Osmotic adjustment in <i>Lycopersicon esculentum</i> and <i>L. Pennellii</i> under NaCl and polyethylene glycol 6000 iso–osmotic stresses

Abstract

Changes in leaf solute contents in response to saline (NaCl) and osmotic (polyethylene glycol, PEG, 6000) stresses were measured in three different salt tolerant cultivars of Lycopersicon esculentum (L.) Mill. (Pera, P‐73 and Volgogradskij), and its wild relative L. pennellii (Correll) D'Arcy accession PE‐47. Iso‐osmotic stresses (–0. 5 MPa) of NaCl (140 m M ) and PEG 6000 (150 g l ‐1 ) were applied to one‐month old plants for 3 weeks. Decreasing leaf dry weight was similar in L. pennellii or L. esculentum cv. P‐73 and Volgogradskij under both stresses, while leaf dry weight of L. esculentum cv. Pera decreased more under PEG stress than under NaCl stress. Water contents decreased in all the PEG treated populations, while their calculated solute potential (Ψ s increased. Under osmotic stress, the total ion contents decreased in relation to control, whereas organic solutes (sugars, amino acids and organic acids) markedly increased in both tomato species, specially in the tomato cultivars, where these solutes represented 50% of the Ψ 5 calculated. Soluble sugar increase was three times higher in leaves of L. esculentum than in the leaves of L. pennellii . Free proline increased under both stresses and its content was highest in L. esculentum and in L. pennellii , respectively, under NaCl and PEG stresses. Nevertheless, the contribution of this metabolite to Ψ s did not exceed 5%, irrespective of treatment and species. The greater organic solute accumulation in L. esculentum than in L. pennellii – which was not reflected in their Ψ 5 values – was not correlated with the tolerances of the two species to osmotic stress. Therefore, osmotic adjustment may not be the only process influencing salt and drought tolerances in tomato; the ability of plants to regulate their metabolic and physiological functions could also play an important role under these harmful conditions. The possible roles of inorganic solutes and metabolites in osmotic adjustment, energetic metabolism and redox regulation are discussed