Effect of Root/Leaf Temperature Differentials on Root/Shoot Ratios in Some Pasture Grasses and Clover

Twelve pasture species were grown in the same aerial environment, but with five constant soil temperatures ranging from 5 to 35 °C, to determine the influence of root temperature on the weight of roots per unit weight of foliage (R/S ratio). This ratio varied by a factor of 2 to 8 within species. Using maximum yield of foliage to indicate the optimum soil temperature for each species, it was found that the R/S ratio was lowest at the optimum soil temperature, and was progressively higher at soil temperatures above and below the optimum with only slight exceptions. This experimental manipulation of R/S ratios suggests that the partitioning of photosynthate is controlled by the relative rates of photosynthesis and root absorption, by inverse proportion: Root mass x rate(abeorption) α Leaf mass x rate(photosynthesis). When other environmental factors were uniform, the relative weight of roots tended to be proportional to the displacement of soil temperature from the optimum for each of the 12 species, the relationship being curvilinear.