6.2.4  Nutrients (nitrogen and phosphorus)

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Leaf expansion is particularly sensitive to nutrient supply (especially nitrogen, phosphorus, potassium (N, P, K) and magnesium) due primarily to the needs of enlarging cells for synthesis of new materials and generation of turgor. Reiterating assumptions made earlier (Section 6.1.3), an initial exponential phase in lamina expansion coincides with an especially active period of cell division, whereas the subsequent asymptotic phase is largely driven by cell enlargement. Relative rate of lamina expansion (r) at the end of that exponential phase is thus taken as indicative of cell division activity, whereas Ax reflects enlargement of that cell population. Nutrient deficiency or imbalance is first detected in leaf growth rather than leaf assimilation, and in terms of canopy development, nutrient supply impacts on phyllochron (Δt0), relative rate of expansion (r) and final leaf size (Ax) (see Equation 6.14).

Such effects are nicely demonstrated by Gmelina arborea Roxb. (colloquially gmelina), a close relative of teak and favoured for tropical plantations by virtue of fast growth.

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Table 6.6

Like teak, G. arborea carries large leaves that commonly grow to 750 cm2 on high-quality sites. Leaves on greenhouse plants are smaller, but their growth dynamics are still informative. Plants grown on high, medium or low N supply (Table 6.6) with leaf [N] 2.94%, 1.18% and 0.63% N (dry mass) respectively show a strong decline in final size (Ax in Equation 6.14). By comparing high N with medium N, and noting little change in r (0.36 d–1 on high N and 0.34 d–1 on medium N), this reduction in Ax must be due mainly to a diminished enlargement of a given population of cells that were generated during the previous exponential phase of leaf growth. By contrast, rate of appearance of new leaves is affected by N supply, due probably to a slower initiation, so that phyllochron (Δt0 in Table 6.6) increased from 4.9 to 7.4 to 9.8 d on high, medium and low N supply respectively.

Phosphorus effects on leaf growth in G. arborea are amenable to a similar analysis. In this case, Ax was less sensitive to reduction from high P to medium P, whereas r was reduced from 0.23 to 0.19 d–1 (and to 0.12 d–1 on low P). Phyllochron was similarly sensitive, and as with N effects, Δt0 became protracted with reduction in P supply (namely 8.5, 11.2 and 18.9 d on high, medium and low P respectively). These plants were taking twice as long to produce new leaves on low P as on low N.

In keeping with common experience on a wide range of plants, nutrient deficiency slowed canopy development in G. arborea, but present analysis has shown that N and P effects are qualitatively different. N deficiency is obvious as a reduction in leaf size, whereas P deficiency impacts to a relatively greater extent on leaf number due to slower appearance. Moderately N deficient plants (leaf [N] c. 1.2 mmol N g–1 dry mass) produced a slower succession of smaller leaves that expanded reasonably quickly, but moderately P deficient plants (leaf [P] c. 50 µmol P g–1 dry mass) produced even fewer leaves (longer phyllochron) that expanded slowly but nevertheless achieved reasonable size. Relative rate of leaf expansion (r) was not different on high N cf. moderate N (r = 0.36 ± 0.03 and 0.34 ± 0.04 respectively) but r was different on high P cf. moderate P (r = 0.228 ± 0.005 and 0.192 ± 0.008 respectively). In the same experiment on G. arborea, Cromer et al. (1993) show dose response curves for r with N saturation ≥ 1.5 mmol N g–1 dry mass and P saturation ≥ 100 µmol P g–1 dry mass.

N, P and K are highly mobile nutrient elements, and even on well-nourished plants individual leaves show considerable nutrient turnover as older (full size) leaves help furnish nutrient requirements of younger expanding leaves at higher nodes. For example, Hopkinson (1964) provided a detailed P budget for cucumber foliage showing a strong import (up to 0.6 mg P leaf–1 d–1) that coincided with rapid expansion, followed by a steady net export (up to 0.15 mg P leaf–1 d–1) in response to the P demands of expanding leaves at higher nodes (Figure 5.12). The time-course of post-maturation senescence will vary according to the overall balance between nutrient supply and demand which depends in turn on root-zone nutrient availability versus requirements for continuing growth and development of new organs.

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Figure 6.12 Gmelina arborea (a relative of teak) is a highly productive tropical tree with large leaves and is favoured as a plantation species. Leaf growth is especially responsive to a step—up from low to medium P supply (subsequent to leaf appearance at node 4, arrow). Relative rate of leaf expansion (r) increased from 0.134 to 0.228 d-1, and phyllochron (Δt0) decreased from 21.4 to 6.2 d. Upper row shows comparative size of leaves from node 6 on two high—P plants. Scale bar = 10 cm. (Further details in Cromer et al. 1993)(Photograph courtesy P E. Kriedemaun) 

Where nutrient supply is restricted, turnover in mature leaves will accelerate (especially in fast-growing species) and senescence will hasten — a common feature under N, P or K deficiency (Chapter 16). Conversely, when such nutrient-deficient plants are restored to full supply, leaf growth response can be dramatic (Figure 6.12) with sharp reduction in phyllochron (from 21.4 to 6.2 d in this example) and major increase in Ax (from 65 to 181 cm2 at node 9; see Cromer et al. 1993).

Growth responses to nutrient supply are usually unmistakable, even spectacular (Figure 6.12) and commonly referenced to nutrient element concentration (e.g. [N] or [P]) on a dry mass basis. However, given the highly dynamic nature of tissue N and P, especially when growth-limiting supply enhances recycling from older organs to new growing points, how meaningful are whole-plant or even leaf values for [N] or [P] as driving variables in growth analysis? In effect, [N] and [P] will vary in both space and time according to patterns of plant growth and development, which are themselves influenced by nutrient supply.

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Figure 6.13 RGR of young seedlings (Eucalyptus grandis) in aeroponic culture (see Ingestad and Lund 1986 for details on technique) can be set by the relative addition rate of a single limiting nutrient (N in this experiment, with all other nutrient elements non-limiting). Clusters of symbols refer to five different RARs, namely 0.04, 0.06, 0.08, 0.10 and 0.12 d-1 (with some minor variation) (Based on Cromer and Jarvis 1990)

Analysis of nutrient-dependent changes in growth indices therefore require test plants where nutrient element concentration can be ‘set’ in space, and also remain stable in time. These prerequisites can be met by aeroponic culture in a constant environment (see Ingestad and Lund 1986 and literature cited). Seedlings are held in aeroponic spray chambers where a small volume of nutrient solution is recirculated continuously, and further nutrients are introduced at a predetermined relative addition rate (RAR). In effect, a steady exponential growth is set by the RAR of a key nutrient (N or P in present examples, but K is equally amenable) while all other essential nutrients are kept non-limiting. RAR thus represents a driving variable for RGR which in turn shows an initially linear response to RAR (Figure 6.13) eventually reaching a point of saturation (not shown here).

Within a plant’s dynamic range of growth response to nutrient supply, RGR and RAR are linearly related so that plants grown this way are well suited to growth analysis. Moreover, whole-plant concentrations of critical nutrients are ‘set’ by RAR such that higher RAR produces higher whole-plant nutrient concentration and remain reasonably stable over time. Cromer and Jarvis (1990) demonstrated this for N in Eucalyptus grandis and Kirschbaum (1991) for P.

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Figure 6.14 Under stable environmental conditions RGR of young seedlings (Eucalyptus grandis) growing in aeroponic culture can be set by the relative addition rate (RAR) of a single limiting nutrient (N in this experiment). If exponential growth is maintained, plant-N concentration [N] will be proportional to RAR, and RGR is then linearly related to [N]. (Based on Cromer and Jarvis 1990)

Using this Ingestad technique, growth and photosynthetic responses to plant-nutrient concentration are not complicated by interactions between ontogeny and nutrient recycling discussed earlier. For example, RGR response to plant [N] (Figure 6.14) can now be resolved into NAR and LAR contributions. Taking data from Cromer and Jarvis (1990) for comparisons (cf. Figure 6.13) their highest RARN, 0.12 d–1, resulted in an RGR of 0.111 d–1, while their lowest RARN, 0.04 d–1, generated an RGR of only 0.039 d–1. Corresponding plant [N] values were 34.1 and 11.7 g N kg–1 dry mass, and resultant values for NAR were 5.55 and 4.45 g m–2 d–1 respectively. Higher [N] thus increased NAR by a factor of 1.247. Leaf weight ratio (LWR) increase was somewhat larger (factor of 1.463) and was accompanied by increased SLA (factor of 1.561).

Combining outcomes from Cromer and Jarvis (1990) with those from P experiments by Kirschbaum et al. (1992), some key differences between N and P in their effects on growth indices in seedlings of E. grandis were apparent. Cromer and Jarvis (1990) concluded, inter alia, that ‘...effects of N on allocation of dry matter to leaves and the way in which dry matter is distributed to intercept light, have a larger influence on seedling growth rate than do effects of N on net rate of carbon gain per unit leaf area’. By contrast, when considering P-dependent effects on RGR, Kirschbaum et al. (1992) conclude that ‘...Carbon fixation rate per unit of plant dry weight increased about 5-fold with increasing nutrient addition rate over the range of addition rates used. That increase was due to a doubling in specific leaf area and a doubling in assimilation rate per unit leaf area, while leaf weight as a fraction of total plant weight increased by about 20 %.’ Unlike N, effects of P on RGR were due more to changes in leaf physiology than to changes in dry matter distribution.

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