previous work (16), NI with NO)-N nutrition decreased shoot fresh and dry weights by 9.6 and 16.5%, re­ spectively (Table I). Mills and Pokorny ( 13) attributed increased tomato growth with NI incorporation in the media to increased NO) concentration when NO,-N comprised 0-75 % of the NO,:NH4 -N nutrition. In the present study, medium NO, con­ centration was increased by Nt incorporation with NO,-N nu­ trition (Fig. I) as shown by others (5, 13, 16). Since both medium NO, concentrations (Fig. I) and shoot NO, concentra­ tion and content (Table 2) were increased by Nt treatment com­ pared to the control treatment with N01-N nutrition, it can be concluded that Nt suppressed denitrification, since NO, uptake was unhindered. The significant interaction of N form and ni­ trapyrin on shoot NO, concentration and content (Table 2) showed that nitrapyrin, compared to its absence, increased shoot NO, concentration and content only with NO)-N nutrition. This shoot NO) accumulation (120 and 86% increase in concentration and content, respectively) may reflect a NImediated reduction in nitrate reductase activity as reported by Notton et al. (14). The decreased shoot Ca concentration and content, and yet the increased shoot uncombined NH. concentration with NI treatTable I. Effect of nitrapyrin and N form on shoot fresh and dry weights. and shoot dry weight as a percentage of fresh weight' Nitrapyrin' Nitrogen form' Shoot fresh wt