Restrictions to carbon dioxide conductance and photosynthesis in spinach leaves recovering from salt stress

Salt accumulation in spinach (Spinacia oleracea L.) leaves first inhibits p hotosynthesis by decreasing stomatal and mesophyll conductances to CO, diff usion and then impairs ribulose-1,5-bisphosphate carboxylase/oxygenase (S. Delfine, A. Alvino, M. Zacchini, F. Loreto [1998] Aust J Plant Physiol 25: 395-402). We measured gas exchange and fluorescence in spinach recovering f rom salt accumulation. When a 21-d salt accumulation was reversed by 2 week s of salt-free irrigation (rewatering), stomatal and mesophyll conductances and photosynthesis partially recovered. For the first time, to our knowled ge, it is shown that a reduction of mesophyll conductance can be reversed a nd that this may influence photosynthesis. Photosynthesis and conductances did not recover when salt drainage was restricted and Na content in the lea ves was greater than 3% of the dry matter. Incomplete recovery of photosynt hesis in rewatered and control leaves may be attributed to an age-related r eduction of conductances. Biochemical properties were not affected by the 2 1-d salt accumulation. However, ribulose-1,5-bisphosphate carboxylase/oxyge nase activity and content were reduced by a 36- to 50-d salt accumulation. Photochemical efficiency was reduced only in 50-d salt-stressed leaves beca use of a decrease in the fraction of open photosystem II centers. A reducti on in chlorophyll content and an increase in the chlorophyll a/b ratio were observed in 43- and 50-d salt-stressed leaves. Low chlorophyll affects lig ht absorptance but is unlikely to change light partitioning between photosy stems.