Norway spruce expressed much lower levels of foliar NR than
Pinus sylvestris (Figure 4.11). Little seasonal variation in
activity was observed, with only slight flushes of activity occurring in April/May and again in August. The low levels of
1400
Nitrate reductase activity (pkat g-1 fw)
1 200
1000
Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Month (1987 - 1988)
Figure 4.11 Seasonal variation in NR activity in (□) one year needles, and (O) current needles of Picea abies
(Norway spruce) at Bencroft Wood 1987-88. Error bars show 95% confidence limits. [Note: all seasonal variation figures are presented at the same scale (0 - 1400 pkat g"^ f.wt.) to emphasize differences between species].
activity could partly be due to the fact that coniferous trees retain their foliage for considerable lengths of time. By retaining foliage longer than one year, coniferous trees have a built-in mechanism to minimize the amount of nitrogen taken up each year per unit of biomass production. Comparisons suggest that coniferous ecosystems need nearly 40% less nitrogen uptake than deciduous ecosystems and thus should be able to out-compete deciduous forests on nitrogen-poor soils
(Cole, 1981).
Interestingly, long periods of foliage retention did not appear to prevent both high levels of, and seasonal variations in NR activities of Pinus sylvestris (section 4.3.3.2). However, pines do tend to replace their needles more regularly than spruces (e.g. Pinus ectinata retains foliage for only 1.25 years; Cole, 1981). This could perhaps account for the differences between P. sylvestris and Picea abies. Norway spruce retains its foliage for about 8 years, although the
length of time varies considerably with factors such as pollution (John Pearson, personal communication). Through needle retention, Norway spruce may minimize the requirement for exogenous nitrogen uptake, with a significant portion of nitrogen required for new growth coming from internal recycling (Sprent, 1979).
However, Norway spruce is specifically grown because of its relatively fast growth rate. Rapid growth rates suggest that Norway spruce would be a species with a high demand for nitrogen, which also suggests that rates of nitrate reduction
would be relatively high.
So why are rates of foliar nitrate reduction so low in Norway spruce? Internal recycling is one reason, but does not necessarily explain the relatively high growth rates of this conifer. Nitrification is often inhibited in the acidic soils of coniferous forests and consequently nitrate concentrations are low (Sprent, 1979; Rice and Pancholy, 1972; 1973; 1974). A reduced nitrate supply has been observed to lead to (i) nitrate reduction being restricted to roots, and/or (ii) to alternative nitrogen sources being used (Stewart et al.,
1989). In such situations, high foliar NR activities would not be expected.
There is much contradictory literature concerning growth and nitrogen source, even on the same species (Titus and Kang, 1982; Kato, 1986), with much of this controversy being related to uncontrolled variables. But, generally, Picea has been observed to grow best when supplied with ammonium rather than nitrate (Dickson, 1989). Induction experiments have shown that, unlike Pinus sylvestris (which is capable of relatively high rates of nitrate reduction after nitrate feeding; see Chapter 8 ), it was impossible to induce NR activity in spruce
needles (data not shown). Therefore, ammonium uptake and assimilation is likely to be more common than nitrate utilization in spruce forests, as well as nutrient availability being fostered naturally by mycorrhizae commonly associated with coniferous roots. Such symbiotic relationships contribute to the nutritional well-being of the trees, with
the tree providing carbohydrate (photosynthate) to the fungus.
Maximum rates of nitrate reduction in spruce needles occurred from late April/early May (Figure 4.11). This peak of activity corresponds to the peak in photosynthetic activity found in spruce needles in a study by Lichtenthaler et al. (1989). These workers examined chlorophyll content and photosynthetic activity in spruce needles from summer 1987 to summer 1988. Their results for photosynthetic activity followed exactly the same pattern as NR activity data of the present study.
Lichtenthaler et al. (1989) observed that photosynthetic activity dropped slightly from 4-8 /imol CO^ m‘^ s'^ in August 1987, to lower levels in October and November. It reached very low levels in December (0-2 pmol), with some recovery in January 1988. From March, the photosynthetic activity increased again to reach maximum values of 6 - 8 [imol CO^ m"* s“^
at the end of April, just as new shoots were beginning to form. The current year needles reached their maximum photosynthetic activity later (May/June) than the one year needles. Thereafter Lichtenthaler and colleagues found that photosynthetic activity was lower again. They suggested that low photosynthetic activities in winter were probably due to closed stomata, but, in part, also due to damage of the photosynthetic apparatus. In addition, lower temperatures experienced in the winter months may also lead to reductions in photosynthetic activity.
demonstrates the similarity between NR activity and photosynthetic activity data of Lichtenthaler et al. (1989). Although fluctuations in NR activity are slight, seasonal patterns are still obvious, and are almost identical to seasonal patterns of photosynthetic rate measured by Lichtenthaler and colleagues. The similarities stress the importance of light energy in the control of NR induction
(Smolander and Oker-Blom, 1989; Chapters 6 and 7).
The total soluble amino acid pool of Picea abies needles was very low (between 3667 and 6420.98 nmol g"^ f.wt.). Similarly
to Pinus sylvestris, main constituents of the pool were
arginine, asparagine, glutamine, glutamate, alanine and serine. These amino acids formed 58.67 to 81.77% of the total pool throughout the year. Seasonal variations in the total concentration of amino acids were associated with changes in concentrations of arginine, asparagine, glutamine and glutamate. This is in agreement with earlier studies on the coniferous species Picea glauca and Pinus banksiana (Durzan, 1968a; Durzan and Steward, 1983). The other major constituent, forming between 14.66 and 24% of the soluble pool in spruce needles, was tyrosine. High concentrations of this plant protectant precursor (up to 1541.04 nmol g"^ f.wt.) suggest the steady turnover of the secondary products and plant metabolites which are required for adequate growth of this species throughout the year (see sections 4.3.1.3 and 4.3.3.2).
4 . 4 P p > K u m é
À huge degree of seasonal variation, both temporal and between species, has been shown in NR activity and amino acid pools. Seasonal trends observed in the pioneer and climax species studied here varied considerably, and it appears that each individual species must be considered separately. However, it was consistently found that pioneers reduced nitrate at much higher rates than climax species, and generally, it was observed that pioneers exhibited a tendency for early seasonal flushes of NR activity. The pioneer species appeared to exhibit more varied seasonal patterns of NR activity than climax species. Deciduous climax species showed a tendency for
late seasonal flushes of activity, although seasonal trends were less pronounced than in the pioneer group. The three evergreen species studied exhibited different seasonal variations in activity from either the climax or pioneer species examined.
Many factors are likely to play important roles in determining the seasonal variations in both NR activity and amino acid concentration. As well as nitrate availability, one of the most important factors determining seasonal variations in foliar NR activity in woody plants is light (Wild and Zerbe, 1977; Bergareche and Simon, 1988). Most of the species studied here were observed to reduce nitrate at increased rates as light intensity increased. In some cases, (i.e. Sambucus nigra
and Rubus fruticosus), the absence of a shading canopy,
dictated by the changing of the s easons, lead to increased availability of light, and appeared to result in higher leaf
^ NR activities. Maximal rates of nitrate reduction in holly corresponded with the seasons when the deciduous canopy was absent, and NR activities of the conifer Picea abies followed identical patterns to seasonal photosynthetic activities measured in this species by Lichtenthaler et al. (1989). Such findings appear to emphasize the importance of light intensity in determining seasonal trends in nitrate reduction. Chapters