Rice can acclimate to lethal level of salinity by pretreatment with sublethal level of salinity through osmotic adjustment

The physiological ability to adapt for various environmental changes is known as acclimation. When exposed to sublethal level of stress, plants develop the ability to withstand severe stress, as acquired tolerance. The present study was conducted to explicate the physiological basis of acquired tolerance in rice. Rice seedlings (variety IR 20) were grown in half strength Hoagland solution, and after 22nd day, they were kept in half strength Hoagland solution containing 50 mM NaCl (sublethal dose) for 7 days followed by half strength Hoagland solution containing 100 mM NaCl (lethal dose) for another 7 days. The non-pretreated 29 days old rice seedlings maintained in half strength Hoagland solution were directly transferred to half strength Hoagland solution containing 100 mM NaCl (lethal dose) solution for 7 days. The control plants were maintained in half strength Hoagland solution without NaCl. Various morphological and physiological parameters were recorded on 29th and 36th days old seedlings from control, pretreated and non-pretreated plants. The results revealed significant reduction in growth parameters (shoot length, root length, leaf area and total dry matter production) of non-pretreated plants below that of pretreated plants. The pretreated plants showed increased values to the extreme of 19.8 per cent in leaf water potential (psi(w)), 9 per cent in relative water content (RWC), 26 per cent in photosynthetic rate (P (N)), 28 per cent in leaf stomatal conductance, and 47 per cent in chlorophyll a over non-pretreated plants. The same trend was also observed in chlorophyll a/b ratio (6.6%) and F (v)/F (m) ratio (19.3%). However, a reverse trend was seen in F (o) value. The pretreated plants showed improved ionic regulation as evident from low Na+, Cl- and high K+ contents, which is attributed to enhanced plant water status and photosynthesis. Both pretreated and non-pretreated plants had higher contents of osmolytes viz., sucrose, leaf soluble sugars and proline contents than control plants. However, starch content revealed an inverse trend. Therefore, the present study reveals that rice can acclimate to lethal dose of salinity stress by pretreatment with sublethal dose of NaCl.