6.2.2  Temperature

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Within a moderate temperature range readily tolerated by vascular plants (say c. 10–35°C) processes sustaining carbon gain show broad temperature optima. By contrast, develop-mental changes are rather more sensitive to temperature, and provided a plant’s combined responses to environmental con-itions do not exceed physiologically elastic limits (i.e. adjustments remain fully reversible) 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 extent (LAR) rather than performance per unit surface area (NAR) was responsible. Such inferences were subsequently validated.

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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 (RGRW) as well as relative rate of canopy area increase (RGRA). Both indices were lowest at 21/10°C. Moreover, variation in RGRW and RGRA was closely correlated across species and treatments (pooled data).

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Figure 6.9 Variation in whole-plant RGR is linked to relative rate of canopy expansion (RGRA) Nine species (including C3 and C4 plants) grown under three temperature regimen (21/10 °C, 32/21 °C and 21/27 °C day/night) expressed wide variation in RGR that showed a strong correlation with RGRA but was poorly correlated with variation in NAR. Extent rather than activity of leaves appears to be more important for RGR response to temperature. (Based on Potter and Jones 1977)

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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 RGRW and RGRA (Figure 6.9). 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 RGRW was poor (Figure 6.9b). By contrast, variation in both RGRW and RGRA was of a similar order and these two indices were closely correlated (Figure 6.9a).

Focusing on canopy expansion as a factor in RGRW response to temperature, RGRA is a composite index and refers to relative rate of canopy area increase by an entire plant. As explained earlier (Section 6.1.3) sources of variation in RGRA include frequency of leaf initiation and appearance, rate of lamina expansion and final size of individual leaves. Temperature effects on whole-plant RGRA can thus be resolved into com-ponent processes which correspond to parameters in Equation 6.14, namely Ax, r, t0 and frequency of leaf appearance (phyllochron, derived by subtraction of t0 for leaves on successive nodes). An example of temperature effects on those component processes is outlined in Table 6.3.

Wheat seedlings were raised at air temperatures of 6, 10 and 18°C and growth in area by successive leaves studied in detail. Recognising that leaf growth dynamics and final size vary with node (Figure 6.4) comparisons between these treat-ments are restricted to equivalent nodes. Ax from node 4 at 6°C is not recorded because plants grew so slowly that leaf 4 had still not emerged by the time this growth experiment was terminated. Leaves at node 2 did, however, attain full size but differed little between temperature treatments, while leaves from node 4 at 10°C and 18°C were also comparable. Unlike the positive effects of daily irradiance on final leaf size (Section 6.2.1), temperature effects on Ax were lacking in these wheat experiments. By contrast, relative rate of area increase (r) was strongly affected by temperature; and because Ax remained unchanged, duration of leaf growth must have been shortened. Similarly, appearance of new leaves was also accelerated under warm conditions; phyllochron decreased from 11 d at 6°C to only 3.5 d at 18°C.

Generalising from data in Table 6.3, positive effects of temperature on r and Dt0 with little contribution from Ax will account for temperature effects on relative rate of canopy expansion by whole plants (RGRA).

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