Ann. appl. Biol. (r966), 57, 345-353 Printed in Great Britain

345

The influence of increased C02 concentration and supplementary illumination on growth of tomato

seedlings during the winter months

BY PETER NEWTON Department of Botany, University of Manchester

(Received 4 February 1966)

S U M M A R Y

During the period November to March a threefold increase in CO, concentration had only a small effect on the growth rate of tomato seedlings, variety Eurocross B. Although net assimilation rates were increased, some inhibitory effects of increased CO, concentration on leaf growth were found when the seedlings were very small. The increase in dry weight was equi- valent to that made in a few days by plants grown with naturally occurring amounts of CO,. There was no increase in the rate of flower initiation. Using supplementary illumination for 17 hr. per day with high-pressure mercury vapour lamps made it possible to produce in November-December seed- lings similar to those grown during March-April with natural illumination.

I N T R O D U C T I O N

In a previous paper (Newton, 1965) the influence of CO, concentration on growth of cucumber seedlings during periods of high solar radiation was described. Further data have been obtained on seedling growth at two levels of CO, when amounts of solar radiation were low, and are presented below. For these experiments the tomato was chosen as the experimental plant because of its greater economic importance. The technique of growth analysis was used to provide information about the following: (I) whether increased growth rates were obtained and if so the parts played by photo- synthetic capacity and efficiency in determining the increase; (2) whether there was a significant effect on rate of flower production; (3) the effect of increased concentration of CO, compared with that of supplementary illumination ; and (4) the combined effect of increased CO, concentration and supplementary illumination. Pure gaseous CO, was used because when hydrocarbons have been burned to produce CO, in glasshouses, damage to plants has occurred which has been attributed to the production of SO, from impurities (Stender, 1951), and the lack of response of illuminated tomato seedlings to increased CO, (Marshall, 1964) may possibly be attributed to other gases produced when propane is burned.

METHODS

Seeds of tomato variety Eurocross B were sown in John Innes potting compost no. I on 31 October, 3 December 1964; 4, 20, 24, 31 January, 4, 1 3 and 19 February, 1965, in a partitioned glasshouse (cf. Marshall, 1964, and Newton, 1965, for details). The air temperature was 60" F. day and night from 31 October to 17 January. From

23 App. Biol. 57

346 PETER NEWTON 18 January onwards the day temperature was increased to 68" F., to determine whether increased temperature improved the effectiveness of the increased CO, concentration. The benches on which the plants were grown were covered with 34 in, of sand, which was maintained at a minimum temperature of 70' F. by electrical heating cables. Ten days after sowing seedlings were transferred into 34 in. clay pots containing John Innes potting compost no. 2. Initially the plant pots were placed touching each other on the benches, but were separated as the plants grew. Half the seedlings were grown with naturally occurring amounts of CO, (untreated seedlings), the other half at a concentration of approximately 1000 p.p.m. CO, (treated seedlings). The CO, supply was turned on at 7.30 a.m. each day, and off at dusk; concentrations in each half of the glasshouse were periodically checked.

Half the seedlings of the first sowing received supplementary illumination from 1 1 November. The 400 W. mercury-vapour lights (G.E.C. type MA/V) were sus- pended approximately 3 ft. above the bench 4 ft. apart and were switched on and off, automatically at 3 a.m. and 8 p.m. respectively. The plant pots under the lights were rearranged on the benches at intervals in an attempt to overcome the variation in growth due to the uneven distribution of the artificial radiation.

At a convenient time after emergence, and subsequently at weekly intervals, ten seedlings from each treatment were removed for dry-weight determination. Leaf areas were measured with either an Eel small area meter or Eel leaf-area meter. Leaf primordia were counted using a low-power binocular microscope. If inflorescence primordia were present their size was assessed on a scale from 1-10, low numbers for those newly initiated, the highest value for inflorescences with at least one flower primordium with its calyx parts separated. Net assimilation rates were calculated in the conventional way (cf. Newton, 1965).

RESULTS

The dry weights of treated seedlings were, with three exceptions, greater than those of untreated seedlings (Fig. I). The exceptions were: ( I ) that treated seedlings grown with supplementary illumination were always lighter in weight than untreated although the difference was not significant; (2) seedlings from the 4 January and (3) the 4 Feb- ruary sowings, where dry weights of untreated seedlings were initially greater than those of treated seedlings. After approximately 5 weeks from sowing the order was reversed in 2 and 3. When the dry weights of treated seedlings were greater than untreated, the assimilates had not been equally distributed between roots, stems and leaves, the overall pattern being that more was translocated to root than stem tissue, but the greater proportion remained in the laminae (Table I).

Total leaf areas of treated seedlings were generally greater than those of untreated seedlings, but there were some exceptions. Leaf areas of initial samples of treated seedlings of four sowings were smaller than untreated, but with time treated became larger than untreated (Fig. 2). The four sowings were 3 December, 4 and 3 I January and 4 February. The total leaf areas of untreated seedlings that received supple- mentary illumination were, however, greater than treated seedlings when each sample was taken. The inhibitory effect of CO, had occurred before the first sample was taken, presumably when cotyledons and the first true leaves were expanding.

CO, and light effects on winter growth of tomato seedlings 347 The effect on growth rates of increasing day temperature from 60" to 68" F. was

negligible, for during two periods with similar light conditions, i.e. November- December (60" F. for seedlings sown on 31 October) and January-February (68" F. for seedlings sown 3 December) growth rates were similar. Dry weights and leaf areas were increased far more by artificially increasing daily radiation than by the increase in CO, concentration or day temperature. This increase in weight and area was largely due to higher relative growth rates during the period from seed sowing to the first

Time from sowing (days) Fig. I . Changes in log, dry weight as influenced by sowing date. Data for some sowings omitted for the sake of clarity. Solid lines treated, broken lines untreated plants. 0 , 31 Oct.; 0, 31 Oct. (plus artificial light); 0, 3 Dec.; ., 4 Jan.; A, 20 Jan.; A, 19 Feb. Significant differences (P = 0.05) between means are indicated by vertical lines.

sample, i.e. when cotyledons and the first true leaves were expanding (Fig. 3). Without supplementary illumination relative growth rates up to the first sample were small and unaffected by sowing date from 3 I October to 3 I January, but they increased with progressively later sowing dates after 3 I January. Relative growth rates during the period between the first and final samples increased progressively with sowing date to values greater than all those of plants that received supplementary illumination.

This suggests that higher growth rates might be obtained during expansion of cotyledons and first true leaves if larger amounts of artificial illumination were used.

Differences in total leaf area between treated and untreated seedlings were not due to differences in the number of expanded leaves. Seedlings with the greater leaf area

23-2

348 PETER NEWTON

Table I. Percentage increase in dry weights, due to CO, treatment approximately 6 weeks after sowing, of roots (R), stems ( S ) and laminae (L) , as injuenced by sawing date

Sowing date Number of days

31 October 47 R IOO s 58 L I79

, 3 December 45 R 4 5 s I8 L 21

4 January 43 R 119 s 51 L 55

20 January 39 R 57 s 59 L 61

13 February 44 R 34 S 18 L 4 4

19 February 44 R 49 s 75

Means L 127

R 67 s 47 L 81

1 I 1 1 I- .30 40 50 60 70

Time from sowing (days)

Fig. 2. Changes in log, total leaf area per plant as influenced by sowing date. Symbols as for Fig. I.

CO, and light effects on winter growth of tomato seedlings 349 had either a similar number of leaves or fewer leaves than those with the smaller leaf area (Table 2). The larger total leaf areas of seedlings from the 31 October sowing, for example, were due to larger individual leaves (Fig. 4). This could be attributed to a higher growth rate before the leaves reached measurable size, growth rates after emergence being similar. A similar effect was found with leaves of cucumber seedlings (Newton, 1965).

The smaller individual leaves of treated compared with untreated seedlings re- ceiving supplementary light could also be attributed to differences in leaf growth rates

0.200 t h

i? 0,150

M > M v

0 Y m h

$ .* Y

2 0'05

-&--0-* > _.- - 70- - - O O

D )'= 0.0703 i- 0.0001 08

0 L J I I I 1 November- 1 December 1 January 1 February

Fig. 3. Relative growth rates over ( I ) the 3-4 week period from seed sowing to the first sample (seed weight used for WJ, broken lines and circles, and (2) the 2-3 week period from the first to the final sample, solid line and squares. Treated plants closed symbols, untreated open symbols. The data for plants receiving supplementary illumination (triangles) were not included when the regressions were calculated ; the two largest values (inverted triangles) were obtained during period (I).

Table 2. Number of expanded leaves and total leaf area per plant when leaves of un- treated plants were larger than those of treated plants

Sowing date Number of leaves

3 December Treated 4'4 Untreated 4'5

4 January Treated I '7

31 January Treated 3'2

4 February Treated 2'0

L.S.D. (p = 0.05) 0.49

Untreated 1.6 L.S.D. 1.15

Untreated 2.8 L.S.D. 0.39

Untreated 2'0 L.S.D. 0.50

Area (an.%)

37.6 43'4

3'52 4'4 443 1.15

21.3 33'9 1.88 8.5

2'73 11'2

3 50 PETER NEWTON before the leaves emerged, i.e. lower rates were here associated with increased CO, concentration. Further experiments are needed before an explanation of this obser- vation can be attempted.

It is not possible to account for the inhibitory effect of CO, on leaf growth that occurred with the sowings of 3 I January and 4 February, because there are insufficient data. It is of interest to note that no effect was found for seedlings sown on 20 January and 24 and 13 February, whose leaves were also expanding during the same period. Obviously further experiments are needed.

Leaf 2

u-u 1 2 3 1 2 3 1 2 3

Sample number

Fig. 4. Changes in log, leaf area of the znd, 3rd and 4th leaves of seedlings sown on 31 October. Seedlings with and without supplementary illumination, open and closed circles respectively. Treated seedlings solid lines, untreated seedlings broken lines.

One reason for lack of difference between total leaf areas of treated and untreated plants was that the cotyledons of the former usually died before those of the latter. This applied to seedlings of the final samples of the 3 December, 20, 24 January and 4 February sowings. Lack of nutrients in the plant pots was unlikely because coty- ledons of all seedlings sown on 31 October, the plants of which reached a far greater size than all the other sowings, were still alive when the final sample was taken. I t is possible that senescence was accelerated by the interaction of high CO, concentration and higher day temperature used for the sowings made after 31 October.

CO, and light eflects on winter growth of tomato seedlings 351 Although total leaf areas of treated seedlings were often significantly less than those

of untreated seedlings, differences in dry weights were less marked because increased CO, concentrations had depressed leaf growth but not net assimilation rates. (Table 3). Over each sample interval for each sowing, net assimilation rates of treated seed-

Table 3. Mean net assimilation rates (g./rn.,/day) of treated and untreated seedlings over the sampling period as influenced by sowing date

Sowing date

31 October

3 December

4 January

20 January

24 January

31 January

4 February

13 February

19 February

*Treated 'Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated Treated Untreated

* Received supplementary illumination.

5 4 8 4.81 1.89 1.61 2438 2'34 2'92 2.65 4 3 0 3'54 3'4'1 3.18 5.61 2'94 5'03

431 3'94 6.14 5.14

4.98

Table 4. Number of expanded and primordial leaves present, and the stage of develop- ment of inflorescences when the first sample was taken, for various sowings

Sowing date

31 October "Treated 'Untreated Treated Untreated

3 December Treated Untreated

4 January Treated Untreated

4 February Treated Untreated

19 February Treated Untreated

No. leaves No. leaves Stage of between

before development first and first of first second

inflorescence inflorescence inflorescence

11.5 2'3 2.3 I 1.4 2.7 3 '0 92 N o flower pri- -

mordia visible - 8-2 10'0 N o flower pri- - 9.3 mordia visible -

11.6 0 8 11'0 0.7 10'2 4'8 2'0

10.3 4' I 0.8 8.9 7'0 3'0 8.9 5'8 2' I

- -

* Received supplementary iilumination.

Stage of develop- ment of second inflor-

e s c e n c e

0.6 1'2 - - - - - - - __

1.0 -

No. days from

sowing

34

34

46

37

33

30

3 52 PETER NEWTON lings were greater than untreated. Values were low, as expected, for plants grown during the winter months, and the increase due to increased CO, concentration was small compared with the increase due to supplementary illumination.

Higher net assimilation rates of cucumber during the summer months due to in- creased CO, concentrations led to both increased production of leaf primordia and increased growth rates of leaf primordia (Newton, 1965). I t seemed probable there- fore that increased net assimilation rates of tomato seedlings would be associated with similar increases in leaf production and, also, flower production. Primordia growth rates were increased (Fig. 4), but rates of leaf production only fractionally increased (Table 4). Sometimes treated pIants had more leaf primordia but the first trusses were less well developed than those of untreated plants.

DISCUSSION

The most striking result was the effect of supplementary illumination on both rate of seedling growth and production of flowers. The high rate of dry-weight increase of these seedlings could be attributed to greater net assimilation rates and in particular to greater photosynthetic capacity. Net assimilation rates of untreated seedlings that received supplementary illumination were three times greater than the rates of those receiving natural daylight only, and leaf areas were approximately ten times greater during the period 3 to 17 December. These data show that supplementary light from high-pressure mercury vapour lamps was more important for promoting leaf growth than for increasing photosynthesis of tomatoes, and indicates that high-intensity illumination from other artificial light sources will be of greatest value when its effect on leaf growth is greater than that on net assimilation rates.

Supplementary illumination made it possible to produce during November and December plants of a similar dry weight and leaf area to those grown during March and April with natural illumination. The plants grown with supplementary illumina- tion had, however, more leaves before the first inflorescence, and the inflorescence was less well developed 30 days after seed sowing than the plants grown with natural illumination. These differences might have been due to the difference in daylengths and spectral composition of the light received by the seedlings.

Increasing CO, concentration for several weeks increased dry weights, by increasing net assimilation rates and leaf areas, only to an amount equivalent to a few days’ growth of seedlings without CO, enrichment. There was an even smaller effect on flower production, which suggests that when total daily radiation is low the competitive ability of the apical region is lower than that of the remainder of the plant, up to the appearance of the first truss.

Compared with supplementary illumination, increased CO, concentration was of little value in increasing growth rates during the winter months. Further experiments are needed to establish under what combination of environmental conditions it could be of value at this time of year. The interaction of increased concentration and supplementary illumination obviously needs thorough investigation.

Increased CO, concentration gave heavier leaf dry weights, with smaller effects on root and stem weights, i.e. not as much carbohydrate was translocated as might be

CO, and &ht eflects on winter growth of tomato seedlings 353 expected. It seems likely, although experiments are needed to verify the hypothesis, that CO, enrichment of tomato plants will be most valuable when high proportions of assimilate are translocated from leaves, e.g. when large fruits are present, for Loomis (1953) has stated that the competitive ability of developing fruit, vegetative buds and flowers, and freshly pollinated fruits decreases in that order. It is also possible that increased yields of fruit obtained when atmospheric COe concentrations are in- creased are partly due to increased rates of photosynthesis in deveIoping fruit.

There was no advantage in using the higher day temperature of 68" F. Death of cotyledons of treated seedlings could be linked with this temperature. It is possible that flower production was not promoted by increased CO, concentration because death of cotyledons was accelerated, cf. Calvert (1959), who found that flowering was delayed if effective cotyledon area was reduced.

Acknowledgements are made to R. Marshall, North Western Electricity Board, Kendal, for raising the seedlings and valuable discussion during the preparation of this paper, and to D. Cranston, Miss A. Webber and Miss J. Pollard for technical assistance.

R E F E R E N C E S

CALVERT, A. (1959). Effect of early environment on the development of flowering in tomato.

LOOMIS, W. E. (1953). Growth and diflerentiution in plants. Iowa State Coll. Press. MARSHALL, R. (1964). C 0 2 does not help tomato propagation. Grower, 61, 812-15. NEWTON, P. (1965). Growth of Cucumis sutivus, variety Butcher's Disease Resister, with two

STENDER, J. A. (1951). Carbon dioxide concentration in an air-proof glasshouse. 3. Inst. Tuinb.

11. Light and temperature interactions. J. hort. Sci. 34, 154-62.

concentrations of carbon dioxide. Ann. uppl. Biol. 56, 55-64.

Tech. Wugeningen, pp. 25-6. (Hort. Abst. 33, no. 684).