During recent years sophisticated techniques are applied in the glasshouse industry for the control of the glasshouse climate. Along with that development, extensive research programs were carried out to establish optimum conditions for growth. Air temperature, radiation, CO 2 -concentration and humidity of the air were the most important factors studied. Relatively little is known about optimum conditions in the root environment. Although some reports are available on the effect of root temperature on growth of tomato plants, the results have only limited applicability and were often contradictory. Therefore, the effect of root temperature on growth of young tomato plants was studied, with two objectives:a. to quantify the effect of root temperature on growth of young tomato plants in order to establish the profitability of root temperature control techniques in practice, andb. to understand the physiological background of the observed effects. | Tomato plants were raised at root temperatures of 12, 15, 20, 25, 30 and 35°C in a glasshouse under natural radiation conditions throughout the year. Air temperature ranged from 17°C in winter to 30°C in summer by day and from 15°C in winter to 20°C in summer by night. Data on plant height, number of leaves, fresh and dry weight of leaves, petioles and stems and on leaf area were recorded periodically during each experiment. | The effect of season on growth was much larger than the effect of root temperature. At root temperatures below 20°C growth was reduced irrespective of the season; above 30°C growth was-reduced during the summer only. An apparent interaction between season and low root temperature could be ascribed to the fact that plants, although of the same age, were at different stages of growth after some time of treatment. | Growth analysis showed, that the reduced growth rate at low root temperature was mainly caused by a decrease of the Specific Leaf Area (SLA). Net Assimilation Rate (NAR) was not affected by root temperature. Daily measurements of leaf length revealed, that especially leaf expansion rate was reduced by low root temperatures, this reduction was not correlated with incoming radiation or evaporation in the glasshouse. | The after-effect of root temperature during raising on subsequent growth, development and yield was studied in three experiments in which plants were raised at either 12, 25 or 35°C root temperature until flowering. After transplanting the plants into a glasshouse normal cultural practices were applied. The first experiment started in very early winter (sowing in September), the second one was a normal early crop (sowing in November) while the third one was a rather late crop (sowing in January). Besides the after-effect of root temperature, the influence of the leaf area per plant was studied by partial defoliation. | The first experiment was too early for normal fruit set and almost no fruits were produced in any of the treatments. Raising the plants at a low root temperature did not adversely affect the yield in the second experiment, but reduced total yield by approximately 10% in the third one. This reduction of yield was caused by a decrease in the number of fruits. Halving the leaf area at transplanting reduced fruit set in January but was without effect later on in the season. Continuous removal of every second leaf accelerated the development during the first weeks but later on weak plants with a much smaller yield were obtained. | The relative effect on air and root temperature was studied under controlled conditions. Leaf growth rate by day and night was measured separately with various combinations of air and root temperatures by day or by night. After 7 days of treatment, Leaf Weight Ratio (LWR) and SLA were determined as well. Air temperature by day was by far the most important factor, followed by air temperature during the night. Leaf growth rate was slightly reduced when root temperature was low during part of the day only. No difference between the effect of root temperature by day and that by night was observed. Growth was reduced more than additional at continuously low root temperatures. | Since the effect of root temperature on growth was independant of season and of time-of-day, the most common hypothesis, that the growth reduction at low root temperatures is due to a reduced rate of water uptake, was doubted. Therefore, some experiments were done in which the relation between water balance, root temperature and leaf growth were studied. One of the results was, that both water stress and a low root temperature decreased leaf growth rate, but this decrease was not accompanied by a decrease in SLA at drought, whereas it was reduced at a low root temperature. These doubts on the primary rôle of the water balance in the root. temperature response of tomato plants was a reason for an investigation into the possible involvement of phytohormones. | Application of phytohormones in foliar sprays on plants at low or optimum root temperatures showed, that complicated interactions exist between these factors. In some cases the growth reduction due to a low root temperature could be partly compensated by addition of gibberellines and cytokinins, but the results were too variable for definite conclusions. | Finally, it may be concluded, that root temperature is not an important factor in the practice of glasshouse tomato growing in the Netherlands. A detailed study into the hormonal balance of tomato plants will be useful for a better understanding of the growth process.