Leaf Water Potential and Gaseous Exchange of Wheat and Tomato as Affected by NaCl and P Levels in the Root Medium

The phosphorus nutritional status of some plants has been shown to influence the plant's response to salinity. The objective of this study was to determine if this interaction influenced the plant's water relations, fixation of CO₂, and yield. Tomato (Lycopersicon esculentum Mill.) and wheat (Triticum aestivum L.) were grown to maturity in nutrient solution cultures containing low to excessive concentrations of NaCl and P. Leaf total water potential (ᴸψₜ) and its components, osmotic (ᴸψₒ) and pressure (ᴸψₚ), decreased for both crops as salinity levels increased. Increases in the P concentration tended to increase ᴸψₜ and ᴸψₒ but decrease ᴸψₚ, although the changes were not always consistent. Interactions between salinity and P treatments caused significant differences in ᴸψₜ and ᴸψₚ for wheat. Statistically significant multiple regression equations were developed for ᴸψₜ, ᴸψₒ, and ᴸψₚ as a function of NaCl and P concentrations for both crops. Plant yields were well-correlated with ᴸψₜ and ᴸψₒ, indicating that these measurements may provide a good characterization of salt stress. Measurements of leaf stomatal diffusion resistance to water vapor (rH₂O) and carbon dioxide (rCO₂) and calculated values of mesophyll resistance (rₘ) increased significantly as salinity increased for tomato. Although the resistance values for wheat also increased as salinity increased, only the rCO₂ measurements rose significantly. The significant increases in rCO₂ and rₘ for tomato with additions of salinity indicate a significant reduction in the rate of photosynthesis. Increasing P values consistently reduced the gaseous resistance levels although the reductions were not always statistically significant. There were no significant interactions between salinity and P treatments for any of these resistance measurements.