Within a moderate temperature range readily tolerated by vascular plants (10–35°C, see Chapter 14) processes sustaining carbon gain show broad temperature optima. By contrast, developmental changes are rather more sensitive to temperature, and provided a plant’s combined responses to environmental conditions do not exceed physiologically elastic limits, temperature effects on RGR are generally attributable to rate of canopy expansion rather than rate of carbon assimilation. In the early days of growth analysis, Blackman et al. (1955) inferred from a multi-factor analysis of growth response to environmental conditions that NAR was relatively insensitive to temperature, but whole plant growth was obviously affected, so that photosynthetic area (LAR) rather than performance per unit surface area (NAR) was responsible. Such inferences were subsequently validated.
Using day/night temperature as a driving variable, Potter and Jones (1977) provided a detailed analysis of response in key growth indices for a number of species (Table 6.2). Data for maize, cotton, soybean, cocklebur, Johnson grass and pigweed confirmed that 32/21°C was optimum for whole-plant relative growth rate (RGR) as well as relative rate of canopy area increase (RGRA). Both indices were lowest at 21/10°C. This was true for C4 as well as C3 species.
C4 species had a higher RGR and RGRA than C3 species, especially under warm conditions (Table 6.2)
All populations described in Potter and Jones (1977) maintained strict exponential growth. NAR could then be derived validly and temperature effects on NAR could then be compared with temperature effects on RGR and RGRA (Figure 6.3). With day/night temperature as a driving variable, most values for NAR fell between 10 and 20 g m–2 d–1. Correlation between NAR and RGR was poor (Figure 6.3). By contrast, variation in both RGR and RGRA was of a similar order and these two indices were closely correlated (Figure 6.3).
A later chapter in this book, Chapter 14, explains the effect of temperature on growth of different plant species, with particular focus on adaptation to very low and very high temperatures.