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3 April, seedlings had low concentrations of Nitrogen, Magnesium and other nutrients, ... force early production of tomato in winter and ..... into fruit production.
Plant and Soil 145:81-91, 1992. © 1992 Kluwer Academic: Publishers. Printed in the Netherlands.

PLSO 9445

Effect of planting date, ventilation and soil temperature on growth and nutrition of tomato in high tunnels MARTIN P.N. GENT Department of Forestry and Horticulture, The Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven, CT 06504, USA Received 19 December 1991. Revised May 1992

Key words: air temperature, growth rate, high tunnel, Lycopersicon esculentum Mill, nutrition, soil temperature, yield Abstract

Growth rates and tissue nutrient concentrations were measured in tomato (Lycopersicon esculentum Mill) grown in unheated high tunnels in the spring in the northeast USA. Two weeks after transplant on 3 April, seedlings had low concentrations of Nitrogen, Magnesium and other nutrients, while later plantings on 17 April and 1 May had adequate nutrition. The low yield and small fruit of the 3 April planting, compared to the later plantings, was likely related to this nutrient stress soon after transplant. Air and soil temperatures were less than 10°C at planting on 3 April. Air and soil were warmed during the day to different extents in tunnels vented at different temperatures. Over all plantings and ventilation regimes, relative growth rates over the two weeks after transplant were correlated to average air temperature. However, there was little uptake of P, N and Mg, when soil was cooler than 12°C. Nutrient concentrations in the shoot became very low because shoot growth continued when soil temperature limited nutrient uptake.

Abbreviations: NUR, nutrient uptake ratio; RGR, relative growth rate; TNC, total nonstructural carbohydrate

Introduction

Unheated high tunnels or clear plastic shelters are used widely in the Mediterranean region to force early production of tomato in winter and spring (Castilla and Fereres, 1990; Orphanos and Papadopoulos, 1980). In colder climates, high tunnels are used less for early production. In Quebec, tomato transplanted into high tunnels in early May ripened in July, only 2 or 3 weeks earlier than when grown in the field (Trudel and Gosselin, 1982). In New Hampshire, tomato transplanted into high tunnels in late May did not ripen earlier but yield and quality were better than when grown outside (Wells, pers.

commun., 1991). In these examples, high tunnels were vented continuously during the day. In Connecticut, planting in early April in high tunnels that were vented during the day only when air warmed above 35°C, forced production 6 weeks earlier than was possible in the field (Gent, 1990; 1991). However, tomato plants grown under these conditions soon showed signs of nutrient deficiencies, and later, leaves were small, stems were thin and flower abortion was excessive. Under controlled conditions, tomato required soil warmer than 12°C for nutrient uptake and vegetative growth (Cannell et al., 1963; Martin and Wilcox, 1963). Uptake of P seemed most

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limited by cool soil (Lingle and Davis, 1959; Tiessen and Carolus, 1963). Inhibition of N uptake depended in part on the form of N available, e.g. NH 4 or NO 3 (Ganmore-Neumann and Kafkafi, 1980). In cool soil, plant growth was inhibited as much or more than nutrient uptake (Tiessen and Carolus, 1963). In high tunnels, the soil was often cooler than 12C in early April, and this may have inhibited nutrient uptake. In other studies in high tunnels, warming the roots increased earliness and total yield of tomato (Devonald and Tapp, 1987; Moss, 1983; Trudel and Gosselin, 1982), but there were no obvious effects on plant nutrition. In heated greenhouses, warming the soil to 25°C increased growth and yield, but effects on plant nutrient concentrations were slight (Gosselin and Trudel, 1983; Papadopoulos and Tiessen, 1987). In these studies, however, the soil never cooled below 12°C. Thus cool soil may have limited the growth and nutrient uptake of tomato in unheated high tunnels in early spring. Either soil or air temperature could limit tomato production in unheated high tunnels. Seedlings were exposed to different soil temperatures by transplanting on 3 dates, and exposed to different air temperatures by growing in tunnels with different set points for ventilation. Plant growth and nutrition were measured two weeks after planting, and related to air and soil temperatures since planting, and to yield later on.

Materials and methods

Growth conditions Four high tunnels (17.1 × 4.3 x 2.6 m high) were set up with the long axis oriented east-west at Lockwood Farm in Hamden, CT, USA (Lat. 42 N, Long. 73 W). For ventilation, rectangular doors at each end ( 2 . 3 x 2 . 0 m high) were opened by a 60 ° rotation on a center pivot. The high tunnels and doors were covered with a single layer of 0.1 mm clear polyethylene. The soil was fine sandy loam, limed to obtain pH 6.0, tilled and covered with 0.03mm black polyethylene mulch. Air temperature was measured with shielded copper-constantan thermocouples

at heights of 0.5 and 1.5 m. Soil temperature was measured at a depth of 10cm next to a plant. These temperatures were monitored continuously by a datalogger (Campbell Scientific model CR10, Logan UT) and day and night time averages, and minimum and maximum temperatures were recorded. The doors were opened automatically when the air temperature exceeded the ventilation set point. Each tunnel had a different ventilation set point; 14, 22, 30 and 38°C. Doors were closed when that air cooled below the setpoint, except if the running average of air temperature since 0600 hr EDT exceeded 25°C.

Plants Seeds of tomato (Lycopersicon esculentum Mill) cultivars 'Early Cascade', 'Early Girl' and 'First Lady' were germinated in peat: vermiculite and grown at about 21°C in a heated greenhouse for 5 to 7 weeks. In 1990, seeds were germinated on 13 February, 6 March and 27 March for planting on 3 April, 17 April and 1 May. In 1989, seeds of 'Early Girl' were germinated on 14 Feb and 23 Mar for planting on 3 April and 1 May. Seedlings were transplanted into level ground to a depth of the first true leaf. In 1989, half the seedlings were planted in 12 liter black plastic pots filled with soil and the other half were transplanted into level ground. Plants were spaced 0.6m apart within rows spaced 0.9m apart. Plants were supported by string and pruned to a single stem as they developed. Plants were watered at transplant and each week thereafter, or when soil moisture tension exceeded 20 kPa. At each watering, a complete fertilizer was applied at 100, 22, 83, and 0.8 mg L i of N, P, K, and Mg respectively, and minor elements (Peters 20-10-20, W.R. Grace Co., Cambridge, MA).

Experimental design The experimental design was a split-split plot. The tunnels or ventilation regimes were main plots, planting dates were sub-plots, and cultivars were sub-sub-plots. There were two randomized complete blocks of sub-plots in each tunnel. In 1990 each sub-sub-plot was a single row of 8 plants across the width of the tunnel. In

Growth and nutrition of tomato in high tunnels" 1989 each sub-sub-plot was 12 plants in a square array.

Harvests Six plants per cultivar were sampled at transplant and others were harvested two weeks later. In 1989, plants in each sub-sub-plot (a total of 24 plants) were harvested at 700, 1200 and 1700 h. Shoots were severed at the cotyledon scar, frozen with dry ice, freeze dried and ground to pass 20 mesh sieve. In 1990, two plants from each sub-sub-plot (total of 48 plants) were harvested at 1200-1300 hours. The root ball was washed free of soil and peat under cold running water. Plants were frozen with dry ice, freeze dried and separated into flower, leaf, stem plus petiole, and root parts. Replicate samples were combined and ground to pass a 20 mesh sieve. Six plants in each sub-sub-plot were grown into fruit production, when ripe fruits were harvested each week, counted and weighed.

Tissue analysis For analysis of total elemental composition, 0.25 g subsamples were digested in 4 mL of boiling HzSO 4 while 10mL H 2 0 2 was added drop wise. The digest was diluted to 100mL and concentrations of macronutrients determined by spectrophotometry following established procedures (Hach 1988). N was determined by the Nestler reaction; P by reaction with molybdate; K by reaction with tetraphenyl borate; and Mg and Ca by reaction with 0.03% Calmagite solution in the presence and absence of EGTA. Analyses were checked and calibrated against results from inductively coupled plasma spectrophotometry, except N was calibrated with Bovine Serum Albumin standards. Soluble NO 3N and PO4-P were extracted by blending 0.25 g subsamples with 100 mL distilled water, adding 0.25 g activated charcoal and filtering. Anions were detected by conductivity after high pressure liquid chromatography (Dionex HPICAS3 column, Dionex, Sunnyvale, CA) with carbonate buffer. Soluble carbohydrates were extracted from 0.10 g subsamples with 12 mL of methanolchloroform-water 12-5-3. The water phase was separated from chloroform and dried at 50°C

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under a stream of air. The dried extract was dissolved in 2 mL distilled water and sucrose, glucose and fructose were detected by differential refractive index after high pressure liquid chromatography (BioRad HPX87C column, Biorad, Richmond, CA) with water. Starch in the residue from the extraction was digested with alpha-amylase, and reducing sugars determined by reduction of K~Fe(CN)(~ (Gent 1986). Total nonstructural carbohydrate in leaves was assayed by the same method without the organic solvent extraction.

Statistical analysis The relative growth rate of the shoot, R G R g g 1 day 1, was defined by: R G R : [log W(t2) - l o g W(t~)]/lt 2 - t,l where W(tl) and W(t2) were shoot dry weights at the time of transplant and 2 weeks later, t~ and t2, respectively. The nutrient uptake ratio over 1 the same growth interval, N U R g nutrient g dry weight, was defined by: N U R : [(W(t2)CONC(t2) - W(t, )CONC(t, )1 / [W(t 2) - W(t, )] where CONC refers to the shoot tissue concentration of a particular nutrient at times t I and t 2. Cultivar and planting date effects were determined by analysis of variance. Linear effects of ventilation were tested within each planting across all cultivars. The R G R and N U R were regressed versus air and soil temperatures over all cultivars and plantings.

Results

Growth and yield Two weeks after transplant, seedlings were larger in tunnels vented at 30 or 38°C than those in tunnels vented at 14 or 22°C (Table 1). Seedlings were 7, 6 and 5 weeks old at transplant on 3 and 17 April and 1 May, respectively, but later plantings grew faster than earlier ones, so the

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0.2

Table 1. Shoot dry weight of tomato seedlings (g p l a n t - ' ) two weeks after transplanting into high tunnels in spring 199(/ Factor

./

Transplant date 3 April

17 April

1 May

Cultivar E. Cascade E. Girl F. Lady

5.5 6.1 4.9

7.2 6.6 6.6

6.1 6.6 5.9

Ventilation a Vent at 14°C Vent at 22°C Vent at 30°C Vent at 38°C

4.0 5.4 6.1 6.5

4.6 5.9 7.6 9.1

5.3 5.1 6.9 7.4

Significance Cultivar Ventilation ~

* ** •

NS ***

NS ***

0.15 '3>, "O '3 O~0.1

O~

{3 IZ O J []

0.05

" A v e r a g e d over cultivars. Interactions between cultivar and ventilation were not significant. b Linear effects of ventilation significant at p < 0.001.

0 seedlings were approximately the same weight two weeks after transplant. The growth response to planting date and ventilation temperature could be related to air temperature, as shown in Figure 1. Within each planting, the average temperature of the air in tunnels vented at 14 or 22°C was cooler than in tunnels vented at 30 or 38°C (x axis in Fig. 1). In later plantings, the air in all tunnels was warmer than in any tunnel in earlier plantings, except for the 1 May planting in the tunnel vented at 14°C. The shoot RGR was best fit with a quadratic response to air temperature averaged over the two week growth interval (Fig. 1). Adding terms for soil temperature, day-to-night variation of air or soil temperature or minimum and maximum air temperatures did not improve regression (results not

shown). Yield of ripe tomatoes was affected more by transplant date than by ventilation (Table 2). In 1990, the average yield of 3 April transplants, 2.2kg plant -l, was much less than that of 17 April and 1 May transplants, 4.0 and 4.1kg plant l, respectively. For 'Early Girl' in a tunnel vented at 30°C, the low yield of the 3 April planting was due to a slow rate of fruit production (as indicated by the linear trends in Fig. 2) and not due to delayed ripening. The same relation held true for the other cultivars and for

5

10

at

14 [] [] •

Transplant

22 Q) Q •

30 38°C date /~ ~ 3Apr A ~ 17 Apr • • 1May

I

r

15

20

25

Air temperature,°C Fig. I. Relative growth rate of tomato seedlings in high tunnels as a function of air temperature averaged over two weeks. The shape of the symbols indicate the ventilation t e m p e r a t u r e in the high tunnel and the shading of the symbol indicates the transplant date. The standard error of the estimate for R G R was 0.009 g g--i day ~. The regression line corresponds to R G R = 0,103 - 0.0169 T + 0.00114 T 2.

plants in a tunnel vented at 38°C. For 3 April transplants, there was an additional reduction in yield in tunnels vented at 14 or 22°C (Table 2), due to a delay in fruit development and ripening (data not shown). The low rate of fruit production of the 3 April planting was not likely due to cool air or soil during fruit growth. During this stage, the mean air temperatures were 18, 20 and 22°C for 3 and 17 April and 1 May plantings, respectively, and the soil was warmer than the air. Thus, differences in yield were likely due to temperatures experienced shortly after planting. Yields from a 3 April planting in 1989 were also low, 1.1 and 1.3 kg plant -1 in tunnels vented at 15 and 35°C, respectively, and plants in pots yielded less than those in level ground (Gent, 1990). The 1 May 1989 planting was not grown into fruit production.

Growth and nutrition o f tomato in high tunnels

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Table 2. Total yield of tomatoes (kg plant ') within 16 weeks after transplanting into high tunnels in spring 1990

Tissue nutrient concentrations

Factor

T h e nutrient concentrations were very low in t o m a t o seedlings two weeks after planting on 3 April 1990 c o m p a r e d to the later plantings. Nutrients in leaves (Table 3) were more sensitive to planting date than were those in the stem (data not shown) or in the roots (Table 4). According to the standards of Roorda van Eysinga and Smilde (1981), the concentrations of total N, N O 3 - N and Mg were deficient in leaves of 3 April transplants. These leaves contained only 50% of the N and 42% of the Mg found in leaves of 1 May transplants. There was little difference between 17 April and 1 May transplants in leaf N, except for plants grown in the tunnel vented at 14°C, where leaf N increased from 40 to 46 mg 1 g . The concentrations of other macronutrients in leaves of 3 April transplants were lower than in later plantings but they were not low enough to be considered deficient. Leaf concentrations of P, K and Ca from 3 April transplants were 83, 78 and 72%, respectively, of those for 17 April transplants. There was little difference in concentration of these nutrients between leaves of 17 April and 1 May transplants. N i t r a t e - N was extremely low in leaves harvested two weeks after transplanting on 3 April; about 0.2 comp a r e d to 1.0 mg g 1 in later plantings. This form of N was more sensitive to planting date than was that of total N. In contrast, phosphate-P was unaffected by planting date. In part, this was due to a high concentration of P in seedlings before I transplanting, 6 mg g Ventilation affected the concentration of nutrients in seedlings transplanted on 3 April. Both the air and soil were warmed by venting at w a r m e r temperatures, but this decreased the nutrient concentrations in leaves, except for N O 3 and Mg (Table 3). There was no linear effect of ventilation in later plantings, except for Mg, which increased in 17 April transplants by venting at w a r m e r temperatures. L e a f total nonstructural carbohydrate concentration, T N C , increased where nutrients decreased (Table 3). In 3 April transplants, T N C averaged 285 mg g ~ and seedlings grown in a tunnel vented at _3 8 °C had twice as much T N C as those in a tunnel vented at 14°C. The composition of T N C was analyzed in 'Early Girl'. Leaves

Transplant date 3 April

17 April

1 May,

Cultivar E. Cascade E. Girl F. Lady

2.2 2.3 2.1

4.1 4.0 4.1

3.9 4.3 4.3

Ventilation ~' Vent at 14°C Vent at 22°C Vent at 30°C Vent at 3g°C

1.6 1.8 3.1 2.7

4.0 2.7 4.8 4.5

4.8 3.2 4.6 3.9

Significance Cultivar

NS

NS

NS

***

NS

NS



.

Ventdatlon

b

~'Averaged over cultivars. Interactions between cultivar and ventilation were not significant• ~'Linear effects of ventilation significant at p < 0.001.

5

4

Transplant date /~ 3 Apr 17 Apr • 1 May

J



!

/

"3 cL

j 1

°"

I

6/1

6/15

6/29

7/13

7/27

t

8/10

i

8/24

Date Fig. 2. Cumulative yield of 'Early Girl' tomato from three plantings in a high tunnel ventilated at 30°C. The standard error of the estimate was 6% of the mean. The linear trend fines correspond to the rate of production.

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Table 3. Nutrient concentrations in leaves of tomato after two weeks in high tunnels in spring 1990

Factor



Nutrient concentrations (mgg

1

dryweight)

pO4-P

NO3-N

P

N

K

Mg

Ca

TNC

1.7 1.5 1.8

0.7 0.8 0.6

3.6 3.8 3.5

31 35 34

20 26 22

4.2 4.4 5.3

23 18 19

225 196 222

2.0 2.1 1.8 1.2

0.2 0.1 0.3 0.3

3.9 3.5 2.8 2.7

25 20 22 18

24 19 17 15

2.6 2.6 2.7 2.4

18 18 15 14

197 259 296 386

**

NS

***

*

***

NS

*

***

1.8 1.2 1.4 1.4

1.0 0.6 1.5 1.4

4.1 3.3 3.8 4.0

40 36 42 43

25 22 23 27

5.3 4.6 6.6 7.0

23 24 21 23

170 179 134 174

Ventilation Plant 1 May Vent at 14°C Vent at 22°C Vent at 30°C Vent at 38°C

NS

NS

NS

NS

NS

**

NS

NS

1.7 1.1 1.7 1.9

1.0 0.5 0.9 1.5

4.6 3.4 3.7 3.9

46 41 41 42

30 25 26 27

6.2 5.6 6.1 6.8

19 21 21 18

164 177 192 177

Ventilation Significance b Cultivar Planting

NS

NS

NS

NS

NS

NS

NS

NS

NS NS

NS ***

NS NS

* ***

** **

** ***

* **

* ***

Cultivar E. Cascade E. Girl F. Lady Plant 3 April Vent at 14°C Vent at 22°C Vent at 30°C Vent at 38°C Ventilation" Plant 17 April Vent at 14°C Vent at 22°C Vent at 30°C Vent at 38°C

*, **, *** linear effects of ventilation within planting significant at 0.05, 0.01 and 0.001 levels respectively. b ,, ** *** differences significant at 0.05, 0.01 and 0.001 levels respectively.

of 3 April transplants of 'Early Girl' contained m o r e s o l u b l e s u g a r s , 68 m g g-1, t h a n d i d 17 A p r i l o r 1 M a y t r a n s p l a n t s , 41 a n d 48 m g g-~, r e s p e c tively. I n 3 A p r i l t r a n s p l a n t s , w a r m e r air r e s u l t e d in m o r e s t a r c h , b u t s o l u b l e sugars w e r e g r e a t e s t in a t u n n e l v e n t e d at 14°C. L e a v e s c o n t a i n e d 92, 60, 71 a n d 51 m g g-X o f s o l u b l e sugars in t u n n e l s v e n t e d at 14, 22, 30 a n d 38°C, r e s p e c t i v e l y . In l a t e r p l a n t i n g s , l e a v e s h a d less T N C a n d v e n t i l a t i o n h a d n o effect o n T N C o r its c o m p o n e n t s . T h e p l a n t i n g d a t e a f f e c t e d t h e n u t r i e n t conc e n t r a t i o n s in r o o t s less t h a n in l e a v e s , e x c e p t for N O 3 - N ( T a b l e 4). R o o t s o f 3 A p r i l t r a n s p l a n t s h a d a b o u t 8 0 % o f t h e M g a n d C a f o u n d in 17 April transplants. Roots of 3 April transplants c o n t a i n e d t h e m o s t P O 4 - P a n d t o t a l P, a n d N a n d K d i d n o t differ b e t w e e n plantings. R o o t s of 3 April transplants contained only a third of the N O 3 - N f o u n d in l a t e r t r a n s p l a n t s . W i t h i n e a c h

p l a n t i n g , t h e r e was n o effect o f v e n t i l a t i o n on r o o t n u t r i e n t s . R o o t s o f 3 A p r i l t r a n s p l a n t s of ' E a r l y G i r l ' c o n t a i n e d 34 mg g-1 o f s o l u b l e s u g a r a n d 77 m g g-1 o f starch, which g r e a t l y e x c e e d e d t h e a m o u n t f o u n d in l a t e r t r a n s p l a n t s , 23 a n d 35 m g g - l , r e s p e c t i v e l y . T h e cultivars d i f f e r e d in t h e c o n c e n t r a t i o n o f n u t r i e n t s in l e a v e s a n d r o o t s ( T a b l e s 3 a n d 4), b u t t h e s e d i f f e r e n c e s w e r e c o n s i s t e n t o v e r all p l a n t i n g s a n d t u n n e l s . ' E a r l y C a s c a d e ' h a d the h i g h e s t c o n c e n t r a t i o n s o f C a a n d t h e lowest conc e n t r a t i o n s o f P O 4 - P a n d N O 3 - N in r o o t s a n d t o t a l N in leaves. ' F i r s t L a d y ' h a d t h e h i g h e s t c o n c e n t r a t i o n o f M g a n d ' E a r l y G i r l ' h a d the m o s t K in s t e m s a n d leaves. I n 1989, t r a n s p l a n t d a t e a f f e c t e d c o n c e n t r a t i o n s o f n u t r i e n t s in t h e w h o l e s h o o t like it a f f e c t e d l e a f n u t r i e n t s in 1990. C o n c e n t r a t i o n s o f all n u t r i e n t s w e r e less in s h o o t s o f 3 A p r i l t h a n 1

Growth and nutrition o[ tomato in high tunnels"

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Table 4. Nutrient concentrations in roots of tomato after two weeks in high tunnels in spring 1990 Factor

Nutrient concentrations (mg g ~ dry weight) POs-P

NO3-N

P

N

K

Mg

Ca

Cultivar E. Cascade E. Girl F. Lady

1.4 2.1 1.9

3.2 4.11 3.9

3.1 3.2 3.1

24 24 25

26 31 31

5.3 4.9 6.4

9 8 7

Plant 3 Vent Vent Vent Vent

April at 14°C at 22°C at 30°C at 38°C

2.5 3.1 2.2 2.1

1.1) 1.6 1.6 1.7

3.6 4.1 3.5 3.2

21 25 26 22

27 27 29 311

4.8 4.4 5.1 5.4

7 7 7 7

Plant 17 April Vent at 14°C Vent at 22°C Vent at 30°C Vent at 38°C

1.6 1.3 1.4 1.2

4.8 3.2 5.6 3.9

2.9 2.9 2.9 2.5

24 28 28 23

31 27 32 32

5.0 7.11 6.4 5.9

8 8 8 llt

Plant 1 May Vent at 14°C Vent at 22°C Vent at 30°C Vent at 38°C

1.8 1.5 1.8 1.3

5.5 4.7 5.5 5.7

3.5 2.5 2.6 2.6

26 26 25 23

36 27 25 33

4.9 6.0 5.9 5.6

8 8 7 8

Significance ~' Cultivar Planting

* **

** ***

NS **

NS NS

NS NS

. . **

.

. **

" * ** *** differences significant at 0.05, 0.01 and 0.001 levels respectively.

May transplants (Table 5). The NO3-N was reduced most by early planting. Starch and soluble sugars were high in 3 April transplants where nutrients were low. In 1989, harvests were made at the beginning, middle and end of the photoperiod, and the concentration of NO 3 in the shoot varied the most with time (Table 5). At noon, NO3-N in 3 April transplants, was only half that at dawn. In 1 May transplants, NO 3 also decreased during the day, but remained above a level indicative of nutrient stress. Soluble sugars showed an opposite trend with time, they increased to a maximum at noon. Starch was highest at the end of the day. Other nutrients generally did not vary with time of day. In both plantings in 1989, plants in pots had higher concentrations of PO4-P, total P and starch compared to plants in level ground, but plants in pots had lower NO3-N and total N after planting on 1 May. The soil in pots had a large diurnal variation in temperature, often ex-

ceeding 30°C during the day but cooling below 10°C at night (Gent 1990). This temperature variation did not improve nutrient status relative to that of the more constant temperature of soil in level ground.

Plant nutrient uptake Some of the nutrients in plants two weeks after transplant were in the seedlings at transplant. The nutrient uptake ratio, NUR, measured actual assimilation as a ratio of dry matter accumulation. The NUR for N, P, and K are plotted against mean soil temperature over each two week interval in Figures 3, 4 and 5. Uptake of each nutrient follows a slightly different pattern. Despite adequate P in shoots of 3 April transplants, NUR was near 0mg P g ~ DW, and within each tunnel it increased in later plantings as the soil warmed (Fig. 3). For N, NUR was about 10mg N g-~ DW in 3 April transplants compared to 40 mg N g ~ DW in later plantings.

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Table 5. Nutrient concentrations in shoots of tomato after two weeks in high tunnels in spring 1989

Factor

Plant 3 April Vent at 15°C Vent at 15°C Vent at 350C Vent at 35°C Time of day 700 h 1200 h 1700 h Significancea Treatment Time Interaction Plant 1 May Vent at 15°C Vent at 15°C Vent at 35°C Vent at 35°C Time of day 700 h 1200 h 1700 h Significance Treatment Time Interaction

Nutrient concentrations (mg g ~dry weight)

Ground Pot Ground Pot

Ground Pot Ground Pot

PO4-P

NO3-N

P

N

K

Sugar

Starch

1.1 1.9 1.1 1.8

0.3 0.5 0.4 0.5

2.7 3.5 2.8 3.4

20 18 18 20

20 22 18 21

133 108 124 88

302 287 233 325

1.5 1.6 1.4

0.6 0.3 0.4

3.1 3.2 3.0

19 18 19

20 22 22

111 125 103

282 281 298

** * *

*** *** ***

** NS NS

* NS *

* NS NS

*** *** NS

* NS NS

1.6 3.1 1.3 3.7

3.0 2.2 3.1 2.4

4.4 5.4 3.4 5.8

37 30 33 28

36 34 34 39

62 62 66 71

190 212 181 223

2.6 2.2 2.5

2.9 2.2 1.9

5.2 4.5 4.6

33 32 32

36 35 35

64 72 60

195 195 215

*** NS NS

*** *** *

*** * NS

** NS NS

NS NS NS

** *** *

* NS NS

" *, **, *** differences significant at 0.05, 0.01 and 0.001 levels respectively. W i t h i n p l a n t i n g s N U R for N did n o t vary with soil t e m p e r a t u r e (Fig. 4). T h e t e m p e r a t u r e dep e n d e n c e of N U R for M g followed that for N; d e p r e s s e d in 3 A p r i l t r a n s p l a n t s b u t a d e q u a t e in later p l a n t i n g s . T h e N U R for K was a b o u t 20 mg K g 1 in 3 A p r i l t r a n s p l a n t s , a n d i n c r e a s e d g r a d u a l l y in later p l a n t i n g s (Fig. 5). T h e beh a v i o r of Ca was similar to that of K. U p t a k e of N a p p e a r e d to be r e l a t e d to a critical t e m p e r a t u r e , u n l i k e shoot growth, which was c o r r e l a t e d with the average air t e m p e r a t u r e . T o define this critical t e m p e r a t u r e , the fraction of t i m e the soil was w a r m e r t h a n 8, 10, 12, 14 or 16°C was c a l c u l a t e d for each p l a n t i n g a n d ventil a t i o n r e g i m e . W h e n this fraction was regressed versus N - N U R , the best c o r r e l a t i o n was with the f r a c t i o n of the growth i n t e r v a l that soil was w a r m e r t h a n 12°C (Fig. 6). This fraction was 0.6 or less for all t u n n e l s after t r a n s p l a n t i n g o n 3 A p r i l , while it was n e a r l y 1.0 for later p l a n t i n g s , e x c e p t i n g the 17 A p r i l p l a n t i n g v e n t i l a t e d at

14°C. F o r all o t h e r n u t r i e n t s except Mg, the c o r r e l a t i o n s were less sensitive to the choice of critical t e m p e r a t u r e .

Discussion A l t h o u g h soil cooler t h a n 12°C d r a m a t i c a l l y reduced growth under controlled conditions (Cannell et al., 1963; C o r n i l l o n , 1974; M a r t i n a n d W i l c o x , 1963), this t e m p e r a t u r e did n o t a p p e a r critical for shoot growth of t o m a t o in high t u n nels. I n the high t u n n e l s , R G R i n c r e a s e d s m o o t h l y with air t e m p e r a t u r e , a n d 3 A p r i l t r a n s p l a n t s that e x p e r i e n c e d soil cooler t h a n 12°C, grew according to the same t r e n d line that d e s c r i b e d the later plantings. T h e e n v i r o n m e n t in high t u n n e l s differed from c o n t r o l l e d condit i o n s in that i r r a d i a n c e was i n t e n s e , a v e r a g i n g 17 to 20 Mj m 2 day l, a n d d a y - t o - n i g h t v a r i a t i o n in

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