ALBERT

Notes: Higher rates of nitrate assimilation are required to support faster growth in enhanced carbon dioxide. To investigate how this is achieved, tobacco plants were grown on high nitrate and high light in ambient and enhanced (700 μmol mol–1) carbon dioxide. Surprisingly, enhanced carbon dioxide did not increase leaf nitrate reductase (NR) activity in the middle of the photoperiod. Possible reasons for this anomalous result were investigated. (a) Measurements of biomass, nitrate, amino acids and glutamine in plants fertilized once and twice daily with 12 mol m–3 nitrate showed that enhanced carbon dioxide did not lead to a nitrate limitation in these plants. (b) Enhanced carbon dioxide modified the diurnal regulation of NR activity in source leaves. The transcript for nia declined during the light period in a similar manner in ambient and enhanced carbon dioxide. The decline of the transcript correlated with a decrease of nitrate in the leaf, and was temporarily reversed after re-irrigating with nitrate in the second part of the photoperiod. The decline of the transcript was not correlated with changes of sugars or glutamine. NR activity and protein decline in the second part of the photoperiod, and NR is inactivated in the dark in ambient carbon dioxide. The decline of NR activity was smaller and dark inactivation was partially reversed in enhanced carbon dioxide, indicating that post-transcriptional or post-translational regulation of NR has been modified. The increased activation and stability of NR in enhanced carbon dioxide was correlated with higher sugars and lower glutamine in the leaves. (c) Enhanced carbon dioxide led to increased levels of the minor amino acids in leaves. (d) Enhanced carbon dioxide led to a large decrease of glycine and a small decrease of serine in leaves of mature plants. The glycine:serine ratio decreased in source leaves of older plants and seedlings. The consequences of a lower rate of photorespiration for the levels of glutamine and the regulation of nitrogen metabolism are discussed. (e) Enhanced carbon dioxide also modified the diurnal regulation of NR in roots. The nia transcript increased after nitrate fertilization in the early and the second part of the photoperiod. The response of the transcript was not accentuated in enhanced carbon dioxide. NR activity declined slightly during the photoperiod in ambient carbon dioxide, whereas it increased 2-fold in enhanced carbon dioxide. The increase of root NR activity in enhanced carbon dioxide was preceded by a transient increase of sugars, and was followed by a decline of sugars, a faster decrease of nitrate than in ambient carbon dioxide, and an increase of nitrite in the roots. (f) To interpret the physiological significance of these changes in nitrate metabolism, they were compared with the current growth rate of the plants. (g) In 4–5-week-old plants, the current rate of growth was similar in ambient and enhanced carbon dioxide (≈ 0·4 g–1 d–1). Enhanced carbon dioxide only led to small changes of NR activity, nitrate decreased, and overall amino acids were not significantly increased. (h) Young seedlings had a high growth rate (0·5 g–1 d–1) in ambient carbon dioxide, that was increased by another 20% in enhanced carbon dioxide. Enhanced carbon dioxide led to larger increases of NR activity and NR activation, a 2–3-fold increase of glutamine, a 50% increase of glutamate, and a 2–3-fold increase in minor amino acids. It also led to a higher nitrate level. It is argued that enhanced carbon dioxide leads to a very effective stimulation of nitrate uptake, nitrate assimilation and amino acid synthesis in seedlings. This will play an important role in allowing faster growth rates in enhanced carbon dioxide at this stage.

Notes: Abstract Downward transport of water in roots, in the following termed “inverse hydraulic lift,” has previously been shown with heat flux techniques. But water flow into deeper soil layers was demonstrated in this study for the first time when investigating several perennial grass species of the Kalahari Desert under field conditions. Deuterium labelling was used to show that water acquired by roots from moist sand in the upper profile was transported through the root system to roots deeper in the profile and released into the dry sand at these depths. Inverse hydraulic lift may serve as an important mechanism to facilitate root growth through the dry soil layers underlaying the upper profile where precipitation penetrates. This may allow roots to reach deep sources of moisture in water-limited ecosystems such as the Kalahari Desert.

Notes: Abstract Four biennial species (Arctium tomentosum, Cirsium vulgare, Dipsacus sylvester and Daucus carota) which originate from habitats of different nutrient availability were investigated in a 2-year experiment in a twofactorial structured block design varying light (natural daylight versus shading) and fertilizer addition. The experiment was designed to study storage as reserve formation (competing with growth) or as accumulation (see Chapin et al. 1990). We show that (i) the previous definitions of storage excluded an important process, namely the formation of storage tissue. Depending on species, storage tissue and the filling process can be either a process of reserve formation, or a process of accumulation. (ii) In species representing low-resource habitats, the formation of a storage structure competes with other growth processes. Growth of storage tissue and filling with storage products is an accumulation process only in the high-resource plant Arctium tomentosum. We interpret the structural growth of low-resource plants in terms of the evolutionary history of these species, which have closely related woody species in the Mediterranean area. (iii) The use of storage products for early leaf growth determines the biomass development in the second season and the competitive ability of this species during growth with perennial species. (iv) The high-resource plant Arctium has higher biomass development under all conditions, i.e. plants of low-resource habitats are not superior under low-resource conditions. The main difference between high- and low-resource plants is that low-resource plants initiate flowering at a lower total plant internal pool size of available resources.

Notes: Abstract Canopy-scale evaporation rate (E) and derived surface and aerodynamic conductances for the transfer of water vapour (gs and ga, respectively) are reviewed for coniferous forests and grasslands. Despite the extremes of canopy structure, the two vegetation types have similar maximum hourly evaporation rates (E max) and maximum surface conductances (gsmax) (medians = 0.46 mm h-1 and 22 mm s-1). However, on a daily basis, median E max of coniferous forest (4.0 mm d-1) is significantly lower than that of grassland (4.6 mm d-1). Additionally, a representative value of ga for coniferous forest (200 mm s-1) is an order of magnitude more than the corresponding value for grassland (25 mm s-1). The proportional sensitivity of E, calculated by the Penman-Monteith equation, to changes in gs is 〉0.7 for coniferous forest, but as low as 0.3 for grassland. The proportional sensitivity of E to changes in ga is generally ±0.15 or less. Boundary-line relationships between gs and light and air saturation deficit (D) vary considerably. Attainment of gsmax occurs at a much lower irradiance for coniferous forest than for grassland (15 versus about 45% of full sunlight). Relationships between gs and D measured above the canopy appear to be fairly uniform for coniferous forest, but are variable for grassland. More uniform relationships may be found for surfaces with relatively small ga, like grassland, by using D at the evaporating surface (D0) as the independent variable rather than D at a reference point above the surface. An analytical expression is given for determining D0 from measurable quantities. Evaporation rate also depends on the availability of water in the root zone. Below a critical value of soil water storage, the ratio of evaporation rate to the available energy tends to decrease sharply and linearly with decreasing soil water content. At the lowest value of soil water content, this ratio declines by up to a factor of 4 from the non-soil-water-limiting plateau. Knowledge about functional rooting depth of different plant species remains rather limited. Ignorance of this important variable makes it generally difficult to obtain accurate estimates of seasonal evaporation from terrestrial ecosystems.

Notes: Abstract Tobacco (Nicotiana tabacum L.) plants transformed with ‘antisense’ rbcS to decrease the expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) have been used to investigate the contribution of Rubisco to the control of photosynthesis in plants growing at different irradiances. Tobacco plants were grown in controlled-climate chambers under ambient CO2 at 20°C at 100, 300 and 750 μmol·m−2·s−1 irradiance, and at 28°C at 100, 300 and 1000 μmol·m−2·s−1 irradiance. (i) Measurement of photosynthesis under ambient conditions showed that the flux control coefficient of Rubisco (C infRubisco supA ) was very low (0.01–0.03) at low growth irradiance, and still fairly low (0.24–0.27) at higher irradiance. (ii) Short-term changes in the irradiance used to measure photosynthesis showed that C infRubisco supA increases as incident irradiance rises, (iii) When low-light (100 μmol·m−2·s−1)-grown plants are exposed to high (750–1000 μmol·m−2·s−1) irradiance, Rubisco is almost totally limiting for photosynthesis in wild types. However, when high-light-grown leaves (750–1000 μmol·m−2·s−1) are suddenly exposed to high and saturating irradiance (1500–2000 μmol·m−2·s−1), C infRubisco supA remained relatively low (0.23–0.33), showing that in saturating light Rubisco only exerts partial control over the light-saturated rate of photosynthesis in “sun” leaves; apparently additional factors are co-limiting photosynthetic performance, (iv) Growth of plants at high irradiance led to a small decrease in the percentage of total protein found in the insoluble (thylakoid fraction), and a decrease of chlorophyll, relative to protein or structural leaf dry weight. As a consequence of this change, high-irradiance-grown leaves illuminated at growth irradiance avoided an inbalance between the “light” reactions and Rubisco; this was shown by the low value of C infRubisco supA (see above) and by measurements showing that non-photochemical quenching was low, photochemical quenching high, and NADP-malate dehydrogenase activation was low at the growth irradiance. In contrast, when a leaf adapted to low irradiance was illuminated at a higher irradiance, Rubisco exerted more control, non-photochemical quenching was higher, photochemical quenching was lower, and NADP-malate dehydrogenase activation was higher than in a leaf which had grown at that irradiance. We conclude that changes in leaf composition allow the leaf to avoid a one-sided limitation by Rubisco and, hence, overexcitation and overreduction of the thylakoids in high-irradiance growth conditions, (v) ‘Antisense’ plants with less Rubisco contained a higher content of insoluble (thylakoid) protein and chlorophyll, compared to total protein or structural leaf dry weight. They also showed a higher rate of photosynthesis than the wild type, when measured at an irradiance below that at which the plant had grown. We propose that N-allocation in low light is not optimal in tobacco and that genetic manipulation to decrease Rubisco may, in some circumstances, increase photosynthetic performance in low light.

Notes: Abstract Wild-type tobacco (Nicotiana tabacum L.) plants and transgenic tobacco transformed with antisense rbcS to decrease expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco; EC 4.1.1.39) were grown at 300 mol-m−2 · s−1 irradiance and 20° C at either 0.1, 0.7 or 5 mM NH4NO3. In high nitrogen (N), growth was reduced in parallel with the inhibition of photosynthesis when Rubisco was decreased by genetic manipulation. In limiting N, photosynthesis was reduced strongly when Rubisco was decreased by genetic manipulation, but growth was hardly affected. At all N levels, decreased expression of Rubisco led to a decrease in the amount of starch accumulated in the leaves. There was a large increase of the specific leaf area (SLA; leaf area maintained per unit dry weight in the leaf) in plants with decreased Rubisco. Increased SLA was associated with an increased inorganic and a decreased carbon contribution to leaf structural dry weight. The increased SLA represents a more efficient investment of photosynthate with respect to maximisation of leaf area and light interception, and partly compensates for the decreased rate of photosynthesis in plants with decreased expression of Rubisco. The changes of starch content and SLA were particularly large in limiting N, when growth rate was effectively independent of the rate of photosynthesis. Increased N availability led to a large increase of the shoot/ root ratio, but only a small increase in SLA. It is argued that N availability and the availability of photosynthate both regulate storage and allocation of biomass to optimize resource utilization, but achieve this via different mechanisms.