Expansion of Bean Leaves and its Suppression by Salinity

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Abstract
Growth of the first trifoliolate leaf of bean (Phaseolus vulgaris) was studied on plants grown in control and saline (72 meq of NaCl/liter) nutrient solutions. Growth was measured as an increase in the length, weight, DNA, RNA and protein of lateral leaflets. DNA content was used as a measure of cell number. Areal growth of both control and salt-stunted leaves was primarily through an increase in cell number; cell enlargement contributed mainly to growth in thickness. The number of cells per unit leaf area tended to remain constant throughout most of the growth period in both control and stunted leaves. The latter were smaller because of fewer cells. Salinity suppressed cell enlargement and cell division proportionately, but it apparently had no effect on the regulatory system that terminated DNA synthesis. This stopped in both control and stunted leaves 8 days after they had emerged from bud. In the controls, RNA synthesis stopped at the same time; protein synthesis continued for several days longer, until cell enlargement was essentially over. Salinity reduced the rate of RNA and protein synthesis, as well as the rate of cell enlargement, but it also prolonged all 3 processes.