Some Effects of Insolation on Mineral Nutrition of Triticum.

Abstract

In an earlier investigation of certain crop plants grown on colloidal humus soils, foliar chlorosis attributable to a physiological insuffciency of iron was reported. Insufficiency of mobile iron and concomitant chlorosis were found to be especially marked in plants exposed to strong insolation. The purpose of the investigation herein reported was to determine if changes in sap acidity induced by light were of sufficient magnitude to reduce the iron supply to the point of chlorosis.

Wheat seedlings for this purpose were grown in an acid humus low in iron. A portion of the cultures were treated with 2000 parts per million of powdered calcium carbonate to correct the known acidity of the soil. Seedlings were grown in glazed pots in a greenhouse at 60° C. until 6 weeks old, at which time analyses were begun. The cultures were evenly divided and placed in adjacent benches subjected to different illumination, one being exposed to full insolation and the other shaded from direct afternoon sunlight by a thin, white muslin screen. Entire plants, freed from soil, were employed and the sap extracted under constant pressure from frozen samples of uniform weight. Acidity of freshly expressed sap was determined potentiometrically in duplicate at 25° C.

The initial analyses, made at 4 hour intervals, disclosed a diurnal cycle of sap acidity which reached a maximum in the early morning and fell steadily to a minimum in the late afternoon in all young wheat cultures. There was some fluctuation from day to day in the hour and amount of the maximal and minimal acidity but the rhythmic diurnal rise and fall continued until the end of the experiment, at which time the plants were 12 weeks old.