Improvement of salt Tolerance in Sunflower (Helianthus Annuus L.) bu Selection and Breeding

Abstract

Some of the major objectives of the present study were: firstly, to assess the amount of variation for salinity tolerance in sunflower, secondly, to determine the extent of variation which is genetically determined, thirdly, to improve the degree of salinity tolerance in sunflower using appropriate selection and breeding techniques, and finally to draw parallels between degrees of salt and various inorganic and organic osmotica. In order to achieve these goals various experiments were conducted step-wise. In the first experiment, forty-five accessions of sunflower collected from different countries, were screened for salinity tolerance after 2 weeks growth in sand culture salinized with 150 meq/l of Nacl+CaCl2 (1:1 ratio equivalent wt. Basis) in half strength Hoagland’s nutrient solution. The results for plant biomass of 45 accessions show that there was considerable variation in salinity tolerance. In a further greenhouse experiment, the salinity tolerance of three tolerant (HO-1, Predovik, Euroflor) and two sensitive (SMH-249UO-985) lines (selected on the basis of their performance in the seedling experiment) was assessed at adult stage to evaluate the consistency of salinity tolerance at different growth stages. All three salt tolerant accessions produced significantly greater plant biomass, seed yield, and seed oil content than the salt sensitive accessions. The tolerant accessions accumulated less Cl- and more K+ in the leaves under saline conditions compared with the salt sensitive accessions. The salt tolerant accessions also maintained relatively high leaf K/Na ratio and K+ versus Na+ selectively. All three tolerant accessions had relatively greater soluble carbohydrates, soluble proteins, total free amino acids and proline in the leaves than the sensitive accessions. A field trial conducted in a salt-affected field confirmed the greenhouse results of the selected accessions. From this study it was clear that salinity tolerance of sunflower does not vary with change in stages of plant cycle, so selection for increased salt tolerance can be carried out at the initial growth stage. Secondly, it was found that there is great variation of salt tolerance in sunflower. Low uptake of Cl-, high uptake of K+, and maintenance of high K/Na ratios and K+ versus Na+ selectivity in the leaves and possibly the accumulation of organic osmotica such as soluble carbohydrates, soluble proteins, proline and free amino acids seem to be important components of salt tolerance in sunflower. In another experiment the effects on two-week-old plants of a salt-tolerant line, Euroflor and a salt-sensitive, SMH-24 of sunflower, of varying Na/Ca ratios in a saline growth medium were assessed after three weeks growth in sand culture under greenhouse conditions. The different Na/Ca ratios of the salt treatment were 36.5, 74.0 and 149 at a constant concentration of 150 mol m-3 NaCl. Euroflor was superior to SMH-24 in fresh and dry matters of shoots and roots at varying external Na/Cl ratios. The leaf Na+ concentration in SMH-24 increased consistently with increase in external Na/Ca ratio, whereas that in Euroflor remained almost unaffected. Although leaf Cl- was significantly greater in SMH-24 than Euroflor, there was no effect of decreasing Ca2+ concentration of saline growth medium on the leaf Cl- concentrations of both lines. The lines did not differ in K+, Ca2+ or Mg2+ concentration of both shoots and roots. The leaf K/Na and Ca/Na ratios, K versus Na selectively were considerably higher in Euroflor than in SMH-24. The lines also did not differ in leaf water potential and gas exchange and these variables were not affected due to decreasing Ca2+ concentration of the saline growth medium. Stomatal conductance and transpiration remained unchanged in Euroflor, whereas those in SMH-24 decreased significantly at the highest external Na/Ca ratio. Euroflor has significantly greater stomatal conductance and transpiration than those of SMH-24 at almost all external Na/Ca ratios, whereas the reverse was true for water use efficiency. It was established that Euroflor was tolerant to low Ca2+ concentration of the saline growth medium as compared to SMH-24. This was mainly attributable to accumulation of relatively low Na and Cl in the leaves, and maintenance of high leaf K/Na and Ca/Na ratios and K versus Na selectively in Euroflor. In a further experiment two lines of sunflower, a salt-tolerant Euroflor and a salt-sensitiveSMH-24, were grown for three weeks in sand culture containing 0 or 150mol m-3 NaCl in full strength Hoagland’s nutrient solution. Distribution of cations in the leaves of varying ages was determined. The older leaves of SMh-24 contained more Na+ in the laminae than the younger leaves at the salt treatment, whereas laminae of leaves varying ages of Euroflor maintained Na+ concentration almost uniform. Distribution of K+, Ca2+ and Mg2+ in the laminae was strongly age-dependent in both lines, i.e., the older leaves contained greater concentration of these cations than those in the younger leaves. The lines did not differ in concentrations of three cations. The older leaves of SMH-24 had significantly lower K/Na ratios than those of Euroflor, but the lines did not differ in lamina Ca/Na ratios. It was concluded that distribution of K+, Ca2+, and Mg2+ in the leaf laminae is age-dependent. Salt tolerance in sunflower is related to exclusion of Na+ in the leaf laminae and to maintenance of almost uniform concentrations of this ion in leaves of all ages. In another experiment the genetic variation for salt tolerance in sunflower was estimated measuring individual plant performance at the vegetative growth stage. Thirty-one half-sib families of a salt -tolerant line, HO-1 and 32 of a salt sensitive line, SMH-24 were chosen randomly. At anthesis each population was separately polycrossed. The progeny of each half-sib family within each population was subjected to 0 (control) or 125 mol m-3 NaCl in full strength Hoagland nutrient solution. Shoot fresh and dry weight data were used to estimate narrow-sense heritability’s within each population. Salt tolerance was heritable, with narrow-sense heritability’s estimated at 0.46 to 0.72 in SMH-24 and 0.28 to 0.47 in HO-1. Differences in heritability estimates could be related to the differential salt tolerance of the two cultivars. These results suggest that a significant advance in salt tolerance in sunflower would be expected through further cycles of selection. In another experiment five thousand seeds of variable material (HO-1 and Euroflor) of sunflower (Helianthus Annuus) were screened at the seedling stage after three weeks growth in saline sand culture containing 250 mol m-3 NaCl in full strength Hoagland’s nutrient solution. Sixty-three and 81 seedlings of HO-1 and Euroflor respectively were selected on the basis of their markedly longer shoot and root lengths. These selected seedlings were grown in normal soil compost till maturity. These selected lines were then compared with the respective unselected lines (base population) in sand culture at varying salt concentrations and also in naturally saline conditions. The selected lines excelled the unselected line in yield in both greenhouse and field studies. In conclusion it was found that a significant advance in salinity tolerance of sunflower is possible through appropriate selection and breeding techniques since the genetic component of variation for salt tolerance is very high in the crop. Low accumulation of chloride and high uptake of K are some of the important adaptive components of salt tolerance in sunflower.