Carbon Dioxide Assimilation and Foliar Ion Concentrations in Leaves of Lemon (Citrus limon L.) Trees Irrigated With Nacl or Na₂SO₄

Abstract

An attempt was made to differentiate between the possible effects of high Cl^- or Na⁺ ions on lemon leaf photosynthesis by treating grafted (Citrus limon (L.) Burm. f. cv. 'Taylor') plants with either NaCl or Na₂SO₄ to establish different foliar concentrations of Cl^- or Na⁺. The rootstocks, Rangpur lime (C. reticulata var. austera hybrid?) and rough lemon (C. jambhiri), were chosen because Rangpur lime is known to be a good Cl^- 'excluder' and rough lemon to be a poor Cl^- 'excluder'. The grafted plants were initially treated for 7 weeks with nutrient solution (control) or nutrient solution containing 50 mol m^-3 NaCl or 25 mol m^-3 Na₂SO₄, after which time there were only marginal reductions in both photosynthetic rates and shoot growth, with the exception of 'Taylor' lemon on rough lemon roostock treated with NaCl where growth was affected more severely than the other treatments. Salinity levels were then doubled to 100 mol m^-3 NaCl and 50 mol m^-3 Na₂SO₄ and the plants treated for a further 8 weeks, causing significant increases in leaf Na⁺ and/or Cl^- concentrations. Mature, 3-4-month-old leaves of 'Taylor' lemon on Rangpur lime rootstock treated with Na₂SO₄ for this period, contained c. 105 mol m^-3 Na⁺ and c. 10 mol m^-3 Cl^- and had photosynthetic rates 60% lower than controls. Similar reductions in assimilation rate were seen in leaves on rough lemon roostock treated with NaCl containing c. 195 mol m^-3 Cl^- and c. 35 mol m^-3 Na⁺. Smaller (35%) but significant reductions in assimilation rate were observed for trees on rough lemon roostock treated with Na₂SO₄, where the tagged leaves contained c. 40 mol m^-3 Na⁺ and 50 mol m^-3 SO₄^2-. Leaf nitrogen concentrations were significantly reduced by treatment of trees with NaCl or Na₂SO₄ but, apart from small reductions in chlorophyll concentrations, there were no visible symptoms of nitrogen deficiency. Leaf turgor was not adversely affected. The data indicate that both Cl^- and Na⁺ can reduce assimilation rates in salt-stressed lemon leaves, possibly because of poor compartmentation within leaves of either ion. Increases in leaf Na⁺ and Cl^- accounted for 54-96% of the reduction in osmotic potential in 'Taylor' lemon leaves on trees treated with NaCl, whereas increases in Na⁺ and SO₄^2- accounted for 33-71% of the reduction in osmotic potential in leaves on trees irrigated with Na₂SO₄. The greater increase in Cl^- compared with the net increase of [Na⁺ + K⁺] in 'Taylor' leaves on rough lemon rootstocks was offset (65%) by reductions in malic and succinic acids. Proline increased significantly only in 'Taylor' leaves on Rangpur lime roostock treated with Na₂SO₄.