PLANT GROWTH UNDER CONTROLLED CONDITIONS. V. THE RELATION BETWEEN AGE, LIGHT, VARIETY AND THERMOPERIODICITY OF TOMATOES

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 has 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 220 in the incubator were intermediate between the higher and lower temperatures. 1 Reeeived for nPblieation Mav 28. 1945. 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 17180C. 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 180 during the day. In each of these temperature conditions one group consisted of plants 250-280 mm. long and grown in onegallon 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 1 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 260C., 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