Gerard M. Bögemann

Survival tactics of Ranunculus species in river floodplains.

Abstract The flooding resistance of four Ranunculus species was studied under controlled conditions and related to the tactics used by these species to survive in their natural habitat in river floodplains. R. bulbosus, a species from seldom-flooded river levées, was relatively intolerant of both waterlogging and complete submergence, due to a constitutively low level of aerenchyma in the root system. This lack of gas spaces resulted in high mortality rates during flooding treatments and an inability to use photosynthetically derived oxygen for root respiration during complete submergence. The pioneer R. sceleratus, predominantly abundant in low lying mudflats, was very resistant to waterlogging and shallow floods. Due to its constitutively high root porosity and its ability to greatly increase the elongation rate of petioles under water this species can ameliorate flooding stress. However, when leaf blades of R. sceleratus were unable to reach the water surface, this species died as quickly as the flooding-intolerant R. bulbosus. This indicates that fast elongation of petioles under water competes for energy and respirable reserves with maintenance processes. R. repens, a species from lower, frequently inundated floodplains, was very tolerant of prolonged waterlogging and submergence. Its high resistance to complete submergence under continuous darkness indicates that this species tolerates hypoxic and/or anoxic tissue conditions via metabolic adjustments. Lysigenous aerenchyma was also induced in the primary root system and in newly developed laterals, and it was able to use oxygen generated by underwater photosynthesis, for root respiration. R. acris, a species from less frequently flooded areas, was as resistant to waterlogging and submergence in the light as R. repens. However, it has a lower resistance than R. repens to complete submergence in the dark. A submergence pre-treatment increased the maximum net underwater photosynthetic rate in R. bulbosus, whereas a significant decrease of light compensation points was observed in R. repens when it had previously been submerged. This study shows that Ranunculus species exhibit various strategies to cope with different flooding conditions. R. repens responds to flooding by its tolerance mechanism and R. sceleratus by avoidance. R. acris ameliorates submergence and R. bulbosus was not able to adapt high water tables.

Measurement of porosity in very small samples of plant tissue

The relative volume of internal gas spaces (i.e., porosity) of the shoot and roots of a plant largely determines its resistance to flooding, as oxygen may diffuse through these cavities from non-flooded parts of the plant into the submerged tissues. The current techniques to measure porosity either need relatively large amounts of plant tissue (200 mg per sample), or are time-consuming and not sufficiently accurate for specific types of plant material. These limitations were the reason to develop a new method of porosity measurement. Small segments of roots were taken from freshly harvested plants, placed in a two-piece hard gelatin capsule and weighed on a microbalance. The root segments were subsequently infiltrated with water under vacuum, blotted carefully and weighed again. Using the increase in weight and the specific weight of infiltrated tissue, derived from a larger sample of roots, it was possible to calculate the porosity of individual root segments as small as 3–5 mg with a length of 5 mm. The new method combines this use of small samples with a high accuracy, and proved useful for a variety of plant species. Porosity data obtained with this method will improve our knowledge of small-scale processes such as aerenchyma development in root tips.

Ethylene Emission and Responsiveness to Applied Ethylene Vary among Poa Species That Inherently Differ in Leaf Elongation Rates

A plants ability to produce and respond to ethylene is essential for its vegetative growth. We studied whole-shoot ethylene emission and leaf growth responses to applied ethylene in fourPoa spp. that differ inherently in leaf elongation rate and whole-plant relative growth rate. Compared with the fast-growingPoa annua and Poa trivialis, the shoots of the slow-growing species Poa alpina and Poa compressa emitted daily 30% to 50% less ethylene, and their leaf elongation rate was more strongly inhibited when ethylene concentration was increased up to 1 μL L−1. To our surprise, however, low ethylene concentrations (0.02–0.03 μL L−1) promoted leaf growth in the two slow-growing species; at the same concentrations, leaf elongation rate of the two fast-growing species was only slightly inhibited. All responses were observed within 20 min after ethylene applications. Although ethylene generally inhibits growth, our results show that in some species, it may actually stimulate growth. Moreover, in the two slow-growingPoa spp., both growth stimulation and inhibition occurred in a narrow ethylene concentration range, and this effect was associated with a much lower ethylene emission. These findings suggest that the regulation of ethylene production rates and perception of the gas may be more crucial during leaf expansion of these species under non-stressful conditions and that endogenous ethylene concentrations are not large enough to saturate leaf growth responses. In the two fast-growing species, a comparatively higher ethylene endogenous concentration may conversely be present and sufficiently high to saturate leaf elongation responses, invariably leading to growth inhibition.

Flooding resistance of Rumex species strongly depends on their response to ethylene : Rapid shoot elongation or foliar senescence

Rumex palustris is a flooding-resistant amphibious species from frequently flooded riversides, whereas Rumex acetosella is flooding-sensitive and grows on dry sandy soils. Upon complete submergence, both species accumulate ethylene to similar levels. After more than four days, however, the ethylene concentration in R. acetosella plants strongly rises to an extremely high level, whereas it remains much lower in R. palustris plants. This latter species responds to ethylene with enhanced leaf elongation, whereas elongation in R. acetosella is insensitive to ethylene. Elongation rates of leaves were measured continuously during the first 8 h of submergence. A comparison of the elongation rates of R. palustris, R. acetosella and silver-treated R. palustris plants demonstrated that R. palustris plants responded to ethylene within 1 h of submergence. In R. acetosella, clear symptoms of senescence and decay were observed within two weeks of submergence. In R. palustris plants, only the oldest leaf was senescent. To investigate the role of ethylene in the senescence process, the effects of silver ions on submerged plants, and the effects of prolonged exposure to an extremely high ethylene level on drained plants were studied in both Rumex species. The results demonstrated that although ethylene accelerated senescence of submerged R. acetosella plants, the process may have been caused by other factors. The slower senescence of R. palustris plants could not be explained by their lower ethylene concentration. Rather, it was caused by a much lower sensitivity of the senescence process to ethylene. Moreover, other factors may be less unfavourable in R. palustris than in R. acetosella plants under submerged conditions.

A lack of aerenchyma and high rates of radial oxygen loss from the root base contribute to the waterlogging intolerance of Brassica napus

The morphology and physiology of the response of two cultivars of Brassica napus to an anaerobic root medium was investigated. The cultivars Chikuzen and Topas showed a large reduction in growth rate when their roots were exposed to a de-oxygenated stagnant nutrient solution containing 0.1% w/v agar. Older seedlings (11 d old) were more sensitive to stagnant agar, expressed as biomass accumulation, than younger ones (5 d old). Brassica napus was characterized by a constitutively low root porosity (3–5%), typical for plant species with a low tolerance to waterlogging. A hypoxia pre- treatment (16 h; 2.25% O2) before exposure to de-oxygenated stagnant agar had no effect on the final number or length of lateral roots and adventitious roots. Brassica napus cv. Chikuzen is characterized by radial oxygen loss being most at the basal portion of the root, when a strong oxygen sink surrounds the root. Oxygen profiles through laterals of Brassica napus cv. Chikuzen show a typical pattern with low oxygen concentrations in the stele and somewhat higher levels in the cortex. Despite the continuum of intercellular air spaces in the root cortical tissue the lack of aerenchyma and therefore low rates of internal oxygen diffusion restricts root growth in anaerobic media and presumably contributes to the sensitivity of Brassica napus to waterlogging.

Partial root drying effects on biomass production in Brassica napus and the significance of root responses

Partial root drying (PRD) has been shown to stimulate stomatal-closure response and improve water-use efficiency and thus biomass production and grain yield under water deficiency. While most studies focus on above-ground responses to PRD, we examined how root responses contributed to effects of partial root drying. In particular, in two experiments with oilseed rape (Brassica napus L.) we investigated whether roots were able to forage for patchily distributed water, and how this affected plant growth compared with uniform watering and alternate watering (in which different parts of the roots receive water alternately). The first pot experiment was carried out in the greenhouse and the second outside under a rain-shelter in which also the watering amount was varied. The results indicate that B. napus roots were able to forage for fixed water patches by selective root placement. In the first experiment with small plants, root foraging was equally effective as enhanced water-use efficiency under alternate watering. Both treatments resulted in about 10% higher shoot biomass compared with uniform watering. Alternate watering generally outperformed uniform watering in the second experiment, but the success depended on the time of harvest and the water supply level. Measurements indicated that only the alternate watering regime effectively reduced stomatal conductance, but lead to a higher shoot biomass only under more severe (50%) rather than under milder water deficiency (70% of a well watered control). Water deficiency strongly reduced leaf initiation rates and leaf sizes in B. napus, but for a given level of water supply the supply pattern (uniform control, fixed patchy or alternate watering) hardly influenced these growth parameters. Although also in the second experiment, the plants selectively placed their roots in the wet parts of the pot, root foraging was not as effective as in the first experiment. Possible reasons for these discrepancies are discussed as well as their implications for the application of PRD effects for crop growth.

Separation of chloroplast polar lipids and measurement of galactolipid metabolism by high-performance liquid chromatography

Procedures are described for the separation of polar lipids from plant chloroplasts by high-performance liquid chromatography, using a polar-modified silica column. Glycolipids and phospholipids were eluted with a gradient of 2-propanol/n-hexane (80:55, v/v) and 2-propanol/n-hexane/water/methanol (80:55:15:10, v/v). The lipids were detected by uv absorbance at 202 nm. Diacylglycerol and mono-, di-, and trigalactosyldiacylglycerol and phosphatidylcholine were separated on a LiChrosorb NH2 column (7-microns particles, Merck, FRG), but acidic lipids were retained. These lipids could be quantified from their 202-nm absorbance recording. The absorption coefficients obtained depended on the mean number of double bonds in the different lipid classes. The separation was applied for a rapid monitoring of the lipid composition in thylakoids and in fractionated inner and outer envelopes. The activities of galactosyltransferases involved in galactolipid metabolism, UDPGal:diacylglycerol galactosyltransferase and galactolipid:galactolipid galactosyltransferase, could be measured quantitatively in specific assays for both enzymes.

Spinach chloroplasts: localization of enzymes involved in galactolipid metabolism

Abstract A shortened procedure for the separation of spinach chloroplast envelope membranes is presented, resulting in high yields of enriched inner and outer envelopes. Fractionated envelopes were tested for activities in galactolipid synthesis by measuring incorporation of UDPGal and by using a specific assay for galactolipid: galactolipid galactosyltransferase (GGGT); additionally, acyl-CoA synthetase activities were tested. UDPGal incorporation and GGGT were found predominantly in enriched inner envelopes, acyl-CoA synthetase was predominantly in enriched outer envelopes. These observations point to a localization of both UPDGal: diacylglycerol galactosyltransferase (UDGT) and GGGT in the inner membrane. Alternatively, chloroplasts were incubated with [14C]acetate under conditions permitting fatty acid synthesis in the presence and absence of UDP[3H]Gal. After incubation, envelope fractions were isolated and analyzed for labeled lipids. Analysis agreed with localization of UDGT in the inner envelope. However, labeled products of GGGT were found both in enriched inner and outer envelopes. Furthermore, inactivation of GGGT by the proteinase thermolysin indicates a location on the cytosolic face of the chloroplast. These apparently contradictory results concerning GGGT might be explained by the involvement of contact sites between inner and outer envelopes or by lipid transformation during membrane fractionation.

Root foraging and yield components underlying limited effects of Partial Root-zone Drying on oilseed rape, a crop with an indeterminate growth habit

We report on two experiments with oilseed rape (Brassica napus L.) to test if partial root-zone drying techniques improve yield in a crop in which vegetative and reproductive growth overlap (indeterminate growth habit), and to investigate what plant morphological responses contribute to the yield that is realized. Deficit irrigation resulted in smaller plants with smaller yields but larger seeds compared to treatments with shallow groundwater (first experiment) and with fully watered conditions (second experiment). Different partial root-zone drying treatments (water supply patterns) under deficit irrigation, however, had little effect on plant growth and yield components (number of branches, branch lengths, number of pods, etc.). Our results suggest that partial root-zone drying doesn’t work well with oilseed rape. Detailed measurements of soil water contents and root distribution indicate that roots were extremely plastic, effectively foraging for water, and these root responses may have overwhelmed physiological effects of partial root drying on the shoot. Furthermore, in crops with indeterminate growth with a short vegetative growth phase, partial root-zone drying may be ineffective in enhancing the major yield components. Further reasons for the lack of success are discussed.

Galactosyltransferase Activities in Intact Spinach Chloroplasts and Envelope Membranes

Two galactosyltransferases producing galactolipids have been studied extensively in chloroplast envelope membranes (1). UDPGal: diacylglycerol galactosyltransferase (UDGT) produces monogalactosyldiacylglycerol (MGDG) (2); and galactolipid: galactolipid galactosyltransferase (GGGT) producess di-, tri- and tetragalactosyldiacylglycerol (DGDG, TGDG and TeGDG) (3-6). In the isolated envelope membranes, UDGT and GGGT co-operate in the production of MGDG, DGDG, TGDG and TeGDG (4, 5).