crease with warmer temperatures (Figure 1) [32]. Growth rates of other cultivars are predicted to be
nonexistent or very low at low temperatures, continuing to increase with temperature until tissue damage
occurs (Table 2). Changes in predicted growth rate (RSG) and efficiency (q/RCO2) with temperature are
similar for a given cultivar [32]. The model, together with measurements of qandRCO2at two or more
temperatures, may aid in selection of cultivars and in understanding adaptation of plants to climatic
changes.
Soybean [Glycine max(L.) Merr.] is grown in many countries and climates. Twenty-two North
American soybean cultivars from six different maturity groups were grown from seed under the same
conditions. Measurement of metabolic heat rate of leaf tissue with a scanning calorimeter revealed that
the slope of heat rate versus temperature showed abrupt changes reflecting shifts in metabolism [33]. The
chilling response temperature for all cultivars was near 17.5°C. The maximum tolerable temperature for
all cultivars was near 43.5°C. Differences in response to temperatures between the extremes relate to ma-
turity group, follow latitudinal trends, and represent adaptation to different climates. Selection of culti-
vars of soybean for best growth in different climates has resulted in relatively rapid adaptation to local
temperatures [33].
6 SMITH ET AL.Figure 1 RSGHBplotted against temperature based on metabolic measurements at 15 and 25°C for 12 cul-
tivars of corn (Zea maysL.). Negative values of RSGHBindicate temperatures at which growth does not oc-
cur and tissues are stressed. (From Ref. 32.)
TABLE 2 Metabolic Heat Rate and Respiration Rate Measured Every 5°C from 5 to
40°C for Corn (Zea mays L.) Seedlings Grown from Seven “Older” Cultivarsa
Temperature response (°C)
Cultivar Low stress Optimal High stress
Pula Janku 5 20 32
Santo Domingo 520 33
Black Popcorn 5 20 30
Loncho 525 38
Black Mexican Sw. 525 30
Santa Ana Blue 520 40
Minipopcorn 520 27
aLow and high stress temperatures are indicated as well as the temperature for optimal growth.