Bibliothek

Notizen: When grown at a low P supply, Hakea prostrata R.Br. (Proteaceae) develops dense clusters of determinate branch roots, termed ‘proteoid’ or ‘cluster’ roots and accumulates Mn in its leaves. The aim of this study was to vary the production of cluster roots and assess the relationship between Mn uptake and cluster-root mass. We collected native soil from a location inhabited by H. prostrata and amended this with ‘high’ and ‘low’ amounts of insoluble or soluble P. After 14 months, we measured the impact of the treatments on cluster-root development and the [P], [Mn], [Fe], [Zn] and [Cu] in young (expanding) and mature leaves. Dry mass and leaf area increased with increasing P availability in the soil, but growth decreased at the highest soluble [P], which caused symptoms of P toxicity. The [P] in young leaves (1.3–2.7 mg g−1 DM) exceeded that in older leaves (0.28–0.85 mg g−1 DM), except when plants were grown with soluble P (3.2–21 mg g−1 DM). Cluster-root formation was inhibited when leaf [P] increased; [P] in young leaves, rather than that in old leaves, appeared to be the factor that determined the proportion of the root mass invested in cluster roots. Old leaves of all treatments had [Mn] from 90 to 120 µg g−1 DM, except for plants grown at high levels of soluble P, when [Mn] decreased below 30 µg g−1 DM. The [Mn] and [Zn] in old leaves and the [Cu] in young leaves were positively correlated with the fraction of roots invested in cluster roots. These findings support our hypothesis that cluster roots play a significant role in micronutrient acquisition, and also provide an explanation for Mn accumulation in leaves of H. prostrata, and presumably Proteaceae in general.

Notizen: The role of gibberellin (GA) in leaf elongation has long been known, however, its involvement in whole shoot growth and biomass allocation is much less clear. We studied the effects of exogenously supplied GA3 and paclobutrazol, an inhibitor of GA biosynthesis, on these processes in Aegilops caudata and Aegilops tauschii, species with contrasting leaf growth characteristics. In both species, addition of GA3 increased leaf elongation rate (LER) through its promoting effect on both cell size and cell number, while paclobutrazol decreased it. Similarly, GA3 increased biomass allocation to the leaves, mainly leaf sheaths, at the cost of allocation to the roots, whereas paclobutrazol had the opposite effect in both species. Despite the increase in LER and biomass allocation to the shoot upon GA3 application, the relative growth rate (RGR) remained constant. Specific leaf area (SLA) was only temporarily affected by GA3 addition. Our results show that the inherent differences in LER and biomass allocation between the slow-elongating A. caudata and the fast-elongating A. tauschii are considerably reduced by the exogenous supply of GA3 to the slow-elongating species, or paclobutrazol to the fast-elongating one. This suggests a role for gibberellins in explaining inherent differences in leaf area expansion and biomass allocation between the two species in this study.

Notizen: The 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 carbon partitioning of plants to predict effects of elevated [CO2] on growth and yield of Triticum aestivum.Although earlier papers have emphasized that elevated [CO2] favours investment of biomass in roots relative to that in leaves, it has now become clear that these are indirect effects, due to the more rapid depletion of nutrients in the root environment as a consequence of enhanced growth. Broadly generalized, the effect of temperature on biomass allocation in the vegetative stage is that the relative investment of biomass in roots is lowest at a certain optimum temperature and increases at both higher and lower temperatures. This is found not only when the temperature of the entire plant is varied, but also when only root temperature is changed whilst shoot temperature is kept constant. Effects of temperature on the allocation pattern can be explained largely by the effect of root temperature on the roots' capacity to transport water. Effects of a shortage in water supply on carbon partitioning are unambiguous: roots receive relatively more carbon.The pattern of biomass allocation in the vegetative stage and variation in water-use efficiency are prime factors determining a plant's potential for early growth and yield in different environments. In a comparison of a range of T. aestivum cultivars, a high water-use efficiency at the plant level correlates positively with a large investment in both leaf and root biomass, a low stomatal conductance and a large investment in photosynthetic capacity. We also present evidence that a lower investment of biomass in roots is not only associated with lower respiratory costs for root growth, but also with lower specific costs for ion uptake. We suggest the combination of a number of traits in future wheat cultivars, i.e. a high investment of biomass in leaves, which have a low stomatal conductance and a high photosynthetic capacity, and a low investment of biomass in roots, which have low respiratory costs. Such cultivars are considered highly appropriate in a future world, especially in the dryer regions. Although variation for the desired traits already exists among wheat cultivars, it is much larger among wild Aegilops species, which can readily be crossed with T. aestivum. Such wild relatives may be exploited to develop new wheat cultivars well-adapted to changed climatic conditions.

Notizen: Abstract A brief survey of the biochemistry of the alternative oxidative pathway (‘cyanide-resistant respiration’) and its occurrence in vivo is given. Several hypotheses about the physiological significance of the alternative chain are discussed. These include a role in (1) heat production, (2) fruit ripening, (3) respiration of plants that contain high levels of cyanogenic glycosides, producing HCN upon wounding, (4) oxidation of NADH that is produced by various causes in excess of that required for ATP production, (5) ion uptake, and (6) osmoregulation.In intact roots of higher plants, the activity of the alternative pathway is more active when less carbohydrate is required for assimilation of N (NH+4 NO-3 or N2) and is less active in those when carbohydrates are being stored in a storage organ or when the availability of photosynthate is reduced. An increase in carbohydrate requirement for osmoregulation is also correlated with decreased alternative chain activity.It is concluded that the alternative pathway in roots plays an important role in oxidation of sugars which are not required for carbon skeletons, energy production for growth and maintenance processes, osmoregulation or storage. However, the significance of this role may vary in different tissues and physiological states.

Notizen: The present authors have shown previously that both respiration rates and in vivo activities of the alternative oxidase (AOX) of leaves of Alocasia odora, a shade species, are lower than those in sun species, thereby optimizing energy production under limited light conditions (Noguchi et al., Australian Journal of Plant Physiology 28, 27–35, 2001). In the present study, mitochondria isolated from A. odora leaves were examined in order to investigate the biochemical basis for the differences in respiratory parameters. Alocasia odora and spinach plants were cultivated under both high and low light intensities, mitochondria were isolated from their leaves, and their respiratory properties compared. Mitochondrial content of leaf extracts from the two species was estimated using fumarase activities and antibody detection of porin (the voltage-dependent anion channel of the outer mitochondrial membrane). On a mitochondrial protein basis, spinach leaves showed higher capacities of the cytochrome pathway and cytochrome c oxidase (COX) than A. odora leaves. However, on a mitochondrial protein basis, A. odora showed higher capacities of AOX, which had a high affinity for ubiquinone when activated by pyruvate. Alocasia odora also had larger amounts of mitochondrial protein per leaf dry weight, even under severely shaded conditions, than spinach. Lower growth light intensity led to lower activities of most pathways and proteins tested in both species, especially glycine-dependent oxygen uptake. In the low light environment, most of the AOX protein in A. odora leaves was in its inactive, oxidized dimer form, but was converted to its reduced active form when plants were grown under high light. This shift may prevent over-reduction of the respiratory chain under photo-oxidative conditions.

Notizen: Shoot activity has been reported to affect rates of ion uptake by plant roots in other ways than merely through supply of assimilates. To elucidate the mechanisms by which a signal from the upper part of the plant controls the rate of K+ and NO3− uptake by roots, both uptake of K+ and NO3− and secretion into the xylem of young sunflower plants (Helianthus annuus L.) were measured after changes in light intensity.No close correlation was observed between the uptake of NO3− and that of K+; an increase in light intensity produced a much greater stimulation of NO3− uptake than of K+ uptake. On the other hand, secretion of NO3− into the xylem was tightly coupled to that of K+, and this coupling was strongly disturbed by excision of the root. The results suggest the involvement of the K2-malate shuttle on the regulation by the shoot of K+ and NO3− secretion in the xylem, which is linked to NO3− uptake, while K+ uptake is independent of this regulation mechanism.

Notizen: Two populations of perennial ryegrass (Lolium perenne L.) S23, selected for contrasting rates of yield and mature leal dark respiration, were used in this study. Since previous work showed that yield decreased severely in population GL66 due to mechanical perturbation, possible effects on respiration rates were examined. Apart from a transient increase in population GL72, there was no effect on the respiration rate but mechanical perturbation did affect other processes. Handling caused transpiration ratesinthe light to increas for GL72, and in the dark to decrease for GL66. In the logger term, the nitrogen content decreased in handled plants grown at high density. These results emphasize that handling plants should be reduced to a minimum since it might have major effect on several processes. It is concluded that the original selection for the Lolium population was indeed on contrasting rates in yield and respiration. Furthermore it is argued that the existence of low- and high-yielding genotypes can be attributed to the dissimilar responses of different genotypes to mechanical influence.

Notizen: Given the close relationship between a plant's growth rate and its pattern of biomass allocation and the effects of abscisic acid (ABA) on biomass allocation, we studied the influence of ABA on biomass allocation and growth rate of wildtype tomato (Lycopersicon esculentum Mill. cv. Moneymaker) plants and their strongly ABA-deficient mutant sitiens. The relative growth rate of sitiens was 22% lower than that of the wildtype, as the result of a decreased specific leaf area. The net assimilation rate and the leaf weight ratio were not affected. The mutant showed a much higher transpiration rate and lower hydraulic conductance of the roots. These two factors resulted in sitiens having a significantly lower leaf water potential and turgor. resulting in reduced leaf expansion and, consequently, a lower specific leaf area relative to the wildtype. Addition of ABA to the sitiens roots resulted in phenotypic reversion to the wildtype. We conclude that the influence of ABA-deficiency on biomass allocation and relative growth rate is the result of altered water relations in the plants, rather than of a direct effect on sink strength of different plant organs.

Notizen: Barneix, A. J., Cooper, H. D., Stulen, I. and Lambers, H. 1988. Metabolism and translocation of nitrogen in two Lolium perenne populations with contrasting rates of mature leaf respiration and yield. - Physiol. Plant. 72: 631–636.Several aspects of nitrogen metabolism and transport were investigated to determine whether these processes could account for the observed differences in the dark respiration rate of mature leaves between two populations of Lolium perenne L. cv. S23: GL72 - a slow respiring, high growth rate line, and cv. GL66 - a fast respiring, low growth rate line.No differences were found in total nitrogen or soluble protein concentrations between the populations, but GL72 showed a higher concentration of soluble amino acids, accounted for mainly by increases in the amounts of asparagine and glutamine. There were no differences in the glutamine synthetase (EC 6.3.1.2) or nitrate reductase (EC 1.6.6.1) activities between populations, but the fast respiring GL66 line showed higher glutamate dehydrogenase (EC 1.4.1.3) and peroxidase (EC 1.11.1.7) activities than GL72. The protein turnover rate, determined from 3H disappearance from leaves labelled with [3H]-acetic anhydride, appeared to be larger in GL66, but the difference was not significant and could not account for the differences in respiration rate.The apparent extent of 15N cycling between roots and shoots was low in Lolium compared to other grass species, and there were no differences between the two populations.It is concluded that the differences in dark respiration rate are not due to differences in demand for ATP by nitrogen assimilatory processes, but may be related to faster leaf senescence in the GL66 population.

Notizen: The root system of wheat seedlings (Triticum aestivum L. SUN 9E) was pruned to two seminal roots. One of the roots was supplied with different levels of NO3, the other was deprived of N. Root respiration and the increment of C and N in roots and shoots were measured to determine the C/N ratio of the phloem sap feeding the N-deprived roots. Thus it was possible to determine translocation of N from the shoots to the roots. It was calculated that the C/N ratio of phloem sap feeding roots of plants growing at optimal and suboptimal N supply was ca 54. A supra-optimal N supply reduced, whilst shading increased, the C/N ratio of phloem sap. At optimal N supply 11% of all N transported to the shoots was retranslocated to the roots. Both a supra-optimal and a limiting N supply increased translocation of N back to the roots to 18% of the N translocated to the shoot, whilst shading of the plants decreased the proportion cycled to 7%. At the optimal N supply, 40% more N was translocated to the roots from the shoot than was incorporated by them. At a lower supply of N, 80% more N was imported from the shoots than was incorporated by these roots. It is suggested that the distribution of N between roots and shoots predominantly occurs in the shoots. The specific mass transfer rate in seminal roots was determined. The highest value was found for roots grown with an optimal N supply: 1.1 mg carbohydrate s−1 cm−2 (sieve tube) which is well within the range observed for other plant organs. Roots supplied with NO3 produced more and longer laterals than N-deprived roots. It is suggested that this is due to the effect of NO3 on import of carbon and other components transported in the mass flow with carbon.