Abstract
In earlier investigations it was observed that foliar chlorosis developed during periods of intense insolation of grain plants grown on humus soils which had been limed to correct acidity. In order to determine the cause of this injury 2 sets of Marquis wheat were grown, one on strongly acid humus soil low in mineral matter and the other on a fertile but acid loam. The acidity of a portion of the soil in each set was corrected by applications of pulverized calcium carbonate. A white muslin screen was installed over one-half of the plants when 6 weeks old, the remainder being left exposed to full illumination.
After symptoms of chlorosis had developed in fully insolated plants, entire tops were cut from several plants in each set at 4-hour intervals, frozen, comminuted and pressed under uniform pressure. The expressed sap was collected and its hydrion concentration measured potentiometrically by means of the calomel half cells and a quinhydrone electrode.
The data indicate that chlorosis was definitely correlated with sap hydrion changes attributable to differences in soil reaction and light intensity. The acidity of sap expressed at 4-hour intervals disclosed a diurnal periodicity in all plants but most pronounced in the plants on limed soil under full insolation. Reversals in the acidity gradient were not conincident with the hour of maximal light intensity, which suggests that the thermal factor in insolation may not be as important as its light effect.
Light intensity and soil alkalinity exercised considerable influence upon the level of acidity attained. Limed cultures in the shaded and unshaded sets maintained a lower degree of acidity and exhibited a greater initial decrease in sap hydrion concentration than the corresponding untreated plants exposed to full light intensity.
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