GROWTH AND BIOCHEMICAL COMPOSITION OF BEAN PLANTS AS CONDITIONED BY SOIL MOISTURE TENSION AND SALT CONCENTRATION

Abstract

Dwarf Red Kidney beans were grown to incipient flower- ing in 10-gallon containers filled with a loam. These soils contained 0, 0.1, 0.2 and 0.4% of added NaCl on the dry soil basis. The 36 cultures were divided into 3 moisture-tension series. Water was added when the soil moisture tension at the 4-inch depth had reached 250 cm. of water and 750 cm. of water for the first 2 series, respectively. Water was added to the 3d series when plants were wilted by mid-morning, corresponding to tension greatly exceeding 800 cm. of water. Plant growth was inhibited as the soil moisture tension at time of irrigation increased, even though in some of the treatments the soil moisture was always above the wilting range. Progressive additions of NaCl to the soil caused progressive decreases in growth and yield of beans. Increasing soil moisture tension or salt cone, tended to cause an increase in the % of nitrate N in the plants, and to have a similar effect, though less pronounced, on the % of soluble organic N. Consequently, increasing intensity of either type of stress tended to cause an increase in nitrate/soluble-organic-N ratio. The % of protein in the leaves increased with increased intensity of either type of stress. Soil moisture tension had no effect on % of reducing sugars whereas increasing salt cone, was associated with a definite decrease in % of these sugars in the stems. The exptl., treatments had little effect on % of nonreducing sugars or % of hemireserves. Increasing either soil moisture tension or salt cone, caused a marked decrease in % of starch in the leaves. The relative effects of salt cone, and soil moisture tension on water availability to these plants is discussed. It is concluded that growth reductions were to a large extent brought about by reduced hydration of protoplasmic proteins, whether water stress was due to osmotic forces or to moisture tension, since adequate percentages of proteinaceous constituents were present for higher rates of meristematic activity. Factors other than varying degrees of water availability were probably operative in conditioning these plants. The data indicate that excessive cones, of the chloride ion per se affect carbohydrate metabolism, possibly through reduced photosynthetic activity. The hyperbolic nature of the relationship between soil moisture % and moisture tension evidently accounts for the frequent finding that, for all practical purposes, plants may not show changes in growth response while reducing the moisture % of soil from field capacity to nearly the wilting %.