BIOCHEMICAL INDICATORS OF SALINITY TOLERANCE IN THE HALOPHYTE Prosopis strombulifera ARE DIFFERENTIALLY AFFECTED BY NaCl AND Na2SO4

LUNA, MARÍA VIRGINIA; LLANES, ANALÍA SUSANA; BERTAZZA, GIANPAOLO; REINOSO, HERMINDA; REGINATO , MARIANA ANDREA

Denizli

Workshop; International Workshop of urbanization, land use, land degradation and environment; 2009

P. strombulifera is an interesting model to study plant tolerance to extreme salinity conditions. Growth parameters, pigments and ion content were diferentially affected by iso-osmotic concentrations of NaCl, Na2SO4 and their combination. Growth was stimulated up to o ?1.9 MPa (500 mM) NaCl, but this species was much less tolerant to Na2SO4 showing growth inhibition and toxicity symptoms. When both salts were combined, the growth curve was intermediate between those with each monosaline solution showing toxicity alleviation. Total leaves area decreased under all treatments at low osmotic potential. P. strombulifera is anphystomatic, showing greater stomata number on adaxial surface. High levels of Na2SO4 caused an increment in stomatal density with lower ostiolar diameter, mainly on the abaxial leaf surface. These morphological adjustments would allow a more efficient CO2 distribution into the mesophyll when stomatal conductance is low. NaCl and Na2SO4 also caused different effects on ion accumulation by plants suggesting specific SO4= effect on membrane permeability. Na+ accumulation under both salts was very high in leaves, being greater with NaCl (21g/100g DW at o ?2,6 MPa). Na2SO4 caused greater Na+ accumulation in roots during the whole experiment. It is remarkable that at the NaCl concentration in which plants showed optimal growth (500 mM) Na+ and K+ content in leaves was iqual (16g/100 g DW) but Cl- content was 5g/100 g DW. K+ accumulation decreased with salt increase in all salt solutions. Salinity notably lowered Ca2+ content favoring Na+ accumulation in treated plants, mainly those under SO4= effects. Cl- accumulation in NaCl monosaline treated plants reached its maximum at o - 2.6 MPa (700 mM): 8g/100g DW in leaves and 5,5g/100g DW in roots. SO4= content in Na2SO4 treated plants was greater in leaves: 6,7g/100g DW at o - 2.6 MPa (530 mM) where accumulated from the beginning of salinization. Thus, anion accumulation was much lower than cation content suggesting that non-ionic compounds may account for osmorregulation and electrochemical balance. Aminoacids, proteins, sugars, polyalcohols and polyamines were determined. High proline accumulation with all treatments was considered a stress metabolic signal not necessarily indicating salt tolerance. There was no increase in glycinbethaine levels in response to any salt. Only NaCl caused extra total protein synthesis at ψo -2.6 MPa; SO4= treated plants had reduced protein content while bisaline treated plants had an intermediate value. Total soluble carbohydrates increased proportionally to salinity but changes in the partitioning root/shoot were observed. Polyols analysis showed mannitol accumulation at moderate and high salinity only in NaCl treated plant leaves, while SO4= and bisaline treated plant leaves produced sorbitol; in the presence NaCl there was no sorbitol production. Inositol was not detected with any treatment suggesting a rapid conversion to pinitol, which accumulated near 280% more in all treatments than in controls. Thus, sorbitol synthesis maybe a manifestation of carbon metabolism disorder under SO4= treatments, and the combination of mannitol and pinitol, a potential biochemical indicator of salt tolerance in this species. Putrecine content in leaves increased considerably in all salinity treatments, being noticeable that cadaverine and 1, 3-diaminopropane, uncommon polyamines, were detected in leaves and roots exposed to the highest salinities. ABA levels varied according to the type of salt, its concentration, organ analyzed and age of the organ. With all treatments ABA decreased in roots and increased in leaves, being the highest levels those found in control plants. The lowest ABA levels were found in roots at ?1.8 MPa NaCl, coincident with maximum seedling growth (halophytic response). Leaves from Na2SO4 treated plants showed high ABA levels coincident with growth inhibition and toxicity symptoms. Carotene peaks coincided with ABA peaks suggesting that the severe stress caused by Na2SO4 demanded for ABA precursors. ABA-GE was the main metabolite found in all plants; instead, DPA levels were very low and PA was not detected in most cases. Highest ABA-GE levels detected in roots under 700 mM Na2SO4 (Ψo= -2.6 MPa), confirm previous observations that ABA-GE would be the stress signal transported to the leaves, in which the highest free ABA levels were measured. Evidence presented here shows the inconvenience of extrapolating results obtained by using NaCl as the sole salinizing agent to field conditions.