Temperature and Biological Development

2.6 Temperature and Biological Development

Now that we have some idea of the behavior of temperature in the natural environment of living organisms, we want to consider temperature from

a biological perspective. For this analysis, we assume that temperatures of plants, microbes, and insects are the same as the temperature of their environment. We need to remember, however, that such is generally not the case. Later we develop the tools to compute organism temperature from environmental temperature and can then consider what effect this will have on the organism.

Temperature strongly influences the rates of all metabolic processes in living organisms, and therefore affects almost all aspects of the growth and development of an organism. Here we want to consider the effect of temperature on the rate of development. We define development as the orderly progress of an organism through defined stages from germination to death. Development differs from growth, which we define as the ac- cumulation of dry matter. Developmental stages vary, depending on the organism being described. In plants, stages such as germination, emer- gence, leaf appearance, flowering, and maturity can be defined, as can intermediate stages within many of these stages. In insects, stages such

as egg, larva, and adult can be identified, and with other living organisms developmental stages can be similarly identified and defined.

Figure 2.6 shows the time taken for completion of the egg stage of Dacus cucurbitae at constant temperatures ranging from 10" to

C. Development time is short at temperatures between

and 30" C, but increases markedly at both higher and lower temperatures. Above

C and below 15" C, development times are very long. We are interested in determining the time taken for completion of the egg stage (or some other developmental stage) under varying temperature conditions. This can be found by computing the reciprocals of the times in Fig. 2.6 to obtain a rate of development. Figure 2.7 shows the rate of development (with units of completed stages per day) as a function of temperature. The shape of this curve is similar for many biological processes, and has been described mathematically using the theory of rate processes (Sharpe and 1977; Wagner, et al. 1984). Such detailed models are written in

of three exponentials, and are therefore difficult to both fit and compute. It is evident, however, that the data in Fig. 2.7 are closely approximated by two straight lines. Again, this is typical of many biological responses to temperature.

Descriptions of the rate of development, such as Fig. 2.7, are the basis for determining the time taken to complete a developmental process when temperature varies. Assume, for example, that one has measurements of soil temperature, and wishes to predict the time required to complete the

Temperature and Biological Development

20 30 Temperature (C)

2.6. Time for development of melon fly (Dacus cucurbitae) eggs at different temperatures.

F IGURE

20 30 Temperature (C)

2.7. Development rate of melon fly eggs showing the almost linear response to temperature.

F IGURE

Temperature

germination stage of a seed. A period of time, usually an hour or a day, is chosen and the average temperature over that time period is determined. The average temperature determines the rate of germination for that time period (Fig. 2.7) and this rate is multiplied by the time period, giving the amount of development which has occurred. The total development is the sum of the products of rate and time for each time period. The total time taken to complete a developmental stage is the time required for the sum of the development increments to reach unity. This is similar to the problem in physics where we are interested in determining the distance traveled by an object which moves at varying speed. There we would write

where is the time-varying rate or speed, and is the total distance traveled. In this analogy, is like the development stage, and the development rate, which is temperature-dependent and may therefore vary in some arbitrary way with time. Since the functional form for the rate is generally not known for development calculations (except in the trivial case where temperature is constant) we approximate the integral with a summation of the products of rate and a finite time increment.

Example 2.5. Suppose the daily mean temperature is

C on day 1, 20" C on day 2 and 25" C on day 3. Using Fig. 2.7, determine how long it would take to complete the egg stage of

cucurbitae.

Solution. The rates for days and 3, estimated from Fig. 2.7, are 0.3,

After two days, 0.3 + 0.6 = 0.9 stages would be

0.6, and 0.8

0.1 days. The total time would therefore be 2.1 days.

complete. The remaining 0.1 stage would take