Salinity increases CO2 assimilation but reduces growth in field-grown, irrigated rice

Salinity is a major yield-reducing factor in coastal and arid, irrigated ri ce production systems. Salt tolerance is a major breeding objective. Three rice cultivars with different levels of salt tolerance were studied in the field for growth, sodium uptake, leaf chlorophyll content, specific leaf ar ea (SLA), sodium concentration and leaf CO2 exchange rates (CER) at photosy nthetic active radiation (PAR)-saturation. Plants were grown in Ndiaye, Sen egal, at a research station of the West Africa Rice Development Association (WARDA), during the hot dry season (HDS) and the wet season (WS) 1994 unde r irrigation with fresh or saline water (flood water electrical conductivit y = 3.5 mS cm(-1)). Relative leaf chlorophyll content (SPAD method) and roo t, stem, leaf blade and panicle dry weight were measured at weekly interval s throughout both seasons. Specific leaf area was measured on eight dates, and CER and leaf sodium content were measured at mid-season on the first (t opmost) and second leaf. Salinity reduced yields to nearly zero and dry-mat ter accumulation by 90% for the susceptible cultivar in the HDS, but increa sed leaf chlorophyll content and CER at PAR- saturation. The increase in CE R, which was also observed in the other cultivars and seasons, was explaine d by a combination of two hypotheses: leaf chlorophyll content was limited by the available N resources in controls, but not in salt-stressed plants; and the sodium concentrations were not high enough to cause early leaf sene scence and chlorophyll degradation. The growth reductions were attributed t o loss of assimilates (mechanisms unknown) that must have occurred after ex port from the sites of assimilation. The apparent, recurrent losses of assi milates, which were between 8% and 49% according to simulation with the cro p model for potential yields in irrigated rice, ORYZA S, might be partly du e to root decomposition and exudation. Possibly more importantly, energy-co nsuming processes, such as osmoregulation, interception of sodium and potas sium from the transpiration stream in leaf sheaths and their subsequent sto rage, drained the assimilate supply.