Carbon use in root respiration as affected by elevated atmospheric 02H a n s L a m b e r s l, I n e k e Stulen 2 and A d r i e v a n der W e r f 3 AbstractThe use of fossil fuel is predicted to cause an increase of the atmospheric CO2 concentration, which will affect the global pattern of temperature and precipitation. It is therefore essential to incorporate effects of temperature and water supply on the carbon requirement for root respiration of plants to predict effects of elevated [CO2] on the carbon budget of natural and managed systems.There is insufficient information to support the contentention that an increase in the concentration of CO2 in the atmosphere will enhance the CO2 concentration in the soil to an extent that is likely to affect root respiration.Moreover, there is no convincing evidence for a direct effect of elevated atmospheric [CO2] on the rate of root respiration per unit root mass or the fraction of carbon required for root respiration. However, there are likely to be indirect effects of elevated [CO2] on the carbon requirement of plants in natural systems.Firstly, it is very likely that the carbon requirement of root respiration relative to that fixed in photosynthesis will increase when elevated [CO2] induces a decrease in nutrient status of the plants. Although earlier papers have emphasized that elevated [CO2] favours investment of biomass in roots relative to that in leaves, these are in fact indirect effects. The increase in root weight ratio is due to the more rapid depletion of nutrients in the root environment as a consequence of enhanced growth. This will decrease the specific rate of root respiration, but increase the carbon requirement as a fraction of the carbon fixed in photosynthesis. It is likely that these effects will be minor in systems where the nutrient supply is very high, e.g. in many managed arable systems, and increase with decreasing soil fertility, i.e. in many natural systems.Secondly, a decrease in rainfall in some parts of the world may cause a shortage in water supply which favours the carbon partitioning to roots. Water stress is likely to reduce rates of root respiration per unit root mass, but enhance the fraction of total assimilates required for root respiration, due to greater allocation of biomass to roots.Increased temperatures are unlikely to affect the specific rate of root respiration in all species. Broadly generalized, the effect of temperature on biomass allocation is that the relative investment of biomass in roots is lowest at a certain optimum temperature and increases at both higher and lower temperatures. The root respiration of some species acclimates to growth temperature, so that the effect of global temperature rise is entirely accounted for by the effect of temperature on biomass allocation. The specific rate of root respiration of other species will increase with global warming. In response to global wanning the carbon requirement of roots is likely to decrease in temperate regions, when temperatures are suboptimal for the roots' cap...

Corn seedlings were transferred 48 h after imbibition to a medium containing [Formula: see text] Hoagland's salts and 5 mM nitrate (K14NO3 or K15NO3). Three treatments were used during the ensuing 6-h induction period: (i) no further additions: (ii) corn mixture of amino acids plus 10 mM glutamine and 10 mM asparagines; and (iii) 10 mM (NH4)2SO4. The shoots, mature root sections (25–35 mm from the tip), and root tip sections (0–10 mm) were then examined for nitrate reductase activity (NR) and the ability to reduce 15NO3in vivo.Both amino acid and ammonium additions resulted in less NR in the shoots. In the roots, the development of NR was inhibited slightly by the amino acids and was enhanced by ammonium additions. Treatment with either corn amino acids or (NH4)2SO4 had no influence on the incorporation of 15N into the ammonium and amino acid fraction. Thus, although the potential end products of NR have slight effects on measured in vitro levels of NR, they have no effect on the reduction of NO3− in the intact corn seedling.

No abstract

With the aims (1) to test whether the different natural occurrence of two Plantago species in grasslands is explained by a different preference of the species for nitrate or ammonium; (2) to test whether the different occurrence is explained by differences in the flexibility of the species towards changes in the nitrogen form;(3) to find suitable parameters as a tool to study ammonium and nitrate utilization of these species at the natural sites in grasslands, plants of Plantago lanceolata and P. major ssp. major were grown with an abundant supply of nitrate, ammonium or nitrate + ammonium as the nitrogen source (0.5 mM). The combination of ammonium and nitrate gave a slightly higher final plant weight than nitrate or ammonium alone. Ammonium lowered the shoot to root ratio in P. major. Uptake of nitrate per g root was faster than that of ammonium, but from the mixed source ammonium and nitrate were taken up at the same rate.In vivo nitrate reductase activity (NRA) was present in both shoot and roots of plants receiving nitrate. When ammonium was applied in addition to nitrate, NRA of the shoot was not affected, but in the root the activity decreased. Thus, a larger proportion of total NRA was present in the shoot than with nitrate alone.In vitro glutamate dehydrogenase activity (GDHA) was enhanced by ammonium, both in the shoot and in the roots. In vitro glutamine synthetase activity (GSA) was highest in roots of plants receiving ammonium.. Both GDHA and GSA were higher in P. lanceolata than in P. major. The concentration of ammonium in the roots increased with ammonium, but it did not accumulate in the shoot. The concentration of amino acids in the roots was also enhanced by ammonium. Protein concentration was not affected by the form of nitrogen.Nitrate accumulated in both the shoot and the roots of nitrate grown plants. When nitrate in the solution was replaced by ammonium, the nitrate concentration in the roots decreased rapidly. It also decreased in the shoot, but slowly.It is concluded that the nitrogen metabolism of the two Plantago species shows a similar response to a change in the form of the nitrogen source, and that differences in natural occurrence of these species are not related to a differential adaptation of nitrogen metabolism towards the nitrogen form.Suitable parameters for establishing the nitrogen source in the field are the in vivo NRA, nitrate concentrations in tissues and xylem exudate, and the fraction of total reduced nitrogen in the roots that is in the soluble form, and to some extent the in vitro GDHA and GSA of the roots. *Grassland Species Research Group. Publ. no 118. 23 24 Blacquibre et al.

Experimental setup to measure shoot photosynthesis and dark respiration. Photograph by E. Leeuwinga. AbstractThe response of Plantago major ssp. pleiosperma plants, grown on nutrient solution in a climate chamber, to a doubling of the ambient atmospheric CO2 concentration was investigated. Total dry matter production was increased by 30 ~o after 3 weeks of exposure, due to a transient stimulation of the relative growth rate (RGR) during the first 10 days. Thereafter RGR returned to the level of control plants. Photosynthesis, expressed per unit leaf area, was stimulated during the first two weeks of the experiment, thereafter it dropped and nearly reached the level of the control plants. Root respiration was not affected by increased atmospheric CO2 levels, whereas shoot, dark respiration was stimulated throughout the experimental period. Dry matter allocation over leaves stems and roots was not affected by the CO2 level. SLA was reduced by 10~o, which can partly be explained by an increased dry matter content of the leaves. Both in the early and later stages of the experiment, shoot respiration accounted for a larger part of the carbon budget in plants grown at elevated atmospheric CO2. Shifts in the total carbon budget were mainly due to the effects on shoot respiration. Leaf growth accounted for nearly 50 ~o of the C budget at all stages of the experiment and in both treatments.Abbreviations: LAR, leaf area ratio; LWR, leaf weight ratio; RGR, relative growth rate; R/S, root to shoot ratio; RWR, root weight ratio; SLA, specific leaf area; SWR, stem weight ratio.