Amer. Jour. Bot. 32(8): 469-479, 1945
Plant growth under controlled conditions. V. The relation between age, light, variety and thermoperiodicity of Tomatoes.
F. W. Went

IN AN earlier paper (Went, 1944a) the conclusion was reached that tomato growth responded more to temperature than to humidity of the air, length of illumination, light intensity, or nutrition. Slight differences in night temperature caused considerable difference in rate of stem elongation or in fruit set. This work bas been continued, and in the present paper a closer analysis of the temperature effect is given, and the temperature response of a number of tomato varieties is described. In the next paper of the series (Went and Cosper, 1945) these data will be compared with the development of these same varieties under practical conditions in the field.

In all the following experiments, except where the contrary is stated, tomato seeds were germinated in sand in an ordinary greenhouse, and the seedlings were watered with nutrient solution. After the first foliage leaves had fully developed, the plants were transferred to one-gallon crocks filled with crushed rock, and they were left outside in a lath house for about two weeks. Then the temperature treatments were started.

More different night temperatures could be tested simultaneously than in the previous experiments, since a few incubators were built into the dark rooms, which were kept at 4° C. above the darkroom temperature. They were not quite satisfactory and the rates of stem elongation did not quite reach those expected. In total weight and fruit weight, however, the values at 22° in the incubator were intermediate between the higher and lower temperatures.

RELATION BETWEEN AGE, TEMPERATURE AND STEM GROWTH RATE.—In a previous article (Went, 1944c) it was concluded that the decrease in growth rate of full-grown tomato plants (variety San Jose Canner) when subjected to night temperatures above 17-18° C. was due to insufficient translocation of sugars from the leaves at these higher temperatures. It also was pointed out that the higher optimum night temperature encountered in seedlings could be explained by their small size so that translocation was less critical than in large plants. If this were actually true, one would expect a gradual lowering of the optimal night temperature as the tomato plant grew, and this was tested.

A group of tomatoes (San Jose Canner) were divided into four groups: two were grown at 26°C. during the day and two at 18° during the day. In each of these temperature conditions one group consisted of plants 250-280 mm. long and grown in one-gallon crocks in crushed rock, and the other group were 60-90 mm. tall, grown in four-inch pots in sand. All were watered twice daily with nutrient solution. One-sixth of each of these groups were placed at six different temperatures each night from 16 :00 to 8:00. Measurements of stem length were taken every two days. Tables I and 2 and figure 1 show the results of these measurements. From table 1 it appears that the steady growth rate at a constant day and night temperature of 26°C., after an initial spurt to 29 mm./day, has settled down to 24.7 mm./day in the period May 1-15. This rate is slightly above the average rate of 23.1 as found in earlier experiments (table 3 in Went, 1944a). The stem growth rates of the other groups over the same 14-day period come remarkably close to the growth rates as expected from the curves in figure 4 from Went (1944a ) as seen in table 2 with the exception of the 22° values, which were too low due to the incubator. This proves again the reproducibility of results obtained in the air-conditioned greenhouses, and it brands the experiment of tables 1 and 2 as thoroughly typical.



TEMPERATURE AND DISEASE INCIDENCE.—As a result of growing tomatoes without interruption in the greenhouse and in the adjoining field for a period of several years, virus diseases gradually became established in the plants, and many of the plants referred to in table 4 were diseased, which is the reason why the experiment was discontinued before the same stage of growth was reached as that shown in table 3. Some marked differences in virus symptoms were observed. When subjected to a day temperature of 18°C., shoestring virus was common, but at 26.5°C. day temperature it did not occur. No apparent connection with night temperature was observed. At the higher day temperature mosaic was slightly more prevalent, but the latter disease was strongly correlated with high night temperatures. At 13° night temperatures no mosaic symptoms were visible; at 16° they were hardly discernible, at 22° and 26° the diseased plants were very apparent, and at 30° night temperature the plants having mosaic showed systemic effects and were dwarfed. These differences persisted in spite of the fact that diseased and healthy plants were in close contact during day or during night. Spotted wilt developed only at high night temperatures.




The thermoperiodicity of tomato plants was studied in detail, considering interrelations between age of plant, light intensity and variety on the one hand, and stem elongation at six night and two day temperatures on the other hand.

It was found that a gradual shift of the optimal night temperature occurred, from 30°C. in small plants to 18°C. for the San Jose Canner and 13°C. for the Illinois T19 in the early fruiting stage.

A similar response was found in 14 other tomato varieties, but they each had slightly different temperature characteristics. In general the English and Greenhouse varieties grew fastest and had the lowest optimal night temperatures. Western varieties had the highest optimal night temperatures, and Eastern varieties were intermediate between the other two as far as night temperature was concerned, but had the lowest absolute growth rates.

When the tomato plants were grown in full sunlight, their optimal night temperature was higher than on cloudy days, provided they were shaded by other plants. In artificial light the optimal night temperature fell off very rapidly with decreasing total illumination. Incidence of virus diseases was greatly modified by both day and night temperature.

In the discussion it is pointed out that these complex interrelationships are examples of a multidimensional causality, which can be presented properly only in multidimensional models.