Karl Dörffling

Leaf K/Na ratio predicts salinity induced yield loss in irrigated rice

Salinity is a major constraint to irrigated rice production, particularly in semi-arid and arid climates. Irrigated rice is a well suited crop to controlling and even decreasing soil salinity, but rice is a salt-susceptible crop and yield losses due to salinity can be substantial. The objective of this study was to develop a highly predictive screening tool for the vegetative growth stage of rice to estimate salinity-induced yield losses. Twenty-one rice genotypes were grown over seven seasons in a field trials in Ndiaye, Senegal, between 1991 and 1995 and were subjected to irrigation with moderately saline water (3.5 mS cm-1, electrical conductivity) or irrigation with fresh water. Potassium/sodium ratios of the youngest three leaves (K/NaLeaves) were determined by flame photometry at the late vegetative stage. Grain yield was determined at maturity. All cultivars showed strong log-linear correlations between K/NaLeaves and grain yield, but intercept and slope of those correlations differed between seasons for a given genotype and between genotypes. The K/NaLeaves under salinity was related to grain yield under salinity relative to freshwater controls. There was a highly significant correlation (p < 0.001) between K/NaLeaves and salinity-induced grain yield reduction: the most susceptible cultivars had lowest K/NaLeaves and the strongest yield reductions. Although there were major differences in the effects of salinity on crops in both the hot dry season (HDS) and the wet season, the correlation was equally significant across cropping seasons. The earliest possible time to establish the relationship between K/NaLeaves under salinity and grain yield reduction due to salinity was investigated in an additional trial in the HDS 1998. About 60 days after sowing, salinity-induced yield loss could be predicted through K/NaLeaves with a high degree of confidence (p < 0.01). A screening system for salinity resistance of rice, particularly in arid and semi-arid climates, is proposed based on the correlation between K/NaLeaves under salinity and salinity-induced yield losses.

Biosynthesis and metabolism of abscisic acid in tomato leaves infected with Botrytis cinerea

Two virulent strains of Botrytis cinerea Pers., one of them (Bc 6) producing abscisic acid (ABA) via 1′,4′-trans-diol-ABA in defined liquid culture, and a second strain (Bc 9) without the ability to form ABA or its fungal precursor, and two near-isogenic lines of tomato were used to study the biosynthesis and metabolism of ABA in infected isolated leaves. The tomato plants used were Lycopersicon esculentum Mill. cv. Ailsa Craig (wild type) and the ABA-deficient mutant flacca. The level of 1′,4′-trans-diol-ABA increased in Ailsa Craig and flacca leaves in a similar pattern to about 4 μg·(gDW)−1 after conidiospore infection with Bc 6, but not after infection with Bc 9. Pulse-feeding experiments showed that [214-C]-1′,4′-trans-diol-ABA was metabolised to ABA and to further plant metabolites of ABA (phaseic acid, dihydrophaseic acid and polar compounds) in both uninfected and infected leaves. Following infection, the turnover of 1′,4′-trans-diol-ABA was reduced. The level of endogenous ABA in leaves infected with the ABA-producing strain Bc 6 rose more than tenfold in Ailsa Craig and twofold in flacca, respectively. Infection of Ailsa Craig leaves with Bc 9 caused a fivefold increase in ABA, and no increase of ABA in flacca. It is concluded that at least four processes control the level of ABA in wild-type tomato leaves infected with Botrytis cinerea: stimulation of fungal ABA biosynthesis by the host; release of ABA or its precursor by the fungus; stimulation of biosynthesis of plant ABA by the fungus; inhibition of its metabolism by the fungus. Application of ABA together with fungal spores to tomato leaves caused a faster development of necrotic leaf area than spore inoculation only.

Salinity increases CO2 assimilation but reduces growth in field-grown, irrigated rice

Salinity is a major yield-reducing factor in coastal and arid, irrigated rice production systems. Salt tolerance is a major breeding objective. Three rice cultivars with different levels of salt tolerance were studied in the field for growth, sodium uptake, leaf chlorophyll content, specific leaf area (SLA), sodium concentration and leaf CO2 exchange rates (CER) at photosynthetic active radiation (PAR)-saturation. Plants were grown in Ndiaye, Senegal, at a research station of the West Africa Rice Development Association (WARDA), during the hot dry season (HDS) and the wet season (WS) 1994 under irrigation with fresh or saline water (flood water electrical conductivity = 3.5 mS cm-1). Relative leaf chlorophyll content (SPAD method) and root, stem, leaf blade and panicle dry weight were measured at weekly intervals throughout both seasons. Specific leaf area was measured on eight dates, and CER and leaf sodium content were measured at mid-season on the first (topmost) and second leaf. Salinity reduced yields to nearly zero and dry-matter accumulation by 90% for the susceptible cultivar in the HDS, but increased leaf chlorophyll content and CER at PAR- saturation. The increase in CER, which was also observed in the other cultivars and seasons, was explained by a combination of two hypotheses: leaf chlorophyll content was limited by the available N resources in controls, but not in salt-stressed plants; and the sodium concentrations were not high enough to cause early leaf senescence and chlorophyll degradation. The growth reductions were attributed to loss of assimilates (mechanisms unknown) that must have occurred after export from the sites of assimilation. The apparent, recurrent losses of assimilates, which were between 8% and 49% according to simulation with the crop model for potential yields in irrigated rice, ORYZA S, might be partly due to root decomposition and exudation. Possibly more importantly, energy-consuming processes, such as osmoregulation, interception of sodium and potassium from the transpiration stream in leaf sheaths and their subsequent storage, drained the assimilate supply.

Effect of abscisic acid on the transport of assimilates in barley

The effect of abscisic acid (ABA) on assimilate transport in barley was investigated in two parallel experiments. First, the effect upon [14C]sucrose transport from the flag leaf to the ear of a single ABA application made at different stages of growth of the fruits was investigated; the effect was measured 24 h after treatment. Second, the effect of a single application of ABA made at the same stages of growth as above on grain weight of the mature plant was investigated. In both types of experiments ABA was applied once to the ear of different plants as an aqueous solution (10-3–10-5 M), one to five weeks after anthesis. [14C] sucrose was applied by means of agar blocks. Parallel to these experiments, the endogenous content of ABA was investigated in the developing grains. When ears were treated with ABA two or four weeks after anthesis, an increase of up to 70% in the 14C-transport from the flag leaf to the ear was observed within a 24-h period after treatment (short duration experiments). At these growth stages the endogenous concentrations of ABA were low. In sharp contrast, ABA, especially in a concentration of 10-3 M, decreased 14C-import from the flag leaf when applied three weeks after anthesis. At this stage the endogenous ABA content had reached its maximum. Long duration experiments with a single application of ABA to the car two weeks after anthesis resulted in a marked increase of weight per thousand kernels. ABA applications made earlier or later than two weeks after anthesis either reduced the grain weight or had no effect. It is concluded that ABA is involved in the regulation of assimilate transport from the leaves to the grains, possibly by influencing the unloading of sieve tubes in the ears. Promotion or inhibition of assimilate import by exogenously applied ABA may depend on the developmental stage of the grains and on the endogenous ABA level.

Genotype‐specific differences in chilling tolerance of maize in relation to chilling‐induced changes in water status and abscisic acid accumulation

Four inbred maize lines differing in chilling tolerance were used to study changes in water status and abscisic acid (ABA) levels before, during and after a chilling period. Seedlings were raised in fertilized soil at 24/22°C (day/night), 70% relative humidity. and a 12-h photoperiod with 200 μmol m-2 s-1 from fluorescent tubes. At an age of 2 weeks the plants were conditioned at 14/12°C for 4 days and then chilled for 5 days at 5/3°C. The other conditions (relative humidity, quantum flux, photoperiod) were unchanged. After the chilling period the plants were transferred to the original conditions for recovery. The third leaves were used to study changes in leaf necrosis, ion efflux, transpiration, water status and ABA accumulation. Pronounced differences in chilling tolerance between the 4 lines as estimated by necrotic leaf areas, ion efflux and whole plant survival were observed. Conditioning significantly increased tolerance against chilling at 5/3°C in all genotypes. The genotypes with low chilling tolerance had lower water and osmotic potentials than the more tolerant genotypes during a chilling period at 5/3°C. These differences were related to higher transpiration rates and lower diffusive resistance values of the more susceptible lines. During chilling stress at 5/3°C ABA levels were quadrupled. Only a small rise was measurable during conditioning at 14/12°C. However, conditioning enhanced the rise of ABA during subsequent chilling. ABA accumulation in the two lines with a higher chilling tolerance was triggered at a higher leaf water potential and reached higher levels than in the less tolerant lines. We conclude that chilling tolerance in maize is related to the ability for fast and pronounced formation of ABA as a protective agent against chilling injury.

Importance of abscisic acid for chilling tolerance of maize seedlings

Summary A hypothesis developed in two earlier papers (Capell and Dorffling 1993, Janowiak and Dorffling 1996), but critically reinvestigated recently by Ristic et al. 1998 was tested again with new material and methods. According to the hypothesis, chilling tolerant maize genotypes accumulate abscisic acid (ABA) faster and in higher amounts than chilling sensitive genotypes when exposed to chilling stress. This relationship is in accordance with a suggested protective role of ABA against chilling injury. The hypothesis was tested in two ways: first by investigating 20 new maize genotypes with defined differences in chilling tolerance, second by manipulating the endogenous level of ABA during a low temperature treatment by applying the ABA biosynthesis inhibitor norflurazon and measuring the resulting chilling tolerance. Results of the first approach showed that the accumulation of ABA in the third leaves determined by an indirect ELISA after two days of chilling at 4 °C was significantly correlated with chilling tolerance as measured by necrotic injury and by ϕ PSII of the 20 tested genotypes. Accumulation of ABA induced by low temperature was significantly higher in the group of chilling tolerant genotypes than in that of chilling sensitive ones. It was inversely related to chilling-induced water loss. Results of the second experiment showed that norflurazon-treated seedlings were less chilling tolerant and accumulated less ABA than untreated ones. Application of ABA compensated the reduction in chilling tolerance by norflurazon. It is concluded that the results support the above mentioned hypothesis that chilling tolerance in maize is related to the ability to accumulate ABA as a protective agent against chilling injury.

In vitro-selection and regeneration of hydroxyproline-resistant lines of winter wheat with increased proline content and increased frost tolerance

Summary An embryogenic callus obtained from immature embryos of a Finnish winter wheat ( Triticum aestivum 1. cv. Jo 3063) was used for in vitro -selection of hydroxyproline (Hyp) resistant calli. Calli were plated on solid Gamborg B5 medium containing 10–20mM Hyp and 2mgL −1 2,4-D. From 6018 embryogenic calli exposed to Hyp in the course of three subcultures nine calli proved to be Hyp-resistant and remained viable and embryogenic. These were transferred to a regeneration medium and finally to Gamborg B5 medium before being transplanted to soil. The regenerated plants were grown at 18°C for 6 weeks and then cold hardened at 2°C for 18 weeks. The mean osmotic potential of the Hyp-resistant cold hardened regenerates was significantly lower than that of hardened controls. At the same time their mean proline content and their mean frost tolerance were significantly higher compared with regenerated controls.

Abscisic acid and water transport in sunflowers

The role of abscisic acid (ABA) in the transport of water and ions from the root to the shoot of sunflower plants (Helianthus annuus) was investigated by application of ABA either to the root medium or to the apical bud. The exudation at the hypocotyl stump of decapitated seedlings was measured with and without hydrostatic pressure (0–0.3 MPa) applied to the root. All ABA concentrations tested (10-10–10-4 mol·l-1) promoted exudation. Maximal amounts of exudate (200% of control) were obtained with ABA at 10-6·mol·l-1 and an externally applied pressure of 0.1 MPa. The effect was rapid and long-lasting, and involved promotion of ion release to the xylem (during the first hours) as well as an increase in hydraulic conductivity. Abscisic acid applied to the apical bud had effects similar to those of the rootapplied hormone. Increased rates of exudation were also obtained after osmotic stress was applied to the root; this treatment increased the endogenous level of ABA in the root as well as in the shoot. Water potentials of the hypocotyls of intact plants increased when the roots were treated with ABA at 5°C, whereas stomatal resistances were lowered. The results are consistent with the view that ABA controls the water status of the plant not only by regulating stomatal transpiration, but also by regulating the hydraulic conductivity of the root.

Changes in the contents of free and conjugated abscisic acid, phaseic acid and cytokinins in xylem sap of drought stressed sunflower plants

Abstract Xylem sap was obtained by pressurizing the root system of sunflower plants ina Scholander bomb. It was found to contain besides abscisic acid (ABA) and cytokinins [determined as zeatin (t-Z) plus zeatin riboside (t-ZR) and isopentenyladenine (2iP) plus isopentenyladenosine (2iPA)], the ABA metabolite phaseic acid (PA) and the conjugated forms of both ABA and PA. The concentrations of ABA, PA and their conjugated forms increased during a drought stress period, while that of the cytokinins decreased.

Effect of new terpenoid analogues of abscisic acid on chilling and freezing resistance

Summary The effect of two new terpenoid analogues of abscisic acid of the acetylene-acetate type coded LAB 144143 and LAB 173711 on chilling and freezing resistance was studied in comparison with that of abscisic acid. The analogues were applied to cucumber plants for tests of chilling resistance, and to wheat plants and wheat cell suspensions for tests of freezing resistance. The analogues increased chilling resistance as well as freezing resistance in a manner comparable to ABA. LAB 144143 proved to be nearly as active as ABA, whereas LAB 173711 was less active. The mechanisms of action of ABA and the analogues as protectants against chilling were studied in more detail in cucumber. It seems to involve several processes: stomatal closure which reduces water loss during chilling, reduction of phospholipid degradation, and promotion of the proline level. Possibilities for practical use of these ABA analogues in horticulture and agriculture are discussed.