Plant and Soil 259: 153–162, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 153 Gas exchange, water relations and osmotic adjustment in two scion/rootstock combinations of Prunus under various salinity concentrations Rossano Massai 1 , Damiano Remorini 1 & Massimiliano Tattini 2,3 1 Dipartimento di Coltivazione e Difesa delle Specie Legnose, Sezione Coltivazioni Arboree “G. Scaramuzzi”, Universit` a degli Studi, Via del Borghetto 80, I-56124, Pisa, Italy. 2 Istituto per la Valorizzazione del Legno e delle Specie Arboree (IVALSA), Consiglio Nazionale delle Ricerche, Via Madonna del Piano, I-50019, Sesto F.no, Firenze, Italy. 3 Corresponding author ∗ Received 26 November 2002. Accepted in revised form 24 September 2003 Key words: chloride, leaf water and osmotic potential, net assimilation rate, sodium, sorbitol, stomatal conductance Abstract Growth, gas exchange and water relations have been studied on hydroponically grown peach (cv. Armking) plants, grafted on GF 677 (Arm/GF) and Mr.S. 2/5 (Arm/MrS), exposed to 0, 40, 80 and 120 mM NaCl concentration, over a four-week period. Plant performance was also evaluated during a subsequent four-week period of relief from stress, by supplying the plants with a complete nutrient solution. Salinity stress reduced growth and net assimilation rate to a greater extent in Arm/GF than in Arm/MrS plants. Salt-induced water stress was more severe in Arm/GF than Arm/MrS leaves, and particularly during the first two weeks of treatment. On the other hand, leaf osmotic potential at full turgor ( π FT ) of salt-treated Arm/MrS received a markedly greater contribution from Na + and Cl − than salt- treated Arm/GF plants. By contrast, divalent cations and K + made a substantially greater contribution to leaf  π FT of salt-treated Arm/GF than to the corresponding  π FT of Arm/MrS plants. Salinity stress markedly altered leaf carbohydrate composition and led to a preferential accumulation of sorbitol, independent on the scion/rootstock combination. Salt-treated Arm/GF but not Arm/MrS plants stored most of the Na + and Cl − loaded in the shoot, into basal (old) leaves. The rootstock ability to control the accumulation of salt in the scion leaves during the salinity stress, determined striking differences in the recovery of photosynthetic performances, during the relief period. A full recovery of A and g s was detected in both 40 and 80 mM salt-treated Arm/GF leaves, but only in 40 mM salt-treated Arm/MrS, at the end of the relief treatment. These data indicate a greater efficiency of GF 677 than Mr.S. 2/5 in protecting peach plants from the deleterious effects of NaCl stress. Introduction An excess of soil salinity affects the performances of hundreds of crop species worldwide, and may be par- ticularly deleterious for most of the fruit-tree crops, which are usually salt-sensitive (Ziska et al., 1990; Banuls and Primo-Millo, 1995; Storey, 1995). A high salinity concentration at root zone salinity may often occur in Mediterranean areas during the long summer season, as a result of high temperatures and both re- duced water availability and quality of irrigation water ∗ E-mail: m.tattini@ivalsa.cnr.it (Margaris, 1981; Tattini et al., 2002). On the other hand, leaching of salts has become an expensive or even an unrealistic horticultural practice (Aboub El- Kashab et al., 1997). The effect of excess salinity at the root zone may markedly decrease the performances of fast-growing Prunus spp., like peach (Tattini, 1990), since high growth and transpiration rates have been shown to be negatively related with the salt exclusion efficiency from the actively growing organs of the shoot (Flowers and Yeo, 1989, 1995; Moya et al., 1999). In “salt- tolerant” fruit-tree crops, like olive, the ability to