Jm. Kinet

Effect of salt and osmotic stresses on germination in durum wheat (Triticum durum Desf.)

In order to determine the relative importance of ionic toxicity versus the osmotic component of salt stress on germination in durum wheat (Triticum durum Desf.), seeds of three cultivars differing in their salt and drought resistance (Omrabi-5, drought-resistant; Belikh, salt-resistant and Cando, salt-sensitive) were incubated in various iso-osmotic solutions of NaCl, mannitol and polyethylene-glycol (PEG) (osmotic potential of −0.15 (control solution) −0.58, −1.05 or −1.57 MPa). Moderate stress intensities only delayed germination, whereas the highest concentration of NaCl and PEG reduced final germination percentages. PEG was the most detrimental solute, while mannitol had no effect on final germination percentages. All osmotica reduced endosperm starch and soluble sugars content as well as α-amylase activities recorded after 48 h of treatment while β-amylase activities were, in contrast, slightly stimulated in all cultivars. Deleterious effects of NaCl and PEG were higher on isolated embryos germinated onto an in vitro Linsmaier and Skoog (LS) medium comparatively to whole seeds. All PEG-treated embryos, however, recovered after the stress relief while NaCl-treated embryos exhibited a lower rate of recovery and some extent of abnormal germination after rinsing. It was concluded that stress inhibition of germination could not be attributed to an inhibition of mobilisation of reserves and that the main effect of PEG occurred via an inhibition of water uptake while detrimental effects of NaCl may be linked to long-term effects of accumulated toxic ions. The behaviour of the three cultivars during germination did not fully reflect their mean level of putative stress resistance in field conditions and germination is, therefore, not recommended as a reliable selection criterion for breeding purposes.

Effects of salt stress on growth, mineral nutrition and proline accumulation in relation to osmotic adjustment in rice (Oryza sativa L) cultivars differing in salinity resistance

Salt-resistant rice cultivars Nona Bokra and IR 4630 exposed at the seedling stage during one or two weeks to 0, 20, 30, 40 or 50 mM NaCl accumulated less Na, Cl, Zn and proline and more K at root and shoot levels than salt-sensitive I Kong Pao and IR 31785. Aiwu, a moderately resistant genotype, exhibited an intermediate behaviour. P transport from root to shoot was inhibited in the most sensitive cultivar IR 31785. Accumulation of Na and Cl and decrease in K content at the shoot level were restricted to the oldest leaves in salt-resistant genotypes while proline accumulated in the youngest leaves in all cultivars. In the presence of NaCl, the osmotic potentials of the roots and of the oldest and youngest leaves were lower in the salt-resistant than in the salt-sensitive genotypes, differences among genotypes increasing with stress intensity. Proline did not appear to be involved in osmotic adjustment in salt-stressed rice plants and the significance of its accumulation is discussed in relation to salinity resistance.

Effects of various salts and of mannitol on ion and proline accumulation in relation to osmotic adjustment in rice (Oryza sativa L.) callus cultures

Mature embryo-derived calli were obtained from three rice (Oryza sativa L.) cultivars differing in salinity resistance at the whole plant level [(I Kong Pao (IKP), (salt-sensitive), Aiwu (moderately resistant) and Nona Bokra (salt-resistant)] and exposed to three iso-osmotic concentrations of NaCl, KCl, Na2SO4, artificial Sea water (ASW) and mannitol. Relative growth rates, ion content and proline accumulation were quantified after 1, 2 and 3 months of stress. hmong salt treatments, KCI was the most detrimental to callus growth in all genotypes and induced a strong increase in Cl content. The NaCl-induced inhibition of growth was lower in calli issued from Nona Bokra than in calli obtained From IKP; Na and CI accumulations as well as internal osmotic potential were lower in Nona Bokra and in Aiwu, thus suggesting a cellular component of salt resistance in these genotypes. Na obvious differences in growth were recorded among genotypes upon mannitol treatment, which appeared more detrimental than NaCl. Proline accumulation was higher in the salt-sensitive cultivar IKP, whatever the nature of the stressing agent: or the stress intensity, and did not appear to be involved in osmotic adjustment. It was concluded that specific ion toxicities are important aspects of salt stress effects on rice cells and chat proline is a symptom of injury in stressed rice calli rather than an indicator of resistance.

Effect of water stress on growth, Na+ and K+ accumulation and water use efficiency in relation to osmotic adjustment in two populations of Atriplex halimus L.

The effect of water stress on growth, Na+ and K+ accumulation and water utilization was investigated in plants of two populations of Atriplex halimus L. originating from Kairouan (Tunisia) and Tensift (Morocco). Water deficit was applied by withholding water for 22 days. All plants remained alive until the end of the treatment although growth was strongly reduced in both populations. Water stress decreased CO2 assimilation in saturating conditions, mainly in the population obtained from Kairouan, suggesting an impact of drought on the dark phase of photosynthesis, beside a decrease in stomatal conductance which was recorded mainly in the population obtained from Tensift. The two studied populations did not differ in their water consumption, as indicated by similar soil gravimetric water content and plant transpiration. However, water use efficiency increased under stress conditions in the population from Tensift but not in the population from Kairouan. Thelatter population displayed a larger capacity for osmotic adjustment. A drought-induced specific increase in Na+ concentration was also reported in both populations. It is concluded that in A. halimus, water stress resistance estimated in terms of biomass production, could be associated with higher WUE rather than with with a greater osmotic adjustment and that sodium may assume a specific physiological function in this xerohalophytic C4 species.

Characterization of rice (Oryza sativa L.) F3 populations selected for salt resistance. I. Physiological behaviour during vegetative growth

Plants belonging to a somaclonal family (R3-1-23, obtained from a salt-resistant callus of the salt-sensitive cultivar I Kong Pao (IKP, salt-sensitive)) were crossed with this intital cultivar IKP or with plants of the elite breeding line IR31785 (IR31, extremely salt-sensitive). Two other crosses were also performed: IR31 × IKP and IKP × Aiwu, where Aiwu is a moderatly salt-resistant cultivar. The physiological behaviour of F3 populations was analysed after 21 and 42 days of exposure to 0 or 30 mM NaCl and compared to the behaviour of plants selected within parental populations. A clear improvement in terms of survival percentages was demonstrated for plants issued from IR31 × 1-23 and IR31 × IKP. These F3 populations exhibited lower Na accumulation in shoots, higher Ca accumulation in roots, higher ratios of shoot P/root P than the parents after 42 days of salt exposure. Salt stress induced a decrease of shoot K, an increase of shoot Cl and proline and a decrease of osmotic potential in all populations. Correlation analysis indicated that no physiological parameter recorded in the absence of salt stress may be used as a reliable selection marker to predict the plant behaviour in salt stress conditions. K/Na ratio recorded in the stressed plants was significantly correlated with surviving percentages recorded after 90 days of salt stress, suggesting that this physiological parameter in stress conditions appears to be the most reliable selection criterion for salt resistance in rice.

Compared effects of sudden and progressive impositions of salt stress in three durum wheat (Triticum durum Desf.) cultivars

The effect of sudden vs. progressive exposure to salt stress at the seedling stage was investigated in three durum wheat cultivars differing in their mean level of sale and drought resistance. Both procedures of stress imposition induced a decrease in relative growth rates, K concentrations and leaf osmotic potential values, as well as an increase in Na, proline and soluble sugar contents. Growth inhibition was largest in the drought-resistant cultivar Omrabi 5, suggesting that drought and salt-resistance are not necessarily linked in Triticum durum. Suddenly-applied NaCl often appeared more detrimental than progressive exposure. Quantitative differences between the effects of the two modes of stress imposition suggested the existence of a precocious adaptative mechanism in response to progressive exposure but varied according to the physiological parameter used to evaluate plant behaviour, the final dose of NaCl and the genotype. The most salt-resistant cultivar (Belikh) was the less influenced by the kinetics of stress application. The modification of the mineral nutrition- and osmotic adjustment-related parameters is discussed in relation to the specific physiological strategy of salinity resistance postulated in Triticum durum and in light of the contrasting evidence in the literature.

Selection of Al-resistant plants from a sensitive rice cultivar using somaclonal variation, in vitro and hydroponic cultures

A selection procedure is described that isolated Al-resistant individuals from a sensitive rice (Oryza sativa L.) genotype. Somaclonal variation was used as the only source of variability and selection pressures were applied at both callus and regenerated plant levels. When 10 week-old, embryo-derived calli were submitted to different Al stresses for a period of 20 weeks. After this in vitro stress period, selected calli were grown during 18 weeks, without selection pressure, on regeneration media. Selection pressures were applied on regenerated plants (R0), and 9 R0 plants which produced seeds were selected. The transmission of the Al-resistant character to the R1, R2, R3 and R4 generations was then investigated, and three plant lines which had an increased percentage of Al-resistant plants till the fourth generation of self-pollination, were identified. One was from a callus maintained on an Al-free medium. The efficiency of in vitro selection pressures was therefore debated.

Evaluation of chilling sensitivity in different rice varieties. Relationship between screening procedures applied during germination and vegetative growth

SummaryRice varieties were screened for chilling tolerance during germination and vegetative growth using different techniques. The effects of temperature during germination were investigated from 10 to 25°C. The screening was most effective at 10°C. Time of data collection has to be considered in order to discriminate slow germinating from chilling sensitive varieties. Chilling survival tests (10°C) applied at the 2-leaf stage revealed that different chilling durations brought complementary results: the longer treatments allowed to identify the most tolerant varieties, while shorter treatments allowed to distinguish intermediate from most sensitive rices. Results of chilling tests applied at the 2- and the 8-leaf stages showed a high correlation. Low-temperature electrolyte leakage and low-temperature fluorescence were also highly correlated to each other and to survival tests applied at the same development stage. By contrast, weak correlations were observed between low-temperature germination and plantlet chilling survival. It is concluded that low-temperature electrolyte leakage and low-temperature fluorescence are good indicators of chilling tolerance during the vegetative growth. The possibility to use a chilling screening procedure applied at this stage in order to predict the chilling tolerance at other development stages is discussed.

Improvement of rice callus regeneration in the presence of NaCl

The effects of abscisic acid (37.8 μM), polyethylene glycol (5%), proline (10 mM), tryptophan (490 μM) and indoleacetic acid (5.7 μM) on rice callus regeneration were studied at various doses of NaCl (0, 50 and 100 mM) on three month-old mature embryo-derived callus of two japonica (I Kong Pao and Aiwu) and two indica (IR 2153 and Nona Bokra) rice cultivars differing in salinity tolerance. NaCl strongly decreased the regeneration frequency of all cultivars but slightly increased the survival of regenerated plantlets. Tryptophan stimulated regeneration and increased subsequent survival rates of regenerated plantlets in all cultivars at all NaCl doses. Abscisic acid and polyethylene glycol, though not affecting the final regeneration percentages, delayed regeneration and reduced the mean number of plantlets produced per regenerating callus in all cultivars, as well as rooting ability and survival of regenerated plantlets in indica genotypes. Proline had no marked effect on regeneration, whatever the NaCl dose or cultivar, while indoleacetic acid reduced shoot regeneration and increased root regeneration.

Evaluation of drought resistance-related traits in durum wheat somaclonal lines selected in vitro

Somaclonal variation associated with in vitro selection has been used as a source of variability to improve drought resistance of 3 durum wheat (Triticum durum Desf.) cultivars (Selbera, Sebou, and Kyperounda). In a previous study, R0 plants with improved drought resistance-related characters were regenerated after selection on culture media containing polyethylene glycol (PEG). This improvement was transmitted to the R1 progeny. The present study analysed the behaviour of the selected tissue culture-derived lines in subsequent R2, R3 and R4 generations. Differences in electrolyte leakage, chlorophyll fluorescence (F-v/F-m), stomatal conductance and days to heading were found between the parental cultivars and most of their in vitro-derived lines. The changes may differ from one cultivar to another. Many promising somaclonal lines still presented improvement for at least 3 of the 4 parameters measured comparatively to initial cultivars. Somaclonal variation thus appears to induce a wide range of modifications among individual components of drought-resistance mechanisms. These improved traits could be valuable if shown to be inherited and to give enhanced agronomic performances in future field studies.