A gene encoding a truncated large subunit of Rubisco is transcribed and salt-inducible in rice

Using the rice salt-tolerant mutant 20 as material, a cDNA library was constructed and two salt-inducible clones, SIR5.5 and SIR8.1, were isolated by differential screening. Homology analysis revealed that the two clones together constituted a chimeric rbcL which encoded a truncated large subunit of Rubisco with 337 amino-acids, plus 64 amino-acids of unknown origin. The expressions of both the normal and the chimeric locus appeared to be developmentally regulated and salt-inducible in shoots of the salt-tolerant mutant 20 and its original variety 77–170. In roots, their expressions were salt-inducible in the salt-tolerant mutant 20 whereas no, or only premature, forms were present in the salt-treated original variety 77–170. Higher concentrations of salt reduced the expressions of both normal rbcL and the chimeric locus. ABA showed no effect on their expression.     

Choice of selection environment for improving crop yields in saline areas

Spatial variability in salt-affected fields is normally very high. Thus, most salinity affected lands are actually comprised of many micro-environments, ranging from low to high salinity in the same field. The evidence on testing genotypes across a broad range of salinity levels shows that the genotype-by-salinity level interaction is commonly large. Thus, breeding for saline areas can be compared to what has been known as breeding for wide adaptation. The target environments both for breeding for saline soils or for wide adaptation are actually a population of many possible environments, for which there exists a significant component of genotype-by-environment(G x E) interaction. Thus it is possible to study the merit of potential strategies for breeding for salinity tolerance using the tools that have been developed for the study of breeding for wide adaptation. The evidence from selection and breeding experiments for wide adaptation seems to favour testing on a representative subset of environments, including stress and non-stress locations; but the choice of these locations is complicated by the multidimensional nature of G x E. However, in the case of salt stress, the crop-yield response functions to salinity are well known. This paper presents an attempt to systematise the choice of the optimum environment(s) to select for improved yield under saline soil conditions, based on the three-piece linear equation presented by Maas and Hoffman (1977) and the theory of direct and indirect responses to selection. It is proposed that three saline levels should be enough to make a valid estimation of the suitability of a number of selection strategies. A worked example with data from a set of grain sorghum inbred lines tested on ten saline levels shows that the same selection strategies would be chosen using the information from the ten saline levels as that obtained using the two extremes and one intermediate level.     

Diallel analysis for mineral element absorption in tropical adapted soybeans [Glycine max (L.) Merrill]

The Brazilian tropical adapted soybeans contains, in addition to superior morphological characters, genetic factors for tolerance to cultivation in acidic, mineral-stressed soils. However, the selection process for these hindrances has been empirical, and information on the genetics of mineral element uptake by the plant is necessary. The objective of this investigation was to identify the mode of inheritance for the absorption of phosphorus, potassium, calcium, magnesium, iron, aluminium, manganese, zinc and copper in a 9 × 9 diallel cross. General combining ability (GCA) was higher than specific combining ability (SCA), with the exception of copper, manganese and zinc, indicating predominantly additive effects. The ratios of GCA/SCA varied between 3.4 (calcium) and 8.5 (magnesium). The regression of covariance (W_r) on variance (V_r) showed that the additive-dominance model explained the genetic differences in this germ plasm. However, the detection of overdominance could be related to possible heterozygosity in the parental varieties for mineral absorption. Broad-sense heritability values were higher than narrow sense heritability values for aluminium, iron, potassium, calcium and magnesium, being in the range of 67.9–86.9% and 42.0–56.6%, respectively. This is an indication that soybeans can be further improved to efficient utilisation of nutrients and to tolerate toxic factors in the soil.     

INFLUENCE OF FLUCTUATING PHOSPHATE SUPPLY ON THE REGULATION OF PHOSPHATE UPTAKE BY THE BLUE-GREEN ALGA ANACYSTZS NIDULANS1

The blue-green alga Anacystis nidulans Drouet (Synechococcus leopoliensis Raciborski) cultivated under phosphate-limited conditions adopts a threshold value in the nanomolar range below which uptake ceases. In this study, we investigated the influence of phosphate pulses on the regulation of uptake behavior during reestablishment of the threshold value. Short-term pulses had only a slight effect on uptake kinetics and, hence, on the threshold value, even if the population had been exposed several times to elevated concentrations above the steady-state level in the growth medium. The threshold value was also practically insensitive to the amount of phosphate stored during these short-term fluctuations. Long-term phosphate pulses resulted in a transition to a metastable state that was characterized by a severalfold higher threshold value. This transition, apparently an adaptation to the transiently elevated phosphate concentrations, was further studied by following the influx of ³²P-phosphate at constant external concentrations and was shown to be complete after a period of 10–20 min. After adaptation to pulses, the uptake behavior followed a linear flow-force relation over a wide range of external concentrations. This behavior was explained by the simultaneous operation of at least two uptake systems with different, but coordinated kinetic parameters. This linear flow-force relation facilitated a direct determination of the threshold value from uptake measurements. For applicability in the field the force-flow relation can be a diagnostic tool to assay for fluctuating phosphate and to establish threshold values below the normal measurable range .     

Effects of plant nutrition on the balance of insect relevant cardenolides and glucosinolates in Erysimum cheiranthoides

The possible effects of environmental stress on plant chemistry that are important to herbivorous insects were examined by growing a wild crucifer, Erysimum cheiranthoides, under different nutrient regimes. Oviposition by the cabbage butterfly, Pieris rapae, is thought to be affected by the balance of glucosinolates (stimulants) and cardenolides (deterrents) at the surface of leaves. E. cheiranthoides seedlings were provided with three levels of nitrogen and two levels of sulfur for a period of 15 days before analysis of semiochemicals in whole leaf tissue and at the surface of the foliage. The ratio of cardenolides to glucosinolates in the plants at elevated C/N ratios followed the carbon/nutrient balance hypothesis. However, a high nitrogen supply enhanced biomass production to the extent that concentrations of secondary compounds were unchanged or reduced. The concentration of glucosinolates (glucoiberin and glucocheirolin) at the surface was positively related to whole tissue levels. However, cardenolide (erysimoside and erychroside) concentrations, which were highest in leaf tissue of nitrogen-deficient plants, had the lowest surface levels on foliage of these plants. Possible reasons for differential expression of cardenolides and glucosinolates in a plant as a result of nutrient deficiency are discussed.     

Increased chilling tolerance and altered carbon metabolism in tomato leaves following application of mechanical stress

We investigated the effects of brushing on the chilling tolerance and metabolism of nonstructural carbohydrates (soluble sugars and starch) in tomato leaves before, during and after a chilling stress. Tomato plants ( Lycopersicon esculentum Mill. cv. Caruso) were cultivated either without mechanical stress application (control plants) or with daily brushing treatments for 15 days (brushed plants), prior to a 7-day chilling treatment (8/5°C day/night). Brushing resulted in shorter plants with a 34% reduction in leaf dry weight per area and a 59% reduction of soluble sugars and starch, on a dry weight basis. The sugar to starch ratio was not affected by brushing. A greater chilling tolerance in the brushed plants was demonstrated by the maintenance of a significantly higher PSII efficiency in brushed plants (42%) compared to that of the control plants (30%) after 7 days of chilling treatment, less visible damage to the leaf tissue, and a more rapid resumption of growth during 3 days of recovery as compared to control plants. During the chilling treatment levels of soluble sugars per leaf dry weight increased 15-fold in the brushed plants and 5-fold in control plants. In the present study we have demonstrated that brushing can increase chilling tolerance in tomato plants. The observed differences in chilling tolerance and concentration of soluble sugars in the leaves may indicate an involvement of soluble sugar levels in acclimation to chilling.     

Plant stress and larval performance of a dipterous gall former

According to the plant vigour hypothesis, galling insects should respond positively and perform better on vigorous plants or plant parts, the opposite of the predictions of the plant stress hypothesis. I carried out field experiments to analyse the effects of sustained abiotic stress on the interactions between the common reed (Phragmites australis) and a gall-forming fly (Lipara lucens). The reed shoot diameter (a measure of plant vigour) is strongly affected by environmental conditions, where dry and/or nutrient-poor habitats produce thinner (stressed) shoots. L. lucens gall density is negatively correlated with shoot diameter. In a survival experiment with a wide range of shoot diameters, larval mortality was also highly correlated with shoot quality. Gall formation was higher on thinner, stressed shoots. An analysis of the gall tissues revealed that galls induced by L. lucens contain a high amount of a nutrient-rich feeding tissue. The impact of L. lucens is higher on thinner shoots. The results of this study showed that L. lucens performs better on stressed hosts, which contradicts the plant vigour hypothesis for galling insects. The low nutrient availability in the stressed shoots can be compensated by the production of galls with a nutrient-rich feeding tissue.     

Oxidative stress and antioxidant content in Chlorella vulgaris after exposure to ultraviolet-B radiation

Growth of Chlorella vulgaris was measured in cultures irradiated with 0, 0.8, 2.0 and 4.4 kJ m² UV-B. Growth expressed as chlorophyll content, declined significantly with increased UV-B dose. Ultraviolet-B irradiated cultures in log phase of growth showed a 284% increase in oxygen radical generation and a 145% increase in lipid peroxidation compared with unirradiated cultures, whereas cultures in the stationary growth phase showed no significant changes in these parameters. The activities of superoxide dismutase and catalase increased by 40 and 500%, respectively, after exposure to a UV-B dose of 4.4 kJ m⁻². Contents of the lipophilic antioxidants α-tocopherol and β-carotene increased by 180 and 63 amol cell⁻¹ respectively, between log and stationary phases in unirradiated cultures; but in UV-B-irradiated cultures these increases were significantly depressed. Photoreducing capacities of chloroplasts were decreased following UV-B irradiation of both isolated chloroplasts and those isolated from irradiated algae. Cells exposed to UV-B exhibited increased size and starch accumulation. These results suggest that oxidative stress conditions related to UV-B exposure trigger an antioxidant response that includes an increase in the activity of the antioxidant enzymes (superoxide dismutase and catalase).     

Effects of moderate drought on ascorbate peroxidase and glutathione reductase activities in mesophyll and bundle sheath cells of maize

Mesophyll and bundle sheath cells of maize leaves ( Zea mays L.) both contain the enzymes ascorbate peroxidase (AP; EC 1.11.1.11) and glutathione reductase (GR; EC 1.6.4.2) which are involved in hydrogen peroxide detoxification. Since bundle sheath cells of maize are deficient in photosystem II and have high CO₂ levels, oxidative stress may be less severe in these cells than in mesophyll cells. The present study was conducted to determine if AP and GR activity levels preferentially increase in mesophyll cells relative to bundle sheath cells when plants are subjected to moderate drought. Although drought inhibited the growth of greenhouse-grown plants, it did not affect the levels of protein, chlorophyll or AP. GR was unaffected by drought in whole leaf tissue and mesophyll cells, but did increase slightly in bundle sheath cells. This slight increase is of questionable biological importance. AP and GR activity levels were similar in mesophyll cells, bundle sheath cells and in whole leaf tissue. The data suggest that moderate drought has little effect on enzymes of the hydrogen peroxide scavenging system and that mesophyll and bundle sheath cells may be exposed to similar levels of hydrogen peroxide.