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Induction of Leaf Red-Speckling by Phosphorus on Sorghum Grown under Controlled Conditions¹

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.

Key Words: Excess phosphorus disorder • Phosphorus nutrition • Phosphorus toxicity • Nutrient solutions • Sorghum bicolor (L.) Moench

² A.M.C.F. and J.W.M., graduate student (presently plant physiologist, Instituto Agronomico, C.P. 28, 13 100 Campinas, S.P., Brazil) and professor, respectively, Dep. of Agronomy, Univ. of Nebraska, Lincoln, NE 68583-0817. R.B.C. and C.Y.S., research plant physiologist, USDA-ARS, Dep. of Agronomy, Univ. of Nebraska, Lincoln, NE 68583-0817.

Received for publication May 28, 1985.