Salt and drought stress differentially affect the accumulation of extracellular proteins in barley

Barley (Hordeum vulgare) was grown for eight days in the presence of a rang e of salt concentrations or subjected to repeated cycles of wilting and reh ydration. Changes in apoplastic protein content, protein pattern, enzymic a ctivities and ion composition were investigated under salinity and drought. The protein content of intercellular washing fluid (IWF) increased 2.5- to 3.0-fold when the NaCl concentration in the growth medium was increased fr om 0 to 100 mM. The elevated protein content was the result of a general in crease in most polypeptides and a pronounced increase in the abundance of s pecific polypeptides of apparent molecular masses of 15, 21, 22, 26, 36, 40 and 62 kDa. Conversely, the IWF protein content decreased during wilting s imilar as after application of colchicin, cytochalasin B or cycloheximide s uggesting that inihibition of protein synthesis or vesicle transport may be the cause for the decrease in apoplastic protein content and enzyme activi ties in dehydrating plant tissue. The changes in apoplastic protein content were accompanied by stress-specific alterations in activities of apoplasti c enzymes. The greater apoplastic protein content was the consequence of st imulated protein synthesis in the presence of NaCl, as evidenced by increas ed incorporation of [S-35]-methionine into IWF protein. The results demonst rate that the leaf apoplast is a compartment which sensitively and differen tially responds to drought and salinity with consequences for plant growth.