A mathematical model of the processes involved in carbon metabolism is described that predicts the influence of temperature on the growth of plants. The model assumes that the rate of production of dry matter depends both on the temperature and the level of nonstructural carbohydrate. The level of nonstructural carbohydrate is determined by the rates of photosynthesis, growth, and maintenance respiration. The model describes the rate of growth and dark respiration, and the levels of carbohydrate seen in vegetative growth of carnation and tomato. The model suggests that the growth of plants at low temperatures is limited by a shortage of respiratory energy, whereas at high temperatures growth is limited by the shortage ofcarbohydrate. Thermoperiodism, wherein a warm day and cool night results in faster growth than does constant temperature, is explained by the model as an increase in the level of nonstructural carbohydrate which promotes the rate of growth relative to the rate of maintenance respiration.Although temperature has a major influence on plant growth, attempts to predict the response are confounded by several phenomena. The heat sum or growing degree day concept (26) predicts the development of a crop growing in the field solely from the accumulation of heat units above a base temperature. A more complex temperature response of growth is seen under controlled, constant-temperature conditions. At 5 to 20°C, the rate of growth increases exponentially; at 20 to 30°C, it levels off; and above 30 to 35°C, the growth rate falls (9,19,27). This has been ascribed to inactivation of an enzyme crucial to growth metabolism both at high and low temperatures which modifies the usual exponential temperature dependence of an enzyme reaction rate (21). Neither model explains the nonadditive effect of fluctuating diurnal temperatures (27) or the influence of light and CO2 (4,9,19) on the growth response.At a constant temperature, the rate of plant growth is linearly related to the rate of photosynthesis (8,13,25). However, the temperature dependence of growth and photosynthesis is not the same. Photosynthesis increases with temperature in an asymptotic manner to a plateau above 15°C (5, 10), while the growth rate increases exponentially in this interval and falls rapidly at temperatures above 25°C (9,19,27). This divergence occurs because only some of the carbohydrate is used to promote growth and the rest is used to maintain the plant in the current state (15,18,20 experimentally from respiration due to growth by plotting the total dark respiration versus the growth rate (13, 23). Growth respiration and maintenance respiration increase exponentially up to 20°C (13, 18). Whereas growth starts to decrease above 25°C, maintenance keeps increasing. This concept accounts for the carbohydrate mass balance of plant metabolism; but, more information is required to explain why maintenance is promoted at higher temperatures and growth is not.In this report, Okhams razor is applied to derive the simplest model that predi...