The effect of soil compaction on wheat during early tillering

Abstract

Wheat ( Triticum aestivum L. cv. Eradu) was grown in the field or glasshouse on a deep loamy sand which had either a compact soil layer (compact soil) between 10 and 55 cm depth or a deep‐tilled profile (loosened soil). The pattern of assimilate transport to the roots was investigated. Previously reported growth rates and solute concentrations of the seminal root axes were coupled with rates of O 2 uptake measured in the field to give estimates of dry weight import into the ethanol‐insoluble (structural), ethanol‐soluble (osmotic) and respiratory pools. The calculated daily import of dry weight was 28% less in mechanically impeded than unrestricted apices. However, the dry weight required to produce 1 cm of root was twice as high for axes in compact soil. In these roots, a greater proportion of imported substrates went into ethanol‐insoluble matter, probably cell walls. Non‐respired 14 C lost from root systems into the rhizosphere was measured in the glasshouse and found to be less than 4% of the 14 C fixed 72 h earlier, even in compact soil. The partitioning of 14 C‐labelled assimilate in plants from the two treatments was compared. The proportion of 14 C in the ethanol‐soluble fraction was hardly affected by tissue age or soil strength. Furthermore, 62–82% of the soluble 14 C was in the neutral (sugar) fraction in the roots. However, the 14 C concentration (dry‐weight basis) in apices of seminal root axes was substantially greater when the roots grew in loosened soil. The greater sink strength of seminal root? growing in loosened soil was reflected in a 3‐fold larger incorporation of 14 C into each gram of new ethanol‐insoluble material formed. In contrast, concentrations of 14 C were similar in mature root tissues, nodal roots and shoots taken from both soil treatments.