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Abstract
Sorghum (Sorghum bicolor (L.) Moench) genotypes were grown in nutrient solutions and in soils at low P under greenhouse conditions to determine genotype differences for dry matter yields, P concentrations and contents, dry matter produced per unit P, P distribution among parts, P uptake rates, and intake root phosphatase activities.
Genotypes grown in nutrient solutions with organic sources of P (ethylammonium-, glyceryl-, and phenyl-phosphates) produced as much dry matter as plants grown with the inorganic sources of P KH(,2)PO(,4) and calcium tribasic phosphate and produced more dry matter when grown in soils. Plants grown with organic P compounds in both nutrient solutions and soil had higher P concentrations and contents than plants grown with inorganic P compounds. Phosphorus from organic sources was more readily available to plates than P from inorganic sources.
Differences in growth and P use were noted among genotypes when a large number of plants (90 to 120 plants per container) were grown together in nutrient solution at 129 (mu)mole P liter('-1). Widest differences among genotypes for dry matter yields were noted for plants grown with ethylammonium-, glyceryl-, calcium tribasic-, and potassium dihydrogen phosphates. Plants grown with KH(,2)PO(,4) and calcium tribasic phosphate and nearly six-fold higher values for dry matter produced per unit P than plants grown with ethylammonium phosphate. Calcium tribasic phosphate and KH(,2)PO(,4) appeared to be the best P compounds to use to screen sorghum genotypes for tolerance to low P in nutrient solutions. NB9040 was the most tolerant and SC33-9-8E4 the least tolerant genotype to low P in both nutrient solutions and in soil. SC369-3-1JB, tolerant to low P in nutrient solution and in low P acid soil, did not grow well in a low P soil at pH 6.8.
Regardless of P source, NB9040 and SC369-3-1JB (tolerant to low P) had lower root phosphatase activities and fewer P deficiency symptoms than SC33-9-8E4 and Ck60-korgi (intolerant to low P) at similar P levels. Plants grown with organic P sources had lower root phosphatase activities and fewer P deficiency symptoms than plants grown with inorganic P sources. Plant responses to P sources were: organic P sources <KH(,2)PO(,4)<inorganic P sources.
Larger genotypic differences for P uptake rates were observed for 24-day-old plants than for older plants. Uptake rates were six to 14 times lower (dependent on genotype) in 52-day-old plants than in 24-day-old plants. NB9040 which had the highest dry matter at each plant age had the lowest rate of P uptake, and CK60-Korgi which had the lowest dry matter at each age had the highest rate of P uptake.
Intact root phosphatase activities and P uptake rates per unit root appear to be good parameters to show differences among genotypes for tolerance to low P; the more tolerant genotypes had lower values than the intolerant genotypes.
Differences in P contents in roots and leaves were only minor among genotypes at 24 and 38 days of age. At 52 days of age, NB9040 translocated more P from the lower to the upper leaves, compared to the other genotypes. Lower P concentrations were found in the grain of the intolerant genotypes SC33-9-8E4 and CK60-Korgi grown to maturity in nutrient solutions.
Hybrids derived from NB9040 and SC120-15 showed an advantage over the hybrids from SC33-9-8E4 and Plainsman for top and root dry matter yields and for dry matter produced per unit P. The hybrids Wheatland x SC120-15 and Wheatland x SC33-9-8E4 had higher root dry matter compared to hybrids made from CK60 and KS35 crosses with SC33-9-8E4 and SC120-15. Tolerance to low P appears to be heritable and related to larger tops and roots and higher dry matter production per unit P during the early stages of growth. Heterosis was observed for most hybrids grown at low P.
