T. McNeilly

Salinity Tolerance in Brassica Oilseeds

Brassica oilseed species now hold the third position among oilseed crops and are an important source of vegetable oil. The most common Brassica oil-seed crops grown for commercial purposes are rape seeds, (Brassica campestris L. and B. napus L.) and mustards (B. juncea (L.) Czern. & Coss. and B. carinata A.Br.). The other Brassica species such as B. nigra (L.) Koch and B. tournefortii Gouan are grown on a very small scale. Brassica napus, B. juncea, and B. carinata are amphidiploids, whereas B. campestris and B. nigra are diploid. Most of the Brassica species have been categorized as moderately salt tolerant, with the amphidiploid species being the relatively salt tolerant in comparison with the diploid species. Due to the higher salt tolerance of the amphidiploids, it has been suggested that their salt tolerance has been acquired from the A (B. campestris) and C (B. oleracea L.) genomes. However, significant inter- and intraspecific variation for salt tolerance exists within brassicas, which can be exploited through selection and breeding for enhancing salt tolerance of the crops. There are contrasting reports regarding the response of these species to salinity at different plant developmental stages, but in most of them it is evident that they maintain their degree of salt tolerance consistently throughout the plant ontogeny. The pattern of uptake and accumulation of toxic ions (Na+ and Cl−), in tissues of plants subjected to saline conditions appears to be mostly due to mechanism of partial ion exclusion (exclusion of Na+ and/or Cl−) in most of the species, although ion inclusion in some cases at intraspecific levels has also been observed. Maintenance of high tissue K+/Na+ and Ca2 +/Na+ ratios has been suggested as an important selection criterion for salt-tolerance in brassicas. Osmotic adjustment has also been reported in Brassica plants subjected to saline conditions, but particularly to a large extent in salt-tolerant species or cultivars. The roles of important organic osmotica such as total soluble sugars, free amino acids, and free proline, which are central to osmotic adjustment, have been discussed. In canola, B. napus, no positive relationship has been observed between salt tolerance and erucic acid content of seed oil in different cultivars. Furthermore, glucosinolate content of the seed meal in canola generally increases with an increase in salt level of the growth medium. This review highlights the responses of potential Brassica crops to soil salinity from the whole plant to the molecular level. It also describes the efforts made during the past millennium in uncovering the mechanism(s) of salinity tolerance of these crops both at the whole plant and cellular levels. The important selection criteria, which are used by researchers to enhance the degree of salinity tolerance in brassicas, are summarized. In addition, the vital role of genetic engineering and molecular biology approaches to the improvement of salt tolerance in brassicas is emphasized.

Responses of four Brassica species to sodium chloride

Abstract Responses in sand culture of four Brassica species, B. campestris, B. carinata, B. juncea and B. napus to NaCl were assessed for growth and yield. B. napus produced significantly greater fresh and dry biomass than the other three species in NaCl treatments and also a higher seed yield in both absolute and relative terms when grown at 125 mol/m3 NaCl. In contrast, B. campestris had the lowest biomass production, seed yield and oil content. B. campestris did not differ significantly from B. juncea and B. carinata in biomass production for growth, but B. carinata had a significantly higher seed yield (1.60 g/plant) than that of B. juncea and B. campestris (0.69 and 0.61 g/plant, respectively). The higher seed yielding species, B. napus (1.74 g/plant) and B. carinata (1.60 g/plant) accumulated lower amounts of Na+ and Cl− in their tissues and had significantly higher K selectivity (SK,Na) in their shoots than did those of B. campestris and B. juncea, B. napus and B. carinata also had higher leaf succulence than that of B. campestris. In view of the results presented here it can be concluded that B. napus and B. carinata are relatively tolerant to NaCl whereas B. campestris and B. juncea are relatively sensitive to NaCl.

The effect of zinc contamination from electricity pylons. Genetic constraints on selection for zinc tolerance

Zinc tolerance, based on root growth in zinc-containing solutions, was examined in samples of 30 Agrostis capillaris L. individuals collected from populations beneath five electricity pylons in North Wales. The populations were found to vary in zinc tolerance. Three of them contained individuals whose tolerance ranged from low to high, one population consisted of individuals with low to moderate tolerance, whilst the fifth population (ZK-180) contained no tolerant individuals. Differences in zinc tolerance were also found between seedlings raised from five seed populations collected from plants growing in the uncontaminated pasture areas adjacent to four of those pylons, and one further pylon in the same area. The population adjacent to pylon ZK-180 consistently showed little sign of tolerant individuals. Samples of seeds collected from the populations in the uncontaminated areas surrounding each pylon, and likely to have been the origin of the pylon populations were grown at a concentration of 15 μg Zn cm−3, and root lengths of 105 randomly chosen seedlings from each were measured. The means and variance of tolerances of four of the five did not differ, but all were significantly higher than that adjacent to pylon ZK-180. Two cycles of selection and breeding for increased zinc tolerance were carried out using seed samples collected from four of the five populations of adults collected beneath pylons examined previously, and including population ZK-180. Marked increases in tolerance were effected in three of the four populations; no corresponding increase in tolerance was achieved for population ZK-180. It is concluded from these findings that the lack of zinc tolerance beneath pylon ZK-180 is due to the absence of appropriate variation in zinc tolerance within that population, and in potential colonizing material adjacent to it, a situation which contrasts with that for the other three pylons.

Accumulation of amino acids, proline, and carbohydrates in response to aluminum and manganese stress in maize

Abstract Synthesis of amino acids, proline, and carbohydrates was studied in roots and shoots of 5 maize accessions, differing in aluminum (Al) and manganese (Mn) tolerance, in response to Al and Mn stress at the seedling stage in solution culture. The concentrations of these metabolites increased in roots and shoots of the seedlings in the nutrient solution with added Al (0.22 mM), and Mn (2.0 mM). Both Al and/or Mn tolerant and non‐tolerant accessions accumulated more metabolites under stress than control. Generally, the tolerant accessions accumulated more solutes than the non‐tolerant maize accessions examined.

Assessment of variability in salt tolerance based on seedling growth in Triticum durum Desf.

Variability in response to salinity was examined in 29 Triticum durum Desf. accessions or cultivars based upon relative root and shoot lengths of 14-day-old seedlings grown in control and 100 mM NaCl solutions. NaCl caused a significant reduction of root and shoot growth for all accessions/cultivars, but the degree of reduction differed between them. Some cultivars had significantly greater relative root and shoot lengths than others, suggesting that there may be potentially useful variability in salinity tolerance within the durum wheats.

Salinity effects on five cultivars/lines of pearl millet (Pennisetum americanum [L] leeke)

The effects of increased salinity [NaCl + CaCl2] on seedlings of five accessions of pearl millet grown for 2 and 7 weeks, respectively, in salinised solution cultures at EC 0.6, 4, 8, 12, 16, and 20 ds m−1 and sand cultures at EC 0.6 and 20 were assessed. There were no consistent relationships between seedlings characters at two and seven weeks in response to increased salinity, and no single character provided an acceptable means of differentiating cultivar/line response. The line having lower shoot mortality had a high root weight, a shoot:root ratio approaching 1, the greatest shoot water content, and the greatest plant height. No relationship was found between these whole-plant characters which suggest greater salinity tolerance, and the pattern of ion distribution, particularly Na+ and Cl−. Sufficient inter-cultivar/line variation in response to salinity was found to suggest that selection of individuals with increased salinity tolerance is possible within pearl millet.

Variation in growth responses to availability of water in Cistus albidus populations from different habitats

Seeds of Cistus albidus L. plants from three populations that are exposed to differing temperature and precipitation in Almería province of south-east Spain, were collected and grown together in a factorial experiment with two irrigation treatments. The aim was to determine whether populations from different habitats differed when exposed to common conditions, or differed in the plasticity of their response to availability of water. Significant differences in growth of branches and in leaf dimensions were found between treatments, indicating phenotypic plasticity. There was also significant variation between populations in growth of branches and leaf dimensions, with a population from a location that is intermediate in terms of precipitation and temperature showing the greatest growth of branches and production of leaves under the well-watered treatment. This population is from a semi-arid climate, where precipitation is unpredictable, and selection may have occurred to favour rapid growth when water is available. This population had the narrowest leaves under both treatments, and the lowest leaf mass in the well-watered treatment. It also maintained the same mass per leaf under the two treatments, whereas the others showed an increased mass of leaves with increased availability of water. Thus, populations differed both in their manner of allocating resources and their response to availability of water.

Between and within cultivar variability in salt tolerance in lucerne, (Medicago sativa L.)

SummaryThe response of two-week-old seedlings of 35 cultivars of lucerne (Medicago sativa L.) to increasing NaCl concentrations was assessed in solution culture under growth room conditions. Shoot length, used as the criterion for assessing salt tolerance, was found to decrease significantly with increasing NaCl concentration for all cultivars, but there was considerable variation in response between, and within cultivars. It is concluded that selection between and within cultivars should lead to increased salt tolerance in this species.

Tolerance ofHolcus lanatus andAgrostis stolonifera to sodium chloride in soil solution and saline spray

SummaryInland and sea cliff populations of bothAgrostis stolonifera L. andHolcus lanatus L. were subjected to soil NaCl treatments, of 100 and 200 mol m−3 NaCl, and tolerance examined using plant dry weight data. A parallel experiment subjected them to salt spray treatments of 2.5%, 5% and 10% NaCl in distilled water, and tolerance assessed from leaf damage.Both populations of each species were equally sensitive to soil NaCl. When subjected to salt spray the sea cliff populations however showed marked resistance to leaf damage. Soil salinity resistance and salt spray resistance thus appear to be independent characteristics in these two species.

Variability in aluminium and manganese tolerance among maize accessions

Tolerance to aluminium and manganese toxicity at the seedling stage for 72 maize accessions was examined in solution culture. 0.22 mM Al and 2.0 mM Mn gave better genotypic separation for aluminium and manganese tolerance assessed on the basis respectively of relative root length, and visual symptoms of leaf chlorosis and necrosis. There was considerable variability among accessions for tolerance to aluminium and to manganese. Three accessions, Bozm 1335, Bozm 1337, and Bozm 1536 showed tolerance to Al, while 4, Chzm 01009, Champ, Bozm 0715, LG 20.80 exhibited tolerance to Mn. Accession, Zea 769 was tolerant to both metals. A significant Al x Mn interaction was found when five accessions were grown in a mixture of 0.22 mM aluminium and 2.0 mM manganese. Root length inhibition in Al alone was slightly ameliorated when the accessions were grown in the Al + Mn solution. Tolerance to aluminium and manganese does not necessarily coincide, different mechanisms being involved in tolerance to the two metals.