Effect of Progressive Drought on Endoproteolytic Activities and Water Status of Brassica napus Leaves

Rapeseed (Brassica napus L. var. oleifera) is known to display adaptive responses under drought stress, especially in leaves. The aim was to study the influence of water deficit on endoproteolytic activities and water status of leaves developed before (L3 leaf) or after the onset of water stress (L2 and L3 leaves). Although the impact of senescence on proteolytic activities is well known, little information is available on the effect of water deficit on such activities. Seedlings of rapeseed were submitted to controlled progressive drought. Soil water potential (T) and soil dry weight humidity (H) were measured daily in the dark. Endoproteolytic activities were assayed both on control and water‐stressed plants using azocasein as substrate after partial purification of the extracts, Two zones of maximal activity were detected, the first between pH 4.5 and 6.0 and the second at pH 8.0 as a sharp peak, in control as well as in drought stressed‐plants. While LI leaf was very sensitive to water stress and wilted when ψ reached −2.8 to −3.2 MPa, L2 leaf began to display signs of senescence for this value whereas L3 leaf which showed the lowest endoproteolytic activities, remained turgid, hardened and developed a typical bluish coloration. The relation between the acquired resistance of young leaves, the low endo‐proteolytic activity and the induction of a 22 kDa protein under progressive drought is discussed.