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Agronomy Journal 92:159-167 (2000)
© 2000 American Society of Agronomy

FIELD-GROWN TOMATO

Nitrogen Stress Effects on Growth and Nitrogen Accumulation by Field-Grown Tomato

Johannes Scholberga, Brian L. McNeala, Kenneth J. Booteb, James W. Jonesc, Sal J. Locasciod and Stephen M. Olsone

a Soil and Water Science Dep., Univ. of Florida, Gainesville, FL 32611-0510 USA
b Agronomy Dep., Univ. of Florida, Gainesville, FL 32611-0510 USA
c Agricultural and Biological Engineering Dep., Univ. of Florida, Gainesville, FL 32611-0570 USA
d Horticultural Science Dep., Univ. of Florida, Gainesville, FL 32611-0690 USA
e North Florida Research and Education Center, Route 3 Box 4370, Quincy, FL 32351-9529 USA

blm{at}gnv.ifas.ufl.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
There are few growth studies evaluating within-season effects of N on vegetative growth and N accumulation of tomato (Lycopersicon esculentum Mill.). Growth analysis of field-grown tomato for a number of Florida locations and management systems is presented here. Severe N stress resulted in fewer and smaller, but thicker, leaves. With increasing N, average leaf area index (LAI) increased from {approx}0.75 to {approx}3, but radiation use efficiency (RUE) typically increased less then 30%. Lower RUE under N-limited conditions reflected a decrease in N concentration of the most recently matured leaves from 40 mg g-1 to as little as 15 mg g-1. Over the life of well-fertilized crops, leaf N concentrations dropped from 55 to 65 mg g-1 during initial growth to 20 to 35 mg g-1 at final harvest. Corresponding N concentrations for fruit and for stems were 30 to 35 mg g-1 and 15 to 25 mg g-1. Severe N stress affected leaf and stem N concentrations most drastically, whereas N in fruits was less variable. With lower N supply (N < 180 kg ha-1) under careful management, nitrogen use efficiency (NUE) for field-grown tomato was {approx}0.4 Mg fresh fruit (kg N)-1 and average crop N accumulation increased from 37 to 210 kg N ha-1 as N fertilization increased from 0 to 333 kg N ha-1. As a fraction of the fertilizer N applied N fertilizer recovery ranged from 0.36 to 0.74 and 0.61 to 0.96 for drip-irrigated and subirrigated crops, respectively.

Abbreviations: ANR, apparent nitrogen recovery • LAI, leaf area index • NUE, nitrogen use efficiency • RUE, radiation use efficiency


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
COMMERCIAL tomato production requires optimal fertilizer and water management for high yields and maximum profits. In many cases, N is the element that most limits crop growth, especially on coarse-textured, low organic-matter soils. Under typical Florida conditions, N is readily lost by leaching and denitrification, with N concentrations in shallow groundwater often dropping below 1 mg L-1 for much of the growing season despite the maintenance of adequate to excessive N levels in the overlying rootzone (McNeal et al., 1994). For subirrigated mulched crops, all N fertilizer is normally applied preplant. For drip-irrigated crops on coarse-textured soil, fertilizer N is typically applied both preplant (20–40% of the total) and with the irrigation water (fertigation) (Locascio et al., 1992; Maynard and Hochmuth, 1995). The lower-limit fertilizer requirement for high-yielding tomato varieties can be as low as 100 to 150 kg N ha-1 (Doorenbos and Kassam, 1986), though in Florida the amount of recommended N fertilization is 180 to 200 kg ha-1 (Maynard and Hochmuth, 1995) and commercial N fertilizer applications are often two- to threefold greater than University of Florida recommendations. Because of the high value of commercial tomato crops and the relatively low cost of fertilizer, producers tend to overfertilize in order to minimize any risk of yield reduction due to nutrient stress (Locascio et al., 1992). To minimize the concurrent risk of groundwater contamination with nitrate N, N supply should be maintained in phase with the crop's requirements. Maximum yields can reportedly be produced with 134 to 224 kg N ha-1 if irrigation management is optimal (Hochmuth, 1988).

Although the body of knowledge on the effects of N fertilization on tomato yield for a specific production system is appreciable, relatively little effort has been made to synthesize this information into a more general knowledge base. Locascio et al. (1992) compiled yield results for the southeastern United States and the Caribbean Islands. However, functional relationships that outline overall yield responses for a range of environmental settings (Bar-Yosef and Sagiv, 1982; Greenwood and Draycott, 1988) have not been established in the humid southeastern USA. The work reported here is part of an overall study designed to provide such information for an associated tomato crop-growth model.

To improve NUE of a specific production system, it is helpful to study the effect of reduced N supply on plant development, canopy characteristics, N accumulation, and dry matter production over time. However, researchers typically only evaluate overall yield responses and associated nutrient contents (Doss et al., 1975; Kaniszewski et al., 1987; Locascio et al., 1992), measure only resultant dry matter accumulation (Bar-Yosef and Sagiv, 1982), or conduct experiments under glasshouse settings instead (Nicola and Basoccu, 1994). Canopy, leaf and rooting characteristics and N uptake dynamics under glasshouse conditions are often vastly different from those for field settings. The supply of relatively constant nutrient concentrations (Huett and Dettmann, 1988) in glasshouses also affects N uptake dynamics.

The research presented here outlines growth and yield responses of field-grown tomato as influenced by N fertilizer for several management systems and geographical regions throughout Florida. Effects of N fertilization on plant growth, canopy characteristics, and N accumulation are discussed and NUE values for these production systems are compared.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Yield data for tomato over a range of N fertilization rates were obtained from the literature for locations throughout the world and from additional recent studies in Florida and were fitted to a quadratic regression model (Table 1) . This model was chosen for its simplicity and its easy parameterization. A number of the data sets had only a limited number of N treatments, which did not allow parameterization of more complex models. In the expression y = a + bN - cN2


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  		Table 1 Overview of reported fresh-fruit yield response of tomato to N fertilizer for a number of production regions

 
where y = fruit yield (Mg ha-1); a, b, and c are regression coefficients; and N = N fertilizer amount (kg N ha-1), it was observed that yields for zero-N treatments were typically well above zero. It was assumed that differences in zero-N response were related to the amount of soil N (mineralization plus residual fertilizer N) available for plant uptake. To estimate this soil contribution, it was assumed that, at lower N levels, the linear component of the quadratic equation would be much larger than the quadratic component (i.e., that the expression ). If we know y0 (the yield with no N fertilizer applied), we can then solve the equation readily by assuming that the value of b (the initial slope of the N-response curve) represents the maximum NUE and the N fertilizer amount (Nmax) at which maximum yield (Ymax) should be attained can be calculated by setting the first derivative of each equation in Table 1 to zero.

Experiments involving a range of N fertilizer treatments also were conducted during the spring 1995 growing season at the Gulf Coast Research and Education Center in Bradenton, FL; during spring 1996 at the Univ. of Florida Horticultural Unit in Gainesville; and in the fall of 1995 at the North Florida Research and Education Center in Quincy. An overview of crop management practices, N fertilizer rates, and planting dates is presented in Table 2 . Average weather data and soil designations for each experimental site were presented by Scholberg et al. (2000). The irrigation method used at Bradenton was a subirrigation (fully enclosed seepage) system, as described by Scholberg et al. (2000). Water use is lower with drip irrigation than with subirrigation (Locascio et al., 1992) and parts of the bed (especially with coarse-textured soils) may remain dry, where nutrients may accumulate and become unavailable for plant uptake.


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  		Table 2 Tomato management practices used in experiments conducted at Bradenton (1995), Gainesville (1996), and Quincy (1995)

 
At each location, plants were sampled at 2- or 3-wk intervals. Two or three representative plants (depending upon location and associated experimental design) were sampled destructively from the experimental plots on each sampling date. Plant growth was measured using procedures described by Scholberg et al. (2000). Radiation interception was estimated using a hedgerow light interception model (Boote and Pickering, 1994), with daily dry weight accumulation then being plotted versus estimated cumulative intercepted daily radiation (Bennett et al., 1993). Leaf photosynthesis was measured 49 d after transplanting on six recently matured, sunlit leaves (PAR > 1500 µE m-2 s-1) per plot at Gainesville (1996) and Quincy (1995) using an LI-6200 Portable Photosynthesis System (LI-COR, Lincoln, NE). This timing was selected so that the measurements would remain within the linear growth phase for fruits at these locations. Plant tissue samples were dried at 65°C, weighed, and ground. A 100-mg plant sample was digested by adding 2.5 mL of 98% sulfuric acid and 1.0 g of potassium sulfate–catalyst mixture. Digestion tubes were heated in a digestion block to 380°C for 4 h. After digestion, samples were diluted to 100 mL and subsamples were transferred to 20 mL scintillation vials and stored at 5°C prior to analysis. Total N was determined using Rapid-Flow Analyzer (RFA) technology (ALPKEM Corp.). Nitrogen accumulation by the plant was calculated by multiplying weights of root, stem, petiole, leaf, and fruit tissue by the corresponding N concentrations.

Experimental data were fitted using linear regression analysis and the slopes of the regression equations were compared using a paired t-test (SAS Institute, 1989).


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Yield Response to N Fertilizer
An overview of tomato yield response to N fertilization is presented in Table 1. The value of the regression equation y-intercept (fresh fruit yield for the zero-N treatment) was typically 20 to 40 Mg ha-1 for coarse-textured soils and 40 to 70 Mg ha-1 for medium-textured soils. By dividing the value of the intercept by the initial (nonquadratic) regression–equation slope, the soil contribution to the N requirement at a given location (N0) could be estimated (as discussed at the beginning of the Materials and Methods section). To our knowledge, this is a novel approach to the estimation of N0 values. The value of N0 then was added to the N fertilizer amount (N) to estimate total plant-available N. Plotting fruit yield vs. total soil + fertilizer N resulted in quite uniform N-response curves for most of the experimental locations (Scholberg, 1996). Under Florida conditions, experimental values for initial NUE (as estimated from the initial linear regression slopes) ranged from 0.12 to 0.40 Mg fresh fruit (kg N)-1, with values of {approx}0.35 being representative (Table 1). Bar-Yosef and Sagiv (1982) reported a NUE value of 0.07, which is very low relative to the values reported here. Using carefully regulated irrigation and greater plant densities increases NUE (Doss et al., 1975; Nassar, 1986), whereas excessive water supply coupled with high initial N fertilization may increase N leaching and reduce NUE (Hochmuth, 1990). With efficient use of both water and nutrients and a narrow row spacing, fruit yields approaching 100 Mg ha-1 reportedly can be attained with 180 kg fertilizer N ha-1 (Hochmuth, 1996). Assuming a value for N0 of 61 kg N (Table 1: Gainesville, 1996: a / b = 22 / 0.36 = 61), this would translate to an NUE of {approx}(100 / (180 + 61) = 0.4 Mg fruit (kg N)-1, consistent with the {approx}0.35 generalization above.

Canopy Characteristics and Radiation-Use Efficiency
In our experiments, LAI was greatest with higher N fertilization, with respective maximum values increasing from {approx}0.75 to {approx}3 (production-area basis, excluding field ditches and drive paths) as N fertilization increased from 0 to {approx}300 kg N ha-1 (Table 3) . Increased N fertilization also resulted in slightly later onset of canopy senescence and thus in higher late-season LAI values. The increase in LAI with increase in N fertilization was associated with increases in mean leaf size (Fig. 1a) and leaf number (Fig. 1b). Specific leaf area (data not shown) ranged from 250 cm2 g-1 early in the season to {approx}150 cm2 g-1 by season's end for the zero-N treatment in one study (Gainesville 1996). As N fertilization increased to 200 to 300 kg N ha-1, specific leaf area for fertilized treatments peaked at {approx}370 cm2 g-1 and declined to a late-season value of {approx}200 cm2 g-1. The most drastic changes in leaf characteristics typically occurred between N fertilizer amounts of 0 and 100 kg N ha-1. The N-stressed plants typically had fewer leaves and leaves were both smaller and thicker (smaller specific leaf area values). Similar results were reported for young tomato plants by Nicola and Basoccu (1994).


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  		Table 3 Summarized growth measurements, N-fertilizer studies at Bradenton (1995), Gainesville (1996), and Quincy (1995)

 


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  		Fig. 1 Average leaf size (a) and leaf number (b) with 0, 100, 200, and 300 kg N at Bradenton (spring 1995)

 
Effects of N fertilization on photosynthesis for Gainesville (1996) and Quincy (1995) are presented in Fig. 2 . Similar trends and correlations were obtained if results were plotted against specific leaf N (g N m-2 leaf area) instead (data not shown). Even though leaf N concentrations with the zero-N treatment at Gainesville in 1996 were in the deficient range (<25 mg g-1; Hochmuth et al., 1991), photosynthesis still averaged 73% of the maximum observed rate. Motta and Medina (1978), using N-depleted media to induce severe N stress in young tomato plants, reported resulting tissue-N concentrations of {approx}13 mg g-1, with observed photosynthetic rates still {approx}60% of maximum observed rates.



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  		Fig. 2 Leaf photosynthesis of recently matured leaves with 0, 133, 200, 266, and 333 kg N at Gainesville during the spring of 1995 (a) and with 0, 66, 133, 200, and 266 kg N at Quincy during the fall of 1995 (b)

 
Maximum photosynthetic rates for tomato leaves (Scholberg, 1996) were attained at tissue concentrations of 38 mg N g-1 and specific leaf N values of {approx}2.0 g N m-2. Although severe N stress in tomato results in the formation of smaller and fewer leaves, overall photosynthetic capacity per unit leaf area is not reduced as readily nor as much as for a crop such as non-nodulated peanut (Sinclair et al., 1993). The concurrent increase in leaf thickness allowed the plant to maintain relatively high photosynthetic rates (on a leaf-area basis) under N-limiting conditions even though N concentration per unit leaf dry weight was lower (Wolfe and Kelly, 1992). Average photosynthetic rates reported here are slightly higher than reported for glasshouse tomato (Jones et al., 1989), but slightly lower than those for peanut (Sinclair et al., 1993).

Effects of N fertilization on RUE (slope of the dry matter versus intercepted radiation curve) of tomato grown at Bradenton and Quincy are shown in Fig. 3 . Radiation interception was calculated using a hedgerow light interception model (Boote and Pickering, 1994) based on measured canopy height, canopy width, LAI, the assumption of an ellipsoid plant shape, plant spacing, time of day, day of year, latitude, and row azimuth. This approach computes direct-beam extinction coefficients hourly as a function of solar elevation and approximate leaf-angle distribution among three angle classes. Once the canopy shadow has been factored in, the model considers sunlit and shaded LAI similar to the corresponding approach for a horizontally uniform canopy. Earlier computations (Scholberg, 1996) had been based on a single-leaf canopy-photosynthesis model, but the hedgerow model was felt to be more appropriate for settings such as these where the crops are grown in distinct rows, with leaves clumped around the main axis of the plant. Observed values for RUE under near-optimal conditions ranged from 0.81 to 1.25 g MJ-1. Higher RUE values at Quincy may be related to the lower overall radiation levels for this fall-season crop (typically crops are more efficient in light utilization at lower radiation levels; e.g., Challa and Heuvelink, 1993). Differences with the amount of N fertilization were nonsignificant above 66 kg N ha-1 and, after the fertilization data were pooled, the RUE of the combined data was higher (p < 0.01) than the value for the zero-N treatments at both Bradenton and Quincy (Fig. 3a and b) but not at Gainesville (data not shown). Although severe N stress reduced light interception and biomass accumulation appreciably, the resulting reduction in RUE was typically less than 20 to 30%. It appears, however, that reduced photosynthetic area and light interception was the prime cause for yield reduction due to N stress, with the reduction in photosynthesis per unit leaf area being of lesser importance. A practical implication of these results is that a reduction in recommended N fertilization may require a revision of recommended plant spacings in commercial settings to maximize light interception and crop yield.



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  		Fig. 3 Total dry weight accumulation over time as a function of cumulative intercepted radiation with 0, 100, 200, and 300 kg N at Bradenton during the spring of 1995 (a) and with 0, 66, 133, 200, and 266 kg N at Quincy during the fall of 1995 (b)

 
Dry Matter Accumulation
Absolute growth rates for tomato at Bradenton and Quincy increased from 0.007 g plant-1 d-1 for transplants to a maximum rate of {approx}17 g plant-1 d-1 70 d after transplanting (Fig. 4) . This translates to maximum dry matter accumulation rates of {approx}190 kg ha-1 d-1. Published values include 12 to 14 g plant-1 d-1 (200–250 kg ha-1 d-1) for glasshouse tomato (Bertin and Gary, 1993) and 200 kg ha-1 d-1 for some other field-grown crops (van Keulen, 1981). With no N fertilizer, growth rates were only 20 to 30% of maximum rates. Reductions in growth rate due to moderate N stress typically became obvious only toward the end of the growing season, when N depletion in the soil started to affect plant growth (e.g., Fig. 4b). The growth response to added N typically was most pronounced between 0 and 100 kg N ha-1, with similar findings reported for solution experiments conducted by Larouche et al. (1989).



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  		Fig. 4 Absolute growth rate with 0, 100, 200, and 300 kg N at Bradenton during the spring of 1995 (a) and with 0, 66, 133, 200, and 266 kg N at Quincy during the fall of 1995 (b)

 
Dry weight of fruit and total dry matter production were generally greatest with the greatest N fertilization (Table 3). However, differences from those for intermediate levels of N fertilization were generally small. Total dry matter accumulation by the plant increased from {approx}190 g plant-1 for the zero-N treatment to {approx}600 g plant-1 with N fertilization of 200 to 300 kg N ha-1. Dry matter accumulation at Quincy (fall crop) was lower compared with that at the other locations (spring crops), probably due to lower late-season radiation values.

Average fruit harvest indices (fruit dry weight / total above-ground dry weight at harvest) were {approx}0.66, 0.56, and 0.50 for Bradenton, Gainesville, and Quincy, respectively. These values were similar to those reported for glasshouse tomato, which range from 0.65 to 0.72 (Challa and Heuvelink, 1993). Typically, harvest indices decreased slightly with increasing N fertilization, but trends were not consistent. High N fertilization resulted in lower initial partitioning of assimilates to fruits and favor the partitioning of dry matter to vegetative growth (Scholberg, 1996). High N rates prolonged the vegetative growth phase and resulted in a slight delay of fruit formation. Fruit set was prolonged but, under Florida conditions (high humidity and temperature), these last-formed fruits are typically smaller and of lower quality. Excess vegetative growth may further reduce crop coverage of pesticides and in that manner prevent optimal pest and disease control. High N fertilization also may prolong the reproductive growth phase of tomato plants, resulting in higher fruit yields during periods of extended fruit harvest (Huett and Dettmann, 1988; Thompson et al., 1976). Similar results have been reported by others (Doss et al., 1975; Huett and Dettmann, 1988; Larouche et al., 1989). Nitrogen stress resulted in a decrease in partitioning of dry matter to stems and leaves and increased transfer of assimilates to fruits near the end of the growing season (Scholberg, 1996). As a result, the fruiting period was shortened and both crop senescence and fruit maturation were enhanced.

Tissue Nitrogen Concentrations
Typically, N concentrations were highest in leaf blades, intermediate in stems and fruit, and lowest in roots (e.g., Fig. 5) . The N concentration of leaf blades decreased from 40 to 60 mg N g-1 (4 wk after transplanting) to 20 to 40 mg N g-1 near the end of the growing season. In most cases, N concentration in stems decreased from 20 to 30 mg g-1 during initial growth to 10 to 20 mg g-1 at harvest. Respective N values in fruit were {approx}30 to 40 mg g-1 and 15 to 30 mg g-1. The linear decrease in N concentration of leaves with time is attributed to leaf aging and N retranslocation to fruit under N-limiting conditions. Similar trends have been observed in field studies with tomato and cotton (Gossypium hirsutum L.) (Thompson et al., 1976).



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  		Fig. 5 Nitrogen concentration of roots (a), stems (b), leaves (c), and fruits (d) with 0, 100, 200, and 300 kg N at Bradenton during the spring of 1995

 
Nitrogen stress affected N concentrations of leaves and stems most drastically, whereas concentrations in fruits and roots were affected less. At Bradenton, initial differences between N fertilization treatments (Fig. 5) were relatively small, but became more pronounced as plants entered the rapid growth phase. Similar results were reported for cotton (Thompson et al., 1976). Because all fertilizer is applied before transplanting for plastic-mulched tomato grown using subirrigation, this typically results in relatively high N concentrations early in the season, followed by a steady decrease in N concentration of the tissue over time (Scholberg, 1996). Tanaka et al. (1974) also reported that both photosynthetic rate and leaf N content of tomato decrease as leaves mature. With the use of drip irrigation typically only 20 to 40% of the N fertilizer is applied preplant, resulting in relatively lower initial N values and a less pronounced decrease in tissue N concentrations with time (Scholberg, 1996). Differences between N treatments also were less pronounced at Quincy than at locations such as Gainesville (results not shown) and even relatively mild N stress symptoms did not appear until {approx}6 wk after transplanting. This was probably related to higher soil N contents and to relatively high moisture- and nutrient-retention capacities for the finer-textured soil at this location.

According to standard values for tissue analysis (Hochmuth et al., 1991), leaf tissue for the subirrigated crops tested high (>50 mg N g-1) until initial flowering and adequate (20 to 30 mg N g-1, depending on stage of growth) throughout the remainder of the growing season. However, the zero-N treatment (Fig. 5c) tested deficient (<20 to 30 mg N g-1) after initial flowering. Except with zero N at Gainesville during the 1996 season (where initial N stress, as indicated by leaf yellowing/thickening and stunted growth, was detected {approx}3 wk after transplanting), all drip-irrigated treatments tested adequate during the entire growing season (Scholberg, 1996). Excessively high N levels for the subirrigated crop could have resulted in excessive accumulation of N by both stems and fruit (Figs. 5b and d).

Nitrogen Accumulation
Total N accumulation (by tomato leaves, stems, fruit, and roots) at Bradenton and Quincy is shown in Fig. 6 . Nitrogen uptake during the first month after transplanting comprised less than 10% of the seasonal N uptake. Total N accumulation was related to crop yield and nutrient supply and ranged from 15 to 50 kg N ha-1 with severely N-stressed plants (zero N) to 140 to 240 kg N ha-1 with nonlimiting amounts of N (Table 4) . Approximately 70% of the total N taken up during initial growth was accumulated in leaves (data not shown), compared with {approx}20% in stems. During fruit development, {approx}35 and 25% of the total N uptake was stored in leaves and stems, respectively. These percentages decreased to {approx}22 and 17%, respectively, near the end of the growing season. Accumulation of N in roots decreased from {approx}20% during initial growth to only {approx}2% near the end of the growing season. Nitrogen stress resulted in a relatively higher fraction of N in the fruit and in a reduction in N accumulation by vegetative plant parts (Scholberg, 1996). With adequate N supply, 50 to 70% of the total N by the end of the season had accumulated in the fruit. However, under severe N stress, this value increased to {approx}75%. Published values for N typically range from 40 to 70% (Stark et al., 1983; Sweeney et al., 1987).



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  		Fig. 6 Nitrogen accumulation by tomato with 0, 100, 200, and 300 kg N at Bradenton during the spring of 1995 (a) and with 0, 66, 133, 200, and 266 kg N at Quincy during the fall of 1995 (b)

 

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  		Table 4 Calculated N accumulation and apparent N recovery (ANR) for tomato crops grown at Bradenton 1995; Gainesville 1995 and 1996; and Quincy 1995

 
Apparent nitrogen recovery (ANR) can be defined as:

where UF and U0 are N uptake by the crop in the presence and absence of fertilizer N, respectively, and NF is the amount of fertilizer applied (Greenwood and Draycott, 1988). Surprisingly, calculated ANR values for subirrigated tomato were higher than for drip-irrigated crops, though all values decreased with increasing N fertilization (Table 4). Lower ANR values with drip-irrigated crops may be related to N accumulation in dry portions of the bed and intense N leaching in the wetted (root) zone. Higher ANR values with subirrigated crops are probably related to the predominant upward movement of water and solutes with this irrigation method, provided that the watertable does not approach the surface so closely that fertilizer dropout can occur (Bonczek and McNeal, 1996).

The decrease in ANR with an increase in N fertilization is in agreement with previous research. Reported ANR values for cabbage using the above approach were 0.78 and 0.59 with N fertilization of 150 and 300 kg ha-1 (Greenwood and Draycott, 1988). Reported cumulative N uptake and ANR in a study using labeled N for drip-irrigated tomato fertilized at a N rate of 220 kg ha-1 were 145 kg ha-1 and 0.40, respectively (Sweeney et al., 1987). These values are comparable with those observed at Gainesville during the 1996 growing season. Total N accumulation by tomato plants grown on a loamy soil in California increased from 240 to 400 kg ha-1 as N rates increased from 160 to 585 kg N ha-1 (Stark et al., 1983). Based on the results presented in Table 4 and reports in the literature, 25 to 60% of the plant N may come from soil N reserves for fine-textured soils or soils with relatively high (>30 g kg-1) organic matter concentrations (Sweeney et al., 1987). Sweeney et al. (1987) also reported that, under Florida conditions, steady state inorganic soil N levels were only 1 to 2 mg kg-1, though 2 to 4% of the organic soil N could become available for plant uptake during the growing season. Based on these results, N supply from mineralization appeared to be {approx}10 to 40 kg N ha-1 during the growing season for soils with an organic matter concentration of 10 to 20 g kg-1, which is in agreement with the results shown in Table 4.

Optimal soil solution N concentrations can be estimated from absolute growth rates. Assuming a maximum dry matter increase of 280 kg dry matter ha-1 d-1 and a plant tissue N content of 0.032 kg N kg-1, maximum daily N uptake would be {approx}9 kg N ha-1 [Bar-Yosef and Sagiv (1982) reported a value of 8 kg N ha-1]. Based on a maximum transpiration rate of 4.8 mm d-1, if N reaches the plant root mainly by mass flow, required N concentration in the soil solution thus should be {approx}150 to 200 mg L-1 N (10–15 mmol NO3–N L-1) for optimal growth. These calculated values appear to be in agreement with results from field and solution culture experiments, where maximum growth and N uptake by tomato occured with solution concentrations of 150 to 200 ppm N (Bar-Yosef and Sagiv, 1982; Larouche et al., 1989). Huett and Dettmann (1988) reported that maximum vegetative growth and highest early fruit yields occurred at 121 mg L-1 N.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Severe N stress reduced tomato LAI, biomass, and fruit yield by 60 to 70%. Reductions in LAI were related to a decrease in leaf size and leaf number and caused a reduction in estimated light interception by {approx}25 to 30%. Reductions in RUE under N-limiting conditions were typically 30% or less and were related in part to a decrease in N concentration of the leaves from 40 to 15 mg g-1, with a concurrent reduction in leaf photosynthesis. Leaf photosynthesis was reduced 30% or less despite the large decrease in N concentration. The increase in leaf thickness may have allowed the plant to maintain relatively high photosynthetic rates (on a leaf-area basis) under N-limiting conditions even though N concentration per unit leaf dry weight was lower. Apparent N fertilizer recovery (as a fraction of the fertilizer N applied) decreased as N rates increased, with values ranging from 0.61 to 0.96 and from 0.36 to 0.74 for subirrigated and drip-irrigated crops, respectively. Nitrogen accumulation for a well-managed tomato was {approx}140 to 200 kg N ha-1, with roughly 70% of this amount accumulated by the fruit. Because less than 10% of the N is taken up during initial growth, it may be concluded that ANR for tomato grown on coarse-textured soils may be improved by reducing the amount of preplant fertilizer to as little as 20% of current production amounts.


   ACKNOWLEDGMENTS
 
Growth analysis of tomato plants was possible through collaboration with C.D. Stanley and A.A. Csizinszky (Gulf Coast Research and Education Center, Bradenton, FL) and G.J. Hochmuth (Horticultural Sciences Dep., Univ. of Florida, Gainesville, FL). This research was supported by joint contributions of the Florida Agricultural Experimental stations and the USDA (ES/SCS/ASCS) via the Lake Manatee Demonstration Project and USDA Special Grant in Tropical Agriculture No. 9-34-34-34135-0641 (Decision Support System for Vegetable Production).


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Florida Agric. Exp. Stn. Journal Paper no. R-06434.

Received for publication February 22, 1999.
   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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The SCI Journals Crop Science Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome