Short- and Long-Term Effects of Salinity on Leaf Growth in Rice (Oryza sativaL.)

Abstract

Addition of 50 mM NaCl to Oryza sativa L. had little effect upon the time of leaf initiation, but leaf mortality prior to the normal phase of senescence was increased and the onset of senescence was advanced. There was no significant effect upon the day-to-day pattern of growth, nor upon the ultimate length, of leaves that were developing at the time of, or shortly after, salinization with 50 mM NaCI. Leaves that developed after prolonged exposure of the plants to salinity were shorter. Addition of NaCl, KC₁ or mannitol to the root medium brought about a cessation of leaf elongation within one minute. Growth at a reduced rate restarted abruptly after a lag period that depended upon the external concentration. Elongation rate recovered its original value within 24 h after exposure to 50 mM NaCl, though not at higher concentrations. Addition of NaCl at concentrations up to 100 mM elicited no short-term effects upon photosynthetic gas exchange. Na uptake contributed to osmotic adjustment of the growing zone. When plants were rapidly exposed to 50 mM NaCl, no change in turgor pressure was detectable in the growing zone with the resolution of the miniature pressure probe used (about 70 kPa). It is concluded that the initial growth reduction in rice caused by salinization is due to a limitation of water supply. A clear distinction is made between the initial effects of low salinity which are recoverable and the long-term effects which result from the accumulation of salt within expanded leaves.