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  1. Vol. 22 No. 2, p. 362-366
     
    Received: May 18, 1981
    Published: Mar, 1982


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doi:10.2135/cropsci1982.0011183X002200020038x

Variation of 14C-labeled Photosynthate Recovery from Roots and Rooting Media of Warm Season Grasses1

  1. D. M. Vietor2

Abstract

Abstract

Variation of photosynthate supply to the grass rhizosphere has been hypothesized to explain variation of acetylene reduction rates among soilplant cores of grasses. Kleingrass (Panicum coloratum L.), pearl millet (Pennisetum americanum (L.) K. Schum.), and bahlagrass (Paspalum notatum Flugge) were grown hydroponically at concentrations of 56, 112, and 224 ppm to determine the proportion of photosynthate translocated to roots and recovered in nutrient solution or leachates from rooting media of 60-day-old plants.

The proportion of photosynthate washed from roots in solution, associated with a fritted clay medium, and respired by roots and bacteria was also determined for hydroponically grown tetraploid and diploid bahiagrass genotypes. Photosynthate was radioactively pulse-labeled via photosynthetic assimilation of 14CO2 to quantify the proportion of photosynthate in bacteria and organic compounds separated from the roots.

Bahiagrass translocated 7% of the 14C-labeled photosynthate to roots, compared to 12% for kleingrass and pearl millet. Of the 14C-labeled photosynthate translocated to roots, nearly 3 times more (1.1%) appeared in nutrient solution or leachates from the rooting medium of bahiagrass compared to kleingrass (0.4%) and pearl millet (0.4 %). The total of 14C washed from roots, associated with fritted clay, and respired comprised an average of 5.7 % of the 14C-labeled photosynthate recovered per plant from the two bahiagrass genotypes.

Diazotrophlc colonization and N2 fixation appeared more probable for the bahiagrass than the other species because in bahiagrass a larger proportion of the photosynthate translocated to roots appeared in bacteria and organic compounds washed from roots at low N levels. However, the low proportion of photosynthate released from roots of the tetraploid and diploid genotypes indicated only a small amount of substrate was available in the rooting medium and rhizosphere during juvenility.

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