The Role of Active and Passive Water Uptake in Maintaining Leaf Water Status and Photosynthesis in Tomato under Water Deficit

Abstract

Tomato (Lycopersicon esculentum Mill. cv. Know-You 301) shoots were grafted onto the rootstock of the same species or Solanum mammosum and grown in nutrient solution. After the grafted tomato plants developed 4–5 leaves, the uppermost fully expanded leaves were used to determine net photosynthetic rate (PN) , transpiration rate and leaf water potential (ΨL) under control (unstressed) and —0.5 MPa water deficit (mannitol was added to nutrient solution). Both PN and leaf conductance (GL) were reduced under water deficit. However, tomato plants grafted onto S. mammosum rootstock had higher ΨL, PN and GL than those grafted onto tomato rootstock under water deficit. This result demonstrates that S. mammosum roots had a greater ability in water uptake under water deficit. Under +0.2 MPa pressure, the root of S. mammosum showed a higher exudation rate than that of tomato. However, the former showed a lower exudation rate than the latter under –0.5 MPa water deficit. It was concluded that the greater ability of water uptake in S. mammosum rootstock under water deficit is related to a lower hydraulic conductivity, which promotes passive, rather than active water uptake.