Seasonal effects of shading on growth of greenhouse lettuce and spinach

Scientia Horticulturae, 24 ( 1 9 8 4 ) 2 3 1 - - 2 3 9

231

Elsevier Science P u b l i s h e r s B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s

SEASONAL EFFECTS OF SHADING LETTUCE AND SPINACH

ON GROWTH

OF GREENHOUSE

E D W A R D P. G L E N N , P A U L A C A R D R A N a n d T. LEWIS T H O M P S O N

Environmental Research Laboratory, Tucson International Airport, Tucson, A Z 85706 (U.S.A.) ( A c c e p t e d for p u b l i c a t i o n 9 A u g u s t 1 9 8 4 )

ABSTRACT G l e n n , E.P., C a r d r a n , P. a n d T h o m p s o n , T.L., 1 9 8 4 . Seasonal effects o f s h a d i n g o n g r o w t h o f g r e e n h o u s e l e t t u c e a n d s p i n a c h . Scientia Hortic., 24: 2 3 1 - - 2 3 9 . L e t t u c e (cultivar ' S u m m e r B i b b ' ) a n d s p i n a c h ( ' M e l o d y ' ) were g r o w n u n d e r various s h a d e t r e a t m e n t s in a g r e e n h o u s e a t T u c s o n , A r i z o n a , U.S.A. Six e x p e r i m e n t s were cond u c t e d w i t h l e t t u c e a n d five w i t h s p i n a c h at d i f f e r e n t t i m e s o f t h e year. T h e o b j e c t i v e was t o c o m p a r e t h e i r g r o w t h p o t e n t i a l s over a wide r a n g e o f P A R f r o m n a t u r a l sunlight. L e t t u c e r e s p o n d e d p o s i t i v e l y t o P A R u p t o t h e h i g h e s t level m e a s u r e d , 45 tool m -2 d a y -~ . T h e m a x i m u m g r o w t h r a t e was 0 . 2 2 1 g g-1 day-1 f r o m D a y 14 t o Day 4 2 a f t e r seeding. S p i n a c h was P A R - s a t u r a t e d a t a p p r o x i m a t e l y 25 tool m -2 d a y -~ a n d t h e m a x i m u m g r o w t h r a t e was 0 . 1 9 4 g g-~ d a y -~ . T h e P A R c o m p e n s a t i o n p o i n t f o r g r o w t h was less t h a n 1 tool m -2 d a y -x f o r b o t h p l a n t s , b u t n e i t h e r p r o d u c e d m a r k e t - q u a l i t y h e a d s b e l o w 8 m o l m -2 d a y -~ d u e t o e t i o l a t i o n effects. T h e g r o u n d c o v e r o f a p l a n t p e r u n i t d r y w e i g h t i n c r e a s e d a t l o w P A R levels, a n d s p i n a c h h a d a 4-fold greater g r o u n d c o v e r p e r u n i t w e i g h t t h a n l e t t u c e a t all P A R l e v e l s . G r o w t h curves were a n a l y z e d b y m e a n s o f d o u b l e r e c i p r o c a l e q u a t i o n s . C o e f f i c i e n t s o f d e t e r m i n a t i o n (r 2 ) were greater t h a n 0.96 in all i n d i v i d u a l e x p e r i m e n t s . P o o l e d l e t t u c e a n d s p i n a c h d a t a h a d r 2 values o f 0 . 7 4 7 a n d 0 . 8 4 6 , respectively. T h e e q u a t i o n s o f b e s t fit h a d similar y - i n t e r c e p t s f o r b o t h p l a n t s , b u t t h e slope o f t h e s p i n a c h e q u a t i o n was significantly greater than that of the lettuce equation, denoting a lower response to PAR for spinach. L e t t u c e was m o r e e f f i c i e n t t h a n s p i n a c h in P A R u t i l i z a t i o n . B o t h p l a n t s grew well u n d e r h i g h P A R c o n d i t i o n s , h o w e v e r , a n d it was c o n c l u d e d t h a t h i g h i n s o l a t i o n d e s e r t regions o f t h e w o r l d c a n b e u s e d f o r g r e e n h o u s e p r o d u c t i o n o f l e t t u c e a n d s p i n a c h . Keywords: greenhouse; spinach.

l e t t u c e ; p h o t o s y n t h e t i c a l l y active r a d i a t i o n ( P A R ) ; s h a d i n g ;

ABBREVIATIONS DW = d r y w e i g h t ; F W -- fresh w e i g h t ; P A R = p h o t o s y n t h e t i c a l l y active r a d i a t i o n ; R G R = relative g r o w t h rate.

0304-4238/84/$03.00

© 1 9 8 4 Elsevier Science P u b l i s h e r s B.V.

232 INTRODUCTION Greenhouse lettuce and spinach are normally grown in the temperate zones, in winter, under poor light conditions (Huyskes, 1971; Large, 1972; Wittwer and Honma, 1979). Radiation is generally the most important factor controlling growth under low light conditions, but the growth rates of lettuce and spinach under the high natural light levels typical of desert regions have n o t been adequately determined. Fifty percent of full sunlight gave the best growth of lettuce in o u t d o o r shade experiments conducted in summer in Rome, Italy (Mattei, 1967; Mattei et al., 1973), whereas full sunlight gave the best growth of lettuce in similar experiments conducted at all times of the year in Japan (Noguchi et al., 1978). Glenn (1984) reported a high statistical correlation between lettuce yield and total radiation up to 550 cal cm -2 day -1 (45 mol m -2 day -1 PAR) in a high insolation greenhouse environment, but similar experiments with spinach have n o t been reported. T h e present experiments determined the growth response of lettuce and spinach to radiation over a wide range of PAR using natural sunlight as the source of illumination. Shading experiments were conducted in a desert greenhouse at different times of year in order to separate the effects of duration and intensity of PAR on growth. MATERIALS AND METHODS Experiments were conducted at Tucson, Arizona, U.S.A., in a 0.6-ha greenhouse described previously (Glenn, 1984). Butterhead lettuce ('Summer Bibb', Harris Seed Co., Inc.) and spinach ('Melody', Northrup-King Seed Co., Inc.) were germinated in peat cubes and transferred into experi m e n t a l treatments 14 days after sowing. The plants were grown in hydroponic culture in 25-1 capacity tanks filled with aerated nutrient solution, which was changed weekly (Jensen and Hicks, 1973). Each tank contained 4 plants spaced 20 cm apart. PAR was varied by hanging black shade cloths of variable mesh size over tanks grouped into blocks of 4. The cloths were hung over the tops and sides of a block of tanks so t h a t sunlight always passed through 1 layer of cloth. Blocks were spaced far enough apart to prevent them from shading each other. Each t r e a t m e n t contained 3 blocks of tanks randomly assigned along tables oriented on a n o r t h / s o u t h axis in the greenhouse. Each shade treatm e n t thus contained 48 lettuce or spinach plants. Control plants were grown in blocks along the same tables exactly as above, but w i t h o u t shade cloth. The d i f f e r e n t cloths gave nominal values of PAR transmission of 8, 27, 34, 45, 53, 70 and 75%. Total radiation in the greenhouse was continuously measured as previously described (Glenn, 1984), using a recording pyranometer (Weathertronics, Inc.) which was calibrated with an Eppley pyranometer operated by the Department of Atmospheric Physics, University of Arizona, Tucson. Total radiation (cal) was converted to PAR (mol) using the relation-

233 ship, 1 mol PAR = 1.22 × 105 cal, based on a series of simultaneous measurements of total radiation and PAR in this greenhouse made concurrently with the present experiments (Glenn, 1984). The percentage of PAR transmitted through each treatment was measured at the start and finish of each experiment by taking readings under each cloth with a PAR quantum sensor (Li-Cor, Inc.). Measurements were made at plant level under each block at 08.00, 12.00 and 15.00 h, with the sensor normal to the sun, and results were averaged to minimize error due to the change in the angle of incidence of sunlight t h r o u g h o u t the day. The percentage transmission and the continuous (unshaded) PAR measurements were multiplied to calculate the total daily PAR received under each treatment. Heating and cooling were controlled by thermostats set at 25°C in the day and 14°C at night. However, between July and September, temperatures exceeded the thermostat settings due to the inefficiency of the fan-and-pan evaporative cooling system during this period of high temperature and humidity. Temperatures were continuously measured at three points in the experimental area by means of thermocouples. Maximum day temperatures and m i n i m u m night temperatures reported here are the average of the 3 readings. Growth rates were determined by the increase in fresh weight of plants, excluding the roots, between Days 14 and 42. Relative growth rates were calculated using the formula for exponential growth: RGR = [In(final wt.) -In(initial w t . ) ] / 2 8 days. The ground cover of a plant was determined by the area of the plant projected on the ground. The area was calculated from the diameter of the plant. The percentage of dry matter was determined by oven-drying plants to constant weight at 60 ° C. The growth response to PAR was analyzed by means of double reciprocal equations, in which the reciprocal of growth rate was regressed against the reciprocal of PAR (Goldman, 1979). This analysis assumes that the growth response to PAR is in the shape of a rectangular hyperbola; a double reciprocal equation converts the relationship into a straight line. The slope (b), y-intercept (a), and coefficient of determination (r 2 ) of the straight line were determined by the least squares m e t h o d (Steel and Torrie, 1962). The maximum growth rate at saturating PAR (~), the PAR needed to produce halfmaximal growth (IK), and the PAR needed to produce 95% maximal growth (Is) were calculated from the following formulae, based on those in Goldman (1979): a = a -1 ; I K = b / a ; I s = b / O . O 5 a . The t-test was used to establish the statistical significance of the difference between slopes of the double reciprocal equations describing pooled lettuce and spinach data (Steel and Torrie, 1962). RESULTS Six shading experiments were conducted from August 1981 to June 1982. Spinach and lettuce were paired in the first five, but spinach plants bolted

234

prematurely in late spring and were excluded from the last experiment. Environmental conditions during each experiment and growth rates of unshaded control plants are given in Table I. Experiment 1, conducted under extremely warm temperatures, produced the highest relative growth rates. The m a x i m u m day temperatures in this experiment exceeded 30°C and night lows exceeded 20°C. Temperatures during Experiments 2--6 were at least 5°C cooler, with m a x i m u m day temperatures ranging from 24.3 to 25.8°C and night lows of 8.8--15.0°C. In these experiments, growth rates increased with decreasing day length and PAR. Growth rates of lettuce plants exceeded those of spinach plants in 4 of 5 experiments, and averaged 16% greater for all experiments combined. TABLE I

Environmental conditions during shade experiments and relative growth rates of unshaded lettuce and spinach plants Experiment No.

Date

1 2 3 4 5 6

8 19 29 23 1 1

Aug.-- 8 Sep. Nov.--17 Dec. Dec.--26 Jan. Mar.--20 Apr. May--29 May June--29 June

Photoperiod (h day -~ )

PAR (mol m -2 day -1 )

Daily air temperature (° C)

RGR

Maximum Minimum

Lettuce

Spinach

12.9 10.2 10.2 12.6 13.8 14.2

23.8 15.5 15.6 33.9 39.9 44.6

30.7 25.8 24.3 25.0 25.2 25.6

0.221 0.172 0.160 0.175 0.178 0.196

0.194 0.147 0.135 0.165 0.182 *

21.4 15.0 13.3 12.2 12.3 8.8

(g g-~, day -~ )

*Spinach bolted prematurely so RGR was not measured.

Control lettuce plants grew faster than shaded lettuce plants in all experiments (Fig. la), whereas 25 or 30% shade produced the best growth of spinach in Experiments 1, 4 and 5 (Fig. lb). The PAR saturation level for lettuce must exceed 40 mol m -2 day -z , while for spinach it is approximately 25 mol m -2 day -1 . At least 3 separate curves for lettuce growth could be discerned, depending upon the day temperatures during an experiment, whereas spinach data all appeared to fall on the same curve. Lettuce and spinach had positive growth rates under all shade treatments at all times of year. Therefore, the PAR compensation points m u s t h a v e been lower than the lowest level of PAR under the heaviest degree of shading; approximately 1 mol m -2 day -1 . Market quality was lowered at low PAR levels, however, due to etiolation of leaves and petioles of spinach, and failure of lettuce to form heads. The lowest level of PAR that would produce marketable lettuce or spinach was approximately 8 mol m -2 day -1 (shown with broken lines in Fig. 1). The ratio of ground cover to dry matter of a plant was also affected by PAR. Plants grown under low light had a much greater ground cover per unit weight at Day 42 than plants grown under high light, and spinach had

235

I

I

e)

I

I

b)

Lettuce

I

Spinach

Exp.I (30.7 C]

0.2 I~

D

"7

0.t

ef

°

! I

20

I

40 0 PAR (molrn-Zd-I)

......

I

I

20

40

Fig. 1. Relative growth rates of (a) lettuce and (b) spinach under different levels of PAR obtained by using shade cloths. Symbols refer to experiments conducted from 8 August t o 8 September (A), from 19 November to 17 December (e), from 29 December to 26 Januaxy (o), from 23 March to 20 April (~), from 1 May to 29 May (D) and from 1 June to 29 June (X).

30(

l

I

120C

a)

'

'

I

i

b)

Lettuce

IOOC

'~: 20(

Spinoch

80C

C:m ° °

60C

~

tO0

4OO e

(..D

200 I

20

I

40 0 PAR (toolm-Zd-I)

°

•

"

,,,,'~ I

20

I

40

Fig. 2. The ground cover of a head of (a) lettuce or (b) spinach divided by the dry weight at Day 42 for plants grown under different levels of PAR. Symbols refer to experiments conducted from 29 December to 26 January (o, e), from 23 March to 20 April (A)and from I M a y t o 29 May (D, =).

236 a m u c h higher g r o u n d cover per u n i t w e i g h t t h a n l e t t u c e at all P A R levels (Fig. 2a,b). The p e r c e n t a g e d r y m a t t e r did n o t vary significantly as a funct i o n o f P A R or a m o n g e x p e r i m e n t s , averaging 7.1% for spinach a n d 3.8% for lettuce. T h e g r o w t h response t o P A R was curvilinear and t h e d a t a were a n a l y z e d using a r e c t a n g u l a r h y p e r b o l a as a g r o w t h m o d e l (Table II). T h e r 2 values were very high, e x c e e d i n g 0.96 f o r a n y individual e x p e r i m e n t . P o o l e d exp e r i m e n t a l d a t a p r o d u c e d l o w e r r 2 values t h a n individual e x p e r i m e n t s , s h o w ing t h a t there were significant differences a m o n g e x p e r i m e n t s . H o w e v e r , the r 2 values o f p o o l e d d a t a were still high, and indicated t h a t a p p r o x i m a t e l y 75% o f the variations in l e t t u c e g r o w t h rates and 86% o f the variations in spinach g r o w t h rates c o u l d be explained b y t h e d o u b l e r e c i p r o c a l e q u a t i o n s relating g r o w t h rates t o P A R at all times o f year. TABLE II RGR (g g-1 day-l) at saturating light levels (fl); PAR (mol m -2 day -1 ) to give halfmaximal RGR (IK); and PAR to give 95% of maximal RGR (Is)for lettuce and spinach. Values were calculated for each experiment separately and for pooled lettuce and spinach experiments. Coefficients of determination (r 2 ) were calculated for the equations giving the best fit of the data to a rectangular hyberbola Experiment No.

Plant

~

IK

Is

r2

1

Lettuce Spinach

0.344 0.568

8.25 24.21

165 484

0.982 0.969

2

Lettuce Spinach Lettuce Spinach Lettuce Spinach Lettuce Spinach

0.192 0:178 0.291 0.233 0.185 0.206 0.193 0.221

1.87 3.27 8.34 8.65 2.00 4.05 3.76 5.58

37 65 167 173 40 81 75 112

0.998 0.992 0.974 0.990 0.999 0.989 0.985 0.993

6 Lettuce Pooled Lettuce Pooled Spinach

0.200 0.220 0.238

3.75 4.10 6.79

75 82 136

0.981 0.747 0.862

3 4 5

N e i t h e r l e t t u c e n o r spinach actually achieved the t h e o r e t i c a l m a x i m u m g r o w t h rates (~) p r e d i c t e d b y t h e g r o w t h m o d e l in a n y individual experi m e n t , a l t h o u g h l e t t u c e a p p r o a c h e d this value in E x p e r i m e n t 6 ( c o m p a r e Tables I and II). P o o l e d d a t a p r e d i c t e d similar ~ values o f 0 . 2 2 0 g g-1 day-1 a n d 0 . 2 3 8 g g-1 day-1 f o r l e t t u c e and spinach, respectively. L e t t u c e g r o w t h rates a p p r o a c h e d or e x c e e d e d 0 . 2 2 0 g g-1 day-1 in E x p e r i m e n t s 1 a n d 6,

237 b u t the highest spinach growth rate, 0.194 g g-1 day-1, was 18% less than the ~ predicted f r om pooled spinach data. Half-saturation levels of PAR (IK) were determined from the slopes of double reciprocal equations. Spinach had a higher IK than lettuce in each experiment, indicating a less steep response to PAR in this t y p e of equation (Goldman, 1979). The difference between slopes of pool ed lettuce and spinach data was statistically significant (t = 2.01, d.f. = 72, *P< 0.05). IK values ranged f r om 1.87 to 8.34 mol m -2 day -1 in lettuce experiments and f r o m 3.27 to 24.21 mol m -2 day -~ in spinach experiments. Thus, in general, half-saturation was achieved at relatively low levels of PAR. E x tr ap o lated values of PAR t ha t p r o d u c e d 95% of ~ (Is) generally exceeded the a m o u n t of PAR available f r o m sunlight. These extrapolated values did n o t appear to be consistent with the experimental data. For example, lettuce growth rates were greater than 95% of the pool ed ~ in Experiments 1 and 6, under m uch lower PAR than expect ed from the pool ed Is (82 mol m -2 day -~ ). Spinach experiments gave Is values ranging from 65 to 484 mol m -2 day -I , even though Fig. l b shows t hat growth rates did not increase at PAR levels greater than 25 mol m -2 day -~ . Despite the significant differences among experiments, the only obvious seasonal effect was at t r i but e d to high temperatures during E x p e r i m e n t 1, which p r o d u c e d m uc h higher a values for both lettuce and spinach than the o t h er experiments. The IK and Is values for spinach, but n o t lettuce, were also higher in this e x p e r i m e n t than in the others. Photoperiods varied f r o m a low of 10.2 h day -1 ( E x p e r i m e n t 2) to 14.2 h day -1 ( E x p e r i m e n t 6), y e t this did n o t appear to be an i m p o r t a n t variable affecting growth rates. This was seen by comparing growth curves p r o d u c e d in different experiments (Fig. 1) with p h o t o p e r i o d (Table I); an equivalent a m o u n t o f PAR p r o d u c e d a p p r o x i m a t e l y the same R G R of lettuce or spinach in all experiments e x c e p t E x p e r i m e n t 1, regardless of p h o t o p e r i o d . DISCUSSION The major conclusion of this study is t hat lettuce and spinach, while capable o f growing at very low levels of PAR, are most productive under high PAR conditions. The growth rates, m arket quality and space utilization efficiency of b o t h plants decreased at low PAR levels. The growth analyses indicated t hat lettuce was m ore efficient in PAR utilization than spinach. L e t t u c e was able to utilize the highest PAR levels measured, 45 mol m -2 day -1 , but spinach was saturated by a p p r o x i m a t e l y 60% o f this a m o u n t . Unshaded lettuce plants had consistently higher RG R values than spinach plants, and IK values of spinach were higher than IK values o f lettuce. In addition, spinach required over 4 times as m u c h ground space to p r o d u c e an equivalent a m o u n t o f dry m a t t e r as lettuce. It should be n o t e d th at a greater ground cover per unit dry weight m ay be a positive ecological adaptation o f shade plants. It enables a plant to intercept the

238 greatest a m o u n t of light per unit of dry m at t er production. It is a negative feature in terms of greenhouse crop pr oduct i on, however, as it results in greater fixed expenses. It was f o u n d that the a m o u n t of PAR necessary to produce marketquality plants, 8 mol m -2 day -1 , was lower than the a m o u n t necessary to support half-maximal growth. Thus, it appeared t hat only rapidly growing plants develop the m o r p h o l o g y desirable in high-quality salad greens. The level o f PAR in the greenhouse in Tucson was always sufficient to produce marketable crops in unshaded treatments. On the other hand, in n o r t h e r n Europe, Great Britain and the northeastern U.S.A., winter PAR levels are f r e q u e n t l y less than 8 mol m -2 day -1 . In these regions, lettuce crops must be held under low greenhouse temperatures during dim weather to prevent damage to the crops from etiolation effects (Large, 1972). L e t t u c e and spinach growth curves bot h fit the model of a rectangular hyperbola, in which the response of R G R to PAR is approxi m at el y linear at low PAR and then approaches a plateau. Double reciprocal equations based on this model were able to explain greater than 96% o f the variation in R GR within experiments. However, the growth model was less successful in explaining variations among experiments. Warm temperatures increased growth rates at a given level of PAR. Other seasonal factors m ay also have operated to reduce the r 2 of pooled data com pared to individual experiments. Variations in p h o t o p e r i o d , however, did n o t appear to be an i m p o r t a n t d e t e r m i n a n t of growth rate at a given level of PAR. The growth model was also inaccurate in extrapolating growth rates at PAR levels higher than those within the experimental range (1--45 mol m -2 day -1 ). The model predicted very high Is values when, in fact, saturation was approached at much lower PAR levels. F o r t u n a t e l y , extrapolation errors will n o t seriously affect the usefulness of the present results, as the experimental data covered a wide range of PAR encompassing the entire range that can be e x p ected within a greenhouse environment. The results with lettuce s uppor t the conclusion of a previous study at this location which r e p o r t e d a high statistical correlation between yield and solar radiation (Glenn, 1984), T h a t study also f o u n d t hat day temperatures as high as 30°C accelerated growth, a conclusion that is also supported by the present experiments. The results are in contrast to field studies t hat show the best growth of lettuce to be under shade cloth, with significant inhibition of growth by full sunlight (Mattei, 1967; Mattei et al., 1973). There were no detrimental effects of high PAR on lettuce or spinach in the present study, although long p h o t o p e r i o d s c a u s e d the prem at ure flowering of spinach. Therefore, desert regions such as Tucson would appear to be well suited for greenhouse p r o d u c t i o n of these cultivars. Yields should be m uch greater than f r o m traditional n o r t h e r n greenhouses considering the greater availability o f PAR.

239 REFERENCES Glenn, E.P., 1984. Seasonal effects of radiation and temperature on growth of greenhouse lettuce in a high insolation desert environment. Scientia Hortic., 22: 9--21. Goldman, J.C., 1979. Outdoor algal mass cultures -- II. Photosynthetic yield limitations. Water Res., 13: 119--136. Huyskes, J.A., 1971. The importance of photoperiodic response for the breeding of glasshouse spinach. Euphytica, 20: 371--379. Jensen, M. and Hicks, N., 1973. Exciting future for sand culture. Am. Veg. Grower (November): 33--35. Large, R., 1972. Glasshouse Lettuce Grower Manual. Grower Books, London, 181 pp. Mattei, F., 1967. The effect of shading on lettuces (Lactuca sativa L.). Riv. Ortoflorofruttic. Ital., 51: 206--215. Mattei, F., Sebastiani, A.L. and Gibbon, D., 1973. The effect of radiant energy on growth of Lactuca sativa L. J. Hortic. Sci., 48: 311--313. Noguchi, M., Kikkawa, M., Hoshino, K., Ikeda, S. and Kobayashi, K., 1978. Analysis of factors determining vegetable crop yield. II. The effect of solar radiation on the growth and dry matter production of lettuce. Bull. Veg. Ornamental Crops Res. Stn., A. Ishinden-Ogoso, TSU No. 4, pp. 55--76. Steel, R.G. and Torrie, J.H., 1962. Principles and Procedures of Statistics. McGraw-Hill, New York, 481 pp. Wittwer, S.H. and Honma, S., 1979. Greenhouse Tomatoes, Lettuce and Cucumbers. Michigan State University Press, Lansing, 225 pp.