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V e g e t a r i a n  N e w s l e t t e r
  
 A Vegetable Crops Extension Publication
    Vegetarian 03-09  grnbullet.gif (839 bytes) September 2003


Effect of Prohexidione-Ca and Mepiquat Chloride on Stolon Production 
and Yield of Florida grown Strawberry (Fragraria ananassa Duch.)

Stolon production by strawberry plants in Florida fruiting fields is highly undesirable. Stolons (runners) act as a sink for photosynthates and nutrients, reducing the amount of resources available for fruit production in an annual hill production system. Stolon removal has shown negative or neutral effects on marketable fruit yields of strawberry plants grown in matted row culture (Pritts and Worden, 1988; Buckley and Moore, 1982). However, stolon removal increased yields per plant in matted row culture without increasing per plot yields for twelve cultivars (Hancock et al., 1982). In annual hill production in Florida, stolon removal twice monthly increased the early and total yields of Tufts while not affecting yields of Dover (Albregts and Howard, 1986). The presence of stolons makes it more difficult for pickers to find berries among the excess vegetation. Hence, manual labor must be used to remove runners in the fruiting field at a cost of $96 to $144 per hectare. If a low cost chemical means could be found to reduce or eliminate runner production in the fruiting field, producers would benefit greatly.

Mepiquat chloride (N, N-dimethylpiperidinium chloride, Pix, Ponnax, BASF Corp.) has long been used in the production of cotton (Gossypium hirsutum L.) to slow or reduce vegetative growth and increase yield and quality of harvested cotton fibers (Biles and Cothren, 2001; Reddy et al.,1996; McConnell et al., 1992; Reddy et al., 1992; Zhao and Oosterhuis, 2000). The effectiveness of mepiquat chloride in reducing vegetative growth and improving reproductive growth has shown mixed results in experimentation. This variation has been attributed to environmental, water, and nutrient factors (Reddy et al. 1992).

Prohexidione-calcium, a relatively new agricultural chemical produced by the BASF Corp. under the trade name Apogee, is a gibberellic acid inhibitor. Currently prohexidione-calcium is registered for use in apples (Malus pumila L.) to control fire blight (Erwinia amylova). In addition, it has shown to have several benefits for apple producers including reduced terminal growth, increased red color and fruit set (Greene and Autio, 2002). It has also been shown to reduced fire blight and increase average fruit weight in pear (Pyrus communis L.) (Costa et al., 2001). In sorghum (Sorghum bicolor L.) ( Lee et al., 1998), plant growth was retarded by applications of prohexidione-calcium but floral initiation was not delayed in comparison to other growth regulators (CCC, uniconazol, and ancymidol). These results suggest that these products may be useful for control of stolons on the 2900 hectares of strawberry production in Florida.

The purpose of this study was to determine the effectiveness of mepiquat chloride and prohexidione-calcium in suppressing runner production of strawberry plants in fruiting fields and the effect of these compounds on yield. Ideally, a suitable chemical would totally control stolon growth, thereby eliminating the need to remove them manually while maintaining or possibly increasing marketable yield.

Material and Methods

'Strawberry Festival' strawberry plants were planted on 19 October 2000 and grown following current University of Florida recommendations (Maynard and Olson, 2000). Treatments consisted of prohexidione-calcium (113.8 and 227.6 g ai/ha), mepiquat chloride (170.6 and 273.5 g ai/ha), and a control. Treatment rates, application timing, and number of applications were chosen according to manufacturers recommendations. Treatments were applied in a volume of water corresponding to 900 liters per hectare. Treatments were arranged in a randomized complete block design replicated 4 times with 14 plants per treatment plot. Spray materials were applied on16 and 30 November and 14 December 2000. Fruit harvest began on 8 December 2000 and continued twice weekly until 1 March 2001 for a total of 26 harvests. Fruit were graded for marketability (not misshapen, > 10g) and disease incidence (Colletoctricum acutatum and Botrytis cinera). Runner length and number per plot were recorded on 8 and 27 December on the plants. Runners were removed after observation.

Yield and yield components were separated by monthly and seasonal totals. Stolon number and length were analyzed for each observation date. All data was analyzed using SAS 8.0 using ANOVA procedures.

Results and Discussion

Significant differences were detectable among treatments during each analysis period for marketable weight and number of berries (Table 1). During December, both prohexidione calcium treatments yielded less than mepiquat chloride at the 273.5 g ai /ha rate, while the control and the lower mepiquat chloride treatments were not significantly different (P < 0.05). Fewer differences were observed in the number of marketable berries with the lower prohexidione calcium rate producing fewer fruit than the control and highest mepiquat chloride application.

Lower yields, during each observation period, for plants treated with prohexidione calcium were due to the fact that these plants never produced a large leaf and crown structure capable of producing abundant fruit. It was an unfavorable year for strawberries due to abnormally low temperatures, which began in December and lasted through March, and resulted in commercial yields reductions of 50%. If the weather had been warmer, these differences might have been less pronounced. However, it may be that a single application of prohexidione calcium at the second spraying date would have been sufficient to suppress runner production and not decrease yields as well. Work by Hicklenton and Reekie (personal communication) in Nova Scotia has shown that the suppression of runner production by prohexidione calcium outlasts suppression of leaf growth. Mepiquat chloride at 273.5 g ai/ha displayed yield enhancement over the control during January (Table 1). This is of special interest as market prices are quite high during January (averaging $16 per 5.5 kg flat compared to a $12 Feb. and $10 March average). A yield increase obtained during this period represents a significant economic advantage to commercial strawberry growers.

Stolon production was dramatically reduced by the application of prohexidione calcium (Table 2). In both prohexidione calcium treatments, no runners were produced at either observation date. At the first observation date, the control produced more runners than all other treatments. However, the average length of stolons as similar between the control and mepiquat chloride treatments. Observations made on 27 December revealed that differences between mepiquat chloride treatments and the control in terms of number of stolons produced had disappeared, but the stolons produced by plants treated with mepiquat chloride were significantly shorter. No differences were detected in disease incidence among treatments.

Prohexidione calcium significantly reduces stolon production at the expense of lowered yields. The product needs to be further studied to determine if altering rates and application timing and frequency can reduce runner production without reducing yield. Mepiquat chloride reduced production of stolons at the first observation and increased yield during January. Its impact on early fruit yield needs to be further evaluated in a multiple year study examining a number of cultivars.

Literature Cited
 

Table 1. Monthly and total yields per plant of 'Strawberry Festival' strawberry plants after application of prohexidione calcium and mepiquat chloride.

Treatment

Marketable Weight
(g/plant)

Marketable Number of Berries per Plant

Number of Cull
Fruit per Plant

 

December 2000

Control

60.4abz,y

3.4a

0.6

Prohexidione-Ca
113.8 g ai/ha

30.1c

1.6b

0.2

Prohexidione-Ca
227.6 g ai/ha

35.1bc

2.3ab

0.5

Mepiquat chloride
170.6 g ai/ha

50.0abc

2.7ab

0.1

Mepiquat chloride
273.5 g ai/ha

65.7a

3.8a

0.5

 

January 2001

Control

132.5b

8.0ab

1.1a

Prohexidione-Ca
113.8 g ai/ha

129.6b

6.9bc

0.9a

Prohexidione-Ca
227.6 g ai/ha

97.3c

6.2c

2.5b

Mepiquat chloride
170.6 g ai/ha

151.0ab

8.9a

1.0a

Mepiquat chloride
273.5 g ai/ha

166.1a

9.4a

0.9a

 

February 2001

Control

193.6a

10.6

2.4

Prohexidione-Ca
113.8 g ai/ha

169.2a

9.9

2.5

Prohexidione-Ca
227.6 g ai/ha

109.4b

6.8

2.6

Mepiquat chloride
170.6 g ai/ha

183.0a

10.1

2.3

Mepiquat chloride
273.5 g ai/ha

173.9a

9.3

2.4

 

March 2001x

Control

37.7ab

2.3a

0.4

Prohexidione-Ca
113.8 g ai/ha

12.1c

0.9b

0.6

Prohexidione-Ca
227.6 g ai/ha

10.9c

0.8b

0.4

Mepiquat chloride
170.6 g ai/ha

42.1a

2.4a

0.4

Mepiquat chloride
273.5 g ai/ha

29.5b

1.8a

0.4

 

Total Season

Control

424.2a

24.2a

4.1a

Prohexidione-Ca
113.8 g ai/ha

341.0b

19.1b

6.1b

Prohexidione-Ca
227.6 g ai/ha

252.6c

16.1b

6.1b

Mepiquat chloride
170.6 g ai/ha

426.1a

24.2a

3.7a

Mepiquat chloride
273.5 g ai/ha

435.2a

24.2a

4.0a

z Means in the same column and the same month followed by same letter are not significantly different. Separation by LSD (p < 0.05).
y Values given represent the mean of four 14 plant plots.
x Values for march represent a single harvest in March.

 

Table 2. Stolon number and average stolon length as affected by prohexidione calcium and mepiquat chloride treatment.

Treatment

Stolon number Average Stolon Length

December 8, 2000

Control

8.75az,y

20.37a

Prohexidione-Ca
113.8 g ai/ha

0.00c

0.00b

Prohexidione-Ca
227.6 g ai/ha

0.00c

0.00b

Mepiquat chloride
170.6 g ai/ha

4.00bc

20.90a

Mepiquat chloride
273.5 g ai/ha

7.50b

17.86a

 

December 27, 2000

Control

15.25a

44.50a

Prohexidione-Ca
 113.8 g ai/ha

0.00b

0.00c

Prohexidione-Ca
227.6 g ai/ha

0.00b

0.00c

Mepiquat chloride
170.6 g ai/ha

11.50a

36.15b

Mepiquat chloride
273.5 g ai/ha

16.75a

39.60b

z Means in the same column followed by same letter are not significantly different. Separation by LSD (p < 0.05).
y Values given represent the mean of four 14 plant plots.

(Duval- Vegetarian 03-09)


Tomato Varieties for Florida

Variety selections, often made several months before planting, are one of the most important management decisions made by the grower.  Failure to select the most suitable variety or varieties may lead to loss of yield or market acceptability. The following characteristics should be considered in selection of tomato varieties for use in Florida.

  • Yield - The variety selected should have the potential to produce crops at least equivalent to varieties already grown.  The average yield in Florida is currently about 1400 25-pound cartons per acre.  The potential yield of varieties in use should be much higher than average.

  • Disease Resistance - Varieties selected for use in Florida must have resistance to Fusarium wilt, race 1, race 2 and in some areas race 3; Verticillium wilt (race 1); gray leaf spot; and some tolerance to bacterial soft rot.  Available resistance to other diseases may be important in certain situations, such as Tomato Spotted Wilt resistance in northwest Florida.

  •  Horticultural Quality - Plant habit, stem type and fruit size, shape, color, smoothness and resistance to defects should all be considered in variety selection.

  • Adaptability - Successful tomato varieties must perform well under the range of environmental conditions usually encountered in the district or on the individual farm.

  • Market Acceptability - The tomato produced must have characteristics acceptable to the packer, shipper, wholesaler, retailer and consumer.  Included among these qualities are pack out, fruit shape, ripening ability, firmness, and flavor.

Current Variety Situation

Many tomato varieties are grown commercially in Florida, but only a few represent most of the acreage.  In years past we have been able to give a breakdown of which varieties are used and predominantly where they were being used but this information is no longer available through the USDA Crop Reporting Service.

Tomato Variety Trial Results

Summary results listing the five highest yielding and the five largest fruited varieties from trials conducted at the University of Floridas Gulf Coast Research and Education Center, Bradenton; Indian River Research and Education Center, Ft. Pierce and North Florida Research and Education Center, Quincy for the Spring 2002 season are shown in Table 1.  High total yields and large fruit size were produced by Fla. 7926 at Bradenton; Florida 47, Fla. 7810, Agriset 761 and Sanibel at Fort Pierce; and SVR 1432427 and BHN 444 at Quincy.   There was very little overlap between locations.  The same entries were not included at all locations.

Table 2 shows a summary of results listing the five highest yielding and five largest fruited entries from trials at the University of Floridas Gulf Coast Research and Education Center, Bradenton; Indian River Research and Education Center, Ft. Pierce and the North Florida Research and Education Center, Quincy for the fall 2002 season.  High total yields and large fruit size were produced by Fla. 7810, Fla. 7885 B, Florida 47 and Florida 91 at Fort Pierce and by Solar Fire and Fla. 7885 B at Quincy.  Solar Fire and Fla. 7885 B produced high yields at all three locations and Fla. 7885 B, Fla. 7810 and Florida 91 produced large fruit at two of three locations.  Not all entries were included at all locations.

Tomato Varieties for Commercial Production

The varieties listed have performed well in University of Florida trials conducted in various locations in recent years.

Large Fruited Varieties

Agriset 761.  Midseason, determinate, jointed hybrid.  Fruit are deep globe and green shouldered.  Resistant: Verticillium wilt (race 1), Fusarium wilt (race 1 and 2), Alternaria stem canker, gray leaf spot.  (Agrisales).

BHN 640.   Early-midseason maturity.   Fruit are globe shape but tend to slightly elongate, and green shouldered.  Not for fall planting.  Resistant: Verticillium wilt (race 1),  Fusarium wilt (race 1, 2 and 3), gray leaf spot, and Tomato Spotted Wilt.  For Trial. (BHN).

Florida 47.  A late midseason, determinate, jointed hybrid.  Uniform green, globe-shaped fruit.  Resistant: Fusarium wilt (race 1 and 2), Verticillium wilt (race 1), Alternaria stem canker, and gray leaf spot.  (Seminis).

Florida 91.  Uniform green fruit borne on jointed pedicels.  Determinate plant.  Good fruit setting ability under high temperatures.  Resistant: Verticillium wilt (race 1), Fusarium wilt (race 1 and 2), Alternaria stem canker, and gray leaf spot.  (Seminis).

Floralina.  A midseason, determinate, jointed hybrid.   Uniform, green shoulder, flattened, globe-shaped fruit.  Recommended for production on land infested with Fusarium wilt, Race 3.  Resistant: Fusarium wilt (race 1, 2, and 3), Verticillium wilt (race 1), gray leaf spot.  (Seminis).

Sebring.  A late midseason determinate, jointed hybrid with a smooth, deep oblate, firm, thick walled fruit.  Resistant: Verticillium wilt (race 1), Fusarium wilt (race1,2 and 3), Fusarium crown rot and gray leaf spot.  For Trial.   (Syngenta).

Solar Fire.  An early, determinate, jointed hybrid.  Has good fruit setting ability under high temperatures.  Fruit are large, flat-round, smooth, firm, light green shoulder and blossom scars are smooth.  Resistant: Verticillium wilt (race 1), Fusarium wilt (race 1,2 and 3) and gray leaf spot.   For Trial.   (University of Florida).

Solar Set.  An early, green-shouldered, jointed hybrid.  Determinate.  Fruit set under high temperatures (92oF day/72o night) is superior to most other commercial varieties.  Resistant: Fusarium wilt (race 1 and 2), Verticillium wilt (race 1), Alternaria stem canker,  and gray leaf spot.  (Seminis).

Sanibel.  A late-midseason, jointless, determinate hybrid.  Deep oblate shape fruit with a green shoulder.  Tolerant/resistant: Verticillium wilt (race 1), Fusarium wilt (race 1 and 2), Alternaria stem canker, root-knot nematode, and gray leaf spot.  (Seminis).

Solimar.  A midseason hybrid producing globe-shaped, green shouldered fruit.  Resistant: Verticillium wilt (race 1), Fusarium wilt (race 1 and 2), Alternaria stem canker, gray leaf spot.  (Seminis).

Sunbeam.  Early midseason, deep-globe shaped uniform green fruit are produced on determinate vines.  Resistant: Verticillium wilt (race 1), Fusarium wilt (race 1 and race 2), gray leaf spot, Alternaria  stem canker.  (Seminis).

Plum Type Varieties

Marina.  Medium to large vined determinate hybrid.  Rectangular, blocky, fruit may be harvested mature green or red.  Resistant: Verticillium wilt (race 1), Fusarium wilt (race 1 and 2), Alternaria stem canker, root-knot nematodes, gray leaf spot, and bacterial speck.  (Sakata).

Plum Dandy.  Medium to large determinate plants.  Rectangular, blocky, defect-free  fruit for fresh-market production.  When grown in hot, wet conditions, it does not set fruit well and is susceptible to bacterial spot.  For winter and spring production in Florida.  Resistant: Verticillium wilt, Fusarium wilt (race 1), early blight, and rain checking.  (Harris Moran).

Spectrum 882.  Blocky, uniform-green shoulder fruit are produced on medium-large determinate plants.  Resistant: Verticillium wilt (race 1), Fusarium wilt (race 1 and 2), root-knot nematode, bacterial speck (race 0), Alternaria stem canker, and gray leaf spot.  (Seminis).

Supra.  Determinate hybrid rectangular, blocky, shaped fruit with uniform green shoulder.  Resistant: Verticillium wilt (race 1), Fusarium wilt (race 1 and 2), root-knot nematodes, and bacterial speck.  (Syngenta).

Veronica.    Tall determinate hybrid.   Smooth plum type fruit are uniform ripening.  Good performance in all production seasons.  Resistant: Verticillium wilt (race 1), Fusarium wilt (race 1 and 2), Alternaria stem canker, nematodes, gray leaf spot and bacterial speck.   (Sakata).

Cherry Type Varieties

Mountain Belle.  Vigorous, determinate type plants.  Fruit are round to slightly ovate with uniform green shoulders borne on jointless pedicels.  Resistant: Fusarium wilt (race 2), Verticillium wilt (race 1).  For trial.  (Syngenta).

Cherry Grande.  Large, globe-shaped, cherry-type fruit are produced on medium-size determinate plants.  Resistant: Verticillium wilt (race 1), Fusarium wilt (race 1), Alternaria stem blight, and gray leaf spot.  (Seminis).

Reference

This information was gathered from results of tomato variety trials conducted during 2002 at locations specified in each table.

Tomato variety evaluations were conducted in 2002 by the following University of Florida faculty:
    D. N. Maynard - Gulf Coast Research & Education Center - Bradenton
    S. M. Olson - North Florida Research & Education Center - Quincy
    P. J. Stoffella - Indian River Research & Education Center - Fort Pierce
 

Table 1.  Summary of University of Florida tomato variety trial results.  Spring 2002.

Location

Variety

Total yield (ctn/acre)

Variety

Average fruit wt. (oz)

Bradenton

Fla. 7973

2967

RFT 0417

7.6

 

Fla. 7926

2799

PX 150535

7.4

 

HMX 1803

2787

XTM 0227

7.3

 

BHN 591

2749

EX1405037

7.3

 

BHN 586

27171

Fla. 7926

7.22

Fort Pierce

Florida 47

3528

Sanibel

6.4

 

Fla. 7810

3286

Florida 47

6.4

 

Agriset 761

3189

Fla. 7810

6.3

 

Sanibel

3108

Florida 91

6.3

 

Fla. 7973

30843

Agriset 761

6.04

Quincy

RFT 0849

2771

BHN 543

8.2

 

BHN 640

2695

SVR 1432427

7.9

 

SVR 1432427

2641

Sunpac

7.6

 

BHN 444

2633

BHN 444

7.5

 

BHN 577

24965

Fla. 7973

7.56

119 other entries had yields similar to BHN 586.
218 other entries had fruit weight similar to PX 150535.
32 other entries had yields similar to Fla. 7973.
42 other entries had fruit weight similar to Agriset 761.
513 other entries had yields similar to BHN 577.
614 other entries had fruit weight similar to Fla. 7973.

 Seed Sources:
            Agrisales:  Agriset 761.
            BHN:  BHN 444, BHN 543, BHN 577, BHN 586, BHN 591, BHN 640.
            Harris Moran:  HMX 1803.
            Seminis:  Florida 47, Florida 91, Sanibel, Sunpac, PX 150535, EX 1405037, SVR 1432427.
            Sakata: XTM 0227
            Syngenta:  RFT 0417, RFT 0849
            University of Florida: Fla. 7810, Fla. 7926, Fla. 7973. 

Table 2.  Summary of University of Florida tomato variety trial results.  Fall 2002.

Location

Variety

Total yield (ctn/acre)

Variety

Average fruit wt. (oz)

Bradenton

Solar Fire

1480

XTM 0231

6.9

 

Solar Set

1461

Florida 91

6.9

 

XTM 0231

1389

HMX 1803

6.8

 

Lucky 13

1387

BHN 650

6.8

 

Fla. 7885 B

13571

XTM 0227

6.72

Fort Pierce

Fla. 7810

1697

Florida 47

5.1

 

Fla. 7885 B

827

Solar Set

5.0

 

Florida 47

781

Florida 91

4.9

 

Florida 91

630

Fla. 7810

4.9

 

Solar Fire

5653

Fla. 7885 B

4.84

Quincy

Solar Fire

1641

Solar Fire

6.4

 

Fla. 7885 B

1548

Fla. 7885 B

6.3

 

Solar Set

1398

XTM 0227

6.2

 

XTM 0230

1321

BHN 640

6.2

 

SVR 145037

13085

Fla. 7810

6.16

1 14 other entries had yields similar to Fla. 7885 B.
2 11 other entries had fruit weight similar to XTM 0227.
3 2 other entries had yields similar to Solar Fire.
4 2 other entries had fruit weight similar to Fla. 7885 B.
5 17 other entries had yields similar to SVR 145037.
6 11 other entries had fruit weight similar to Fla. 7810.

 Seed Sources:
            Agrisales:  Lucky 13
            BHN:  BHN 640, BHN 650.
            Harris Moran: HMX 1803.
            Sakata: XTM 0227, XTM 0230, XTM 0231.
            Seminis: Florida 47, Florida 91, Sanibel, Solar Set, SVR 1405037.
            University of Florida: Solar Fire, Fla.. 7810, Fla. 7885 B.

(Olson and Maynard - Vegetarian 03-09)


Alleviation of the Impact of Fertilization and Irrigation on the Nitrogen
Cycle in Vegetable Fields with Bmps - Part I. The Nitrogen Cycle

The nitrogen (N) cycle is a set of transformations that affect N in the biosphere. Through a series of microbial transformations in the soil, N is made available to vegetables. Thus, knowledge of this cycle by which N passes from air to soil to organisms and back to air, and how the components of the cycle are affected by human activities, is required to design effective strategies for decreasing undesirable losses of N from vegetable production to the environment.

Adequate management of fertilization and irrigation has always been recognized as one of the keys to successful vegetable production in Florida. Thus, fertilization and irrigation practices have aimed at supplying enough nutrients and water to ensure economical yields. Since up to 200lbs/A of exogenous N are recommended for vegetable production in Florida, and fertilizer use efficiency seldom exceeds 75%, it is likely that fertilization affects the N cycle. Best Management Practices (BMPs) aim at reconciling the needs of economical vegetable crop production with those of environmental protection. Efficient BMP implementation, therefore, requires an understanding of how current cultural practices affect the N cycle in commercial vegetable fields.  It is likely that a complete understanding of these issues by farmers and vegetable professionals will be a prerequisite for the success of the BMP program.

The goal of this article is to present the N cycle as it relates to crop production.  A description of (1) how fertilization and irrigation practices affect the N cycle, and (2) how the proposed BMPs may help reduce the environmental impact of these cultural practices will be provided in the next issue of the Vegetarian.

The Nitrogen Cycle in a Typical Ecosystem

Because the N cycle is a cycle, it has no clear beginning and no end. Hence, for the sake of presentation, this description of the cycle starts with N in the soil organic matter where N is in the form of amino acid, proteins, and nucleic acids (Fig. 1). In the soil, N found in decomposing organic matter may be converted into inorganic N forms by soil microorganisms (bacteria and fungi) in a process called mineralization (step 1). Those bacteria and fungi, also called decomposers, may be found in the upper soil layer. They chemically transform the N found in organic matter from amino-N (NH2) to ammonium (NH4+).
            Step 1: Organic matter
Ammonium
                    R-NH2 
NH4+ 

Nitrogen in the form of NH4+ can then be adsorbed (step 2) onto the surfaces of clay particles in the soil. The NH4 ion that has a positive charge may be held by soil colloids because they have a negative charge. This process is called micelle fixation.
            Step 2: Ammonium in solution adsorbed ammonium ammonium back into solution
                    NH4aqueous 
 NH4+ - soil colloid 
NH4aqueous

As this fixation is reversible, NH4+ may be released from the colloids by way of cation exchange. When released, NH4+may be chemically altered into nitrite (NO2-) by a specific type of autotrophic bacteria belonging to the genus Nitrosomonas. Nitrosomonas are autotrophic bacteria that can synthesize their own organic N compounds from inorganic N sources (step 3a). Then, NO2- may be quickly converted into nitrate (NO3-) by another type of bacteria belonging to the genus Nitrobacter (step 3b). Both of these processes involve chemical oxidation and together are known as nitrification (Pidwirny, 2002). Both bacteria utilize the energy released by the oxidation of N compound in their metabolism of which NO2- and NO3- are by-products. This 2-step process involves a complex series of reactions that can be summarized as:
Step 3a: Ammonium in solution nitrite in solution
55NH4+ + 76 O2 + 109HCO3- 
C5H7O2N + 54NO2-+ 57H2O + 104H2CO
            Step 3b: Nitrite in solution
Nitrate in solution
                    400 NO2- + NH4+ + 4 H2CO3 + HCO3-+ 195 O2
C5H7O2N + 3 H2O  + 400 NO3-

These equations highlight two important points: nitrification requires oxygen and it affects bulk soil pH. First, approximately 4.3 mg O2 are consumed for every mg of NH4+ oxidized into NO3-. Second, a quite substantial amount of alkalinity in the form of HCO3- is consumed when NH4+ is oxidized, thereby, indirectly decreasing soil pH.

The rate of step 3a (NH4+ transformed in NO2-) is slower than that of step 3b. Hence, NO2- does not normally accumulate in soils, but NO3- may. Because NO3- has a negative charge, it may not be adsorbed onto the soil colloids. As most NO3- salts (such as potassium nitrate, calcium nitrate, magnesium nitrate) have high solubility (high Ksp), most NO3- stays in the soil solution.

If NH4+ is neither adsorbed onto soil colloids nor transformed in NO3- , it may be volatilized (step 3c). However, this occurs rather in agricultural ecosystems where fertilizers (urea and manure) are added, than in undisturbed ecosystems.
Step 3c: NH4+ in the soil  NH3 in the air
Nitrate and NH4+ in the soil solution are the most common forms of N taken up by vegetable crops.  Nitrogen uptake is the most important step of the N cycle in vegetable production.
            Step 4a: Ammonium in solution
 Ammonium inside the root
                  NH4+ aqueous
NH4+ inside the root
                  Nitrate in solution 
 Nitrate inside the root
                  NO3- aqueous
NO3- inside the root

In plant nutrition, N is an essential element.  Nitrogen is involved in the composition of all amino acids, proteins and many enzymes. Nitrogen is also part of the puric and pyrimidic bases, and therefore is a constituent of nucleic acids. Typically, N content in plant range between 1.0 and 6.0% of the dry weight in leaf tissues (this means that 1 to 6 g of N may be found in 100g of dry tissue). High N contents, however, can cause growth stimulations which can produce deficiencies of other elements (if these are not supplied additionally) due to dilution effects. Under N shortage, plants grow slowly and are weak and stunted.

Nitrate and NH4+ are the main N forms taken up by plants. However, NO3- and NH4+ should be regarded as two different nutrients because they affect plant metabolism differently. Nitrate is negatively charged, while NH4+ is positively charged.  As nutrient uptake is a process that is electrically neutral, it does not involve any net change in plant electric charge. The absorption of NO3- requires the concomitant uptake of a cation or the release of an anion (OH- or organic acid). Similarly, the absorption of NH4+ when the accompanying ions are H+ or OH-, affects soil pH. Hence, NH4+ uptake may depress the uptake of the essential cations (K+, Ca2+, Mg2+).

Another difference between NO3- and NH4+ is that NO3- may be stored in the plant before it is used, whereas NH4+ needs to be detoxified. Ammonium must be rapidly incorporated into organic molecules because free NH4+ disorganizes the photosynthesis mechanism by uncoupling redox reactions and affecting the photosynthetic membrane stacks (grana) in chloroplasts. On the contrary, free NO3- is not toxic and it can be stored in the plant until utilized or incorporated into organic molecules by the light-activated enzyme nitrate reductase (NR), after being reduced into NH2 group.  Reduced NO3- is added to a glutamic acid residue in a transammination reaction that generates glutamine. Differences in NO3- and NH4+effects on plant growth can be summarized in the old saying: NH4+ greens a plant, while NO3- grows a plant.  

Consequently, an optimum NO3-N: NH4-N ratio exists for vegetable production. The optimum NO3-N : NH4-N ratio for vegetable growth is 75 : 25. When NH4+ is the dominant form of N available for plant uptake, a smaller plant will result. When the root system is in fact overloaded in its ability to detoxify the NH4+ absorbed, then NH4+ will be translocated to the top portion of the plant. There, carbon sources otherwise used for leaf and stem growth are instead used into detoxification of the NH4+. Protein synthesis pathway dominates the production of the cell.

If NO3- is not taken up, it can be transported below the root zone and leached (step 4b) or denitrified (step 4c). As NO3- is soluble in water, it is easily leached from the rootzone by excessive rainfall or irrigation (step 4b). In Florida sandy soils, the bottom of the root zone is typically 12 inches for shallow-rooted crops and 3 feet for deep-rooted vegetable crops. The actual rooting depth of vegetables may be limited by the presence of hard pans, acid layers, or a spodic horizon.
            Step 4b: Nitrate in the root zone Nitrate in the groundwater
                    NO3- in the root zone NO3- in the groundwater

Because the water holding capacity of Florida sandy soils is typically 10% (v : v), the top 12 inches soil can hold 1 inch of water.  Hence, rainfall of 3 inches in 3 days, or 4 inches in 7 days are considered to be leaching rains that take NO3- below the root zone.

Once below the root zone, NO3- easily enters the hydrologic system. Karst geology is commonly found throughout Florida. A sand layer of variable thickness covers a limestone base (Fig. 2). Through repeated wet/dry cycles, limestone slowly dissolves, creating swales and sinkholes. Through sinkholes, leaching rain is directly in contact with groundwater and is not filtrated; NO3- may be found in underground water, springs and in the streams.

Elevated NO3- concentration in ground water has been associated with water quality/health issues and eutrophication. First, when it reaches groundwater, NO3- concentration may exceed the water quality standard of 10 mg/L NO3- -N. Short-term exposure to drinking water with a NO3- concentration above 10 mg/L NO3-N is a potential health problem primarily for infants. Their immature digestive systems are more likely than adult digestive tracts to allow the reduction of NO3- to NO2-.  In some rare cases, the presence of NO2-  in the digestive tract of newborns has lead to a disease called methemoglobinemia or blue baby.

The second impact of NO3- on water quality is when it accumulates into waterways and causes the eutrophication of N-limited ecosystems. Eutrophication is a condition in an aquatic ecosystem when exogenous quantities of the limiting factor (N in north Florida and P in South Florida) result in algae blooms.
            Step 4c. NO3- in waterways 
NO3- in algae blooms

Algae blooms cloud the water making it difficult for larger submerged aquatic vegetation (SAV) to get enough light and compete for dissolved oxygen.  The SAV may dieback thereby reducing available habitat of aquatic animals, which in turns affects the whole food chain in the aquatic ecosystem. In addition, algae blooms increase the Biological Oxygen Demand (BOD), thereby competing with other aquatic animals.

Nitrate that is neither taken up by the plant nor leached may be denitrified. Denitrification (step 4c) occurs commonly in anaerobic soils and is carried out by heterotrophic bacteria. This kind of bacteria must consume energy rich organic molecules for survival. The most common denitrifying bacteria include several species of Pseudomonas, Alkaligenes and Bacillus. The process of denitrification involves the reduction of NO3- into dinitrogen (N2) or nitrous oxide (N2O) gas. Both of these gases then diffuse into the atmosphere. No oxygen is required for this process that occurs in anoxic conditions. On the contrary, oxygen is produced and may be used by nitrifying bacteria in another layers of the soil. Denitrifying bacteria use N as the final electron acceptor in their metabolism. Denitrified N in the form of N2O or N2 forms joins the largest store of N in the cycle found in the atmosphere. The atmospheric store is estimated to be approximately one million times larger than the total N contained in all living organisms.
           
Step 4c: Nitrate in soil    N oxides gases in the atmosphere +Oxygen
                  
NO3- in soil  N2O and N2  forms in the atmosphere+ O2

Dinitrogen in the atmosphere may return to earth by three ways: rain (step 5), fertilizer production (step 6), or nodules fixation (step 7). Small proportions of atmospheric N2 return to the soil in rainfall or through the effects of lightning; only 1013 g per year of N2 (22,000 Million lbs per year of N2) is fixed and transformed in ammonia by lightning. Nitrogen fertilizers are produced by condensation of N2 and H2 which produces NH3.
            Step 6: Dinitrogen + Dihydrogen  Ammonia + energy.
                    N2 (g) + 3H2(g)
 2NH3(g) + energy (Anon b, 2003)

The bulk N2 returned to earth, however, is biochemically fixed in the soil by specialized micro-organisms like bacteria, actinomycetes, and cyanobacteria. This process is called nitrogen fixation (step 5). It may occur in plants that harbor nitrogen-fixing bacteria within their nodules. Free-living bacteria may also fix N2, but on a smaller scale. The amounts of N fixed by free-living, non-photosynthetic bacteria in the soil may achieve an approximate maximum of 15 Kg/ha/year (13.4 lbs/A/year).
            Step 7: Dinitrogen in the air Ammonia for the plant
                   Nin the air 
 NH3 for the plant

Biological nitrogen fixation can be represented by the following equation, in which two moles of ammonia are produced from one mole of nitrogen gas, at the expense of 16 moles of ATP (energy) and a supply of electrons and protons (hydrogen ions):
           
Step 7: N2 + 8H+ + 8e- + 16 ATP   2 NH3 + H2 + 16ADP + 16 Pi

The low N contribution of the free-living, non photosynthetic bacteria, is the result of limited availability of suitable organic substrates (energy sources) and low bacterial populations in the soil environment. Nitrogen fixation is characteristically higher in tropical soils, where substrate availability, temperature and moisture are more favorable to the maintenance and activity of an actively growing bacterial population.

The best-studied example of N fixation is the association between legumes and bacteria in the genus Rhizobium. The main legume crops commercially grown in Florida as snap bean (Phaseolus vulgaris) and pink-eyed and black-eyed pea (Vigna unguiculata). These Rhizobium and legumes are able to survive independently (soil nitrates must then be available to the legume), but this association is beneficial to both organisms. In exchange for some N, the bacteria receive from the plants carbohydrates. Special structures (nodules) in roots allow them to be connected with the roots of the plant.  Scientists estimate that biological fixation globally adds approximately 140 million metric tons of N to soil and sea ecosystems every year.  However, the actual amount of N fixed in each ecosystem depends on the environmental conditions and the nature of biological system(s) present, which are capable of N fixation.  Nitrogen fixation rates may vary from almost 0 up to 1,000 Kg/ha/year (892 lbs/A/year).

The last step of the N cycle is the return of organic matter to the soil (Step 8). Organic matter returns to the soil in the form of crop residues, incorporation of cover crops, and/or organic amendments such as compost or manure. This organic matter will be mineralized and then, follow the steps of the cycle, again.

The N cycle described above, (from the mineralization of organic matter to the return to the soil of organic matter) occurs in an undisturbed ecosystem. However, higher vegetable yields may be achieved with intensive production practices, fertilization and irrigation. Therefore, vegetable production may affect some steps of the N cycle.

(
Eve-Marie Cockx, short-term scholar, and Simonne - Vegetarian 03-09)

Extension Vegetable Crops Specialists

Daniel J. Cantliffe
Professor and Chairman

Mark A. Ritenour
Assistant Professor, postharvest

John Duval
Assistant Professor, strawberry

Steven A. Sargent
Professor, postharvest

Chad Hutchinson
Assistant Professor, vegetable production

Eric Simonne
Assistant Professor
and EDITOR, vegetable nutrition

Elizabeth M. Lamb
Assistant Professor, vegetable production
William M. Stall
Professor, weed science
Yuncong Li
Assistant Professor, soils
James M. Stephens (retired)
Professor, vegetable gardening
Donald N. Maynard (retired)
Professor, varieties
Charles S. Vavrina
Professor, transplants

Stephen M. Olson
Professor, small farms

James M. White
Associate Professor, organic farming

Related Links
University of Florida
Institute of Food and Agricultural Sciences
Horticultural Sciences Department
Florida Cooperative Extension Service
North Florida Research and Education Center - Suwannee Valley

Gulf Coast Research and Education Center - Dover
UF/IFAS Postharvest

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