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.