Induction of Leaf Red‐Speckling by Phosphorus on Sorghum Grown under Controlled Conditions 1

Studies were conducted to better understand the enhancement and properties of a red‐speckling that was induced on the lower leaves of young sorghum [ Sorghum bicolor (L.) Moench] plants grown in nutrient solutions with various levels of P. Severity of leaf red‐speckling was enhanced as solution P level increased. Phosphorus as low as 1.2 to 2.5 μnol per plant induced red‐speckling. At comparably low levels of P, organic sources (phenyl‐, glyceryl‐, and ethylammonium phosphate) induced more severe symptoms than did inorganic sources [potassium dihydrogen, calcium (dibasic‐, tribasic‐, meta‐, and pyro‐), ferrous, ferric, and aluminum phosphate]. A redspeckled fourth leaf (from the plant base) of ‘Martin’ had lower photosynthetic rates, chlorophyll concentrations, chlorophyll a/b ratios, and leaf areas than a comparable nonspeckled NB9040 leaf. Except at high P, a comparable nonspeckled fifth leaf of Martin and NB9040 had similar photosynthetic rates. Reduced leaf photosynthetic rates were associated with severe red‐speckling, which in turn was induced in Martin plants grown at high levels of P. Martin accumulated more P, developed severe leaf red‐speckling, and was more adversely affected by P than NB9040. Some of the unusual or abnormal red colorations that appear on leaves of young sorghum plants may be caused by P. Excess P disorders or toxicities occurred on sorghum leaves at P levels lower than had normally been considered.