1954of Pacific Coast Geographers55 THE PHOTOSYNTHESIS POTENTIAL OF THE ENERGY COMPONENTS OF CLIMATE Robert W. Pease University of California, Los Angeles There is need for quantitative measurement of the energy components of climate, especially as they apply to plant growth. Precipitation is measured as a quantity, but temperature gets only numerical description, and light is usually ignored. Quantitative measure of these components would seem as desirable as for precipitation, and a method for making it should be a useful addition to the techniques of climatic study. The following is a preliminary report of thought in this direction. Several systems for relating temperature to plant growth have been devised . They have not received widespread use, however, for they do not achieve good correlation with nature and are awkward to use. In general: 1.they fail to recognize all cardinal points of plant temperature response, 2.they assume all plants have the same temperature response, 3.they do not consider daily range of temperature which varies results significantly, 4.last, but not least, they do not consider the light factor, and thus are only partially rational with nature. A plant does not directly respond to energy as a "quantity", and thus usual direct energy measures, such as the gram-calories, will not suffice. Instead, it responds to energy "intensity" and the rate of its reaction is in accordance with illumination and temperature. We are here concerned with measuring the "effect" of energy, rather than energy itself, and a unit of measurement would involve the "rate of response" over a time period. It might well be an hour of maximum response, or its equivalent in hours of partial response. The simplest but least accurate application of this approach considers temperature response only, but, unlike earlier methods, takes into consideration the daily march of temperature. If growth at the optimum temperature is taken as unity, and if the fractional response at other temperatures is known, the growth value of hourly temperatures of any daily march can accumulate into the "equivalent hours of maximum growth" for that day. Temperature response curves for a number of crop plants have been derived experimentally. The most used is Lehenbauer's curve for corn seedlings.(3) With such physiologic curves and with appropriate climatic data, monthly and yearly growth potentials for any site can be accumulated in the above temperature-growth response terms. Temperature-growth summations fit nature's reality only moderately well, however, for they omit the light factor. They will not show, for example, the 56Yearbook of The AssociationVol. 16 rapid growth which occurs in the long days of high latitude summer, or the inhibiting effect of excessive cloudiness. Further, growth is primarily concerned with the vegetative phase of a plant's life cycle and may be only indirectly related to crop yield. The validity of its use hinges upon the fact that in its terminal position in the physiologic sequence growth summarizes environmental forces acting on the plant. Growth cannot occur, however, when food is not available ; so would it not be more valid to apply climatic controls to the food-making process itself, that is, to photosynthesis, This makes possible the consideration of the light factor and, further, the inhibiting effect of plant respiration. In fact, carbon dioxide fertilization experiments demonstrate that photosynthesis correlates closely with crop yields. To understand better how this can be accomplished, let us review briefly this food-making process. Water absorbed by roots is translocated to leaves where, in intercellular spaces, it mixes with carbon dioxide gas from the air. The carbonic acid thus formed osmotically enters specialized leaf cells where, in the presence of light, it is converted into the plant food, sugar. The complete process consists of several sub-reactions. One of these requires light, is photo-chemical in character, and is termed the "light reaction". The others are strictly chemical in character and are termed "dark reactions". Essentially , the light reaction responds to light intensity, but not to temperature, while the dark reactions are sensitive to temperature but not to light. The energy components of climate directly control the rate of the process by their respective influences on these sub- reactions. The carbon dioxide enters the intercellular...