Site, Scale and Time-course for Adjustments in Lateral Root Initiation in Wheat Following Changes in C and N Supply

Abstract

The pattern of lateral root initiation in seminal roots of wheat (Triticum aestivumL. cv. Alexandria) and the location, scale and time-course for adjustments in initiation were studied after changes in C and N supply. Macroscopically visible primordia appeared in a non-acropetal sequence with the frequency (number per unit length) increasing with distance behind the main root apex to a maximum at 40–50 mm behind the root tip. Pruning the root system to a single seminal axis increased the primordia frequency by 23% within 15 h. After longer periods, the effect of root-pruning was greater. The enhanced primordia frequency was first observed in tissue located 0–10 mm behind the apex at the start of treatment. Feeding glucose (50 mM) also increased primordia frequency within 15 h, but to a greater extent, and here additional primordia were initiated in tissue located 0–10and10–20 mm behind the apex at the start of treatment. Withdrawing NO3−from one part of a split-root system, whilst maintaining the supply to the other, reduced primordia frequency in the non-fed roots and, in some cases, a compensatory increase in the NO3−-fed roots was observed. The location and scale of the adjustments were similar to those found with root-pruning and glucose-feeding, but were slightly slower to appear. In spite of some differences in detail, there was a broad similarity in site, scale and time-course for adjustments in lateral root initiation with these treatments, which is consistent with the operation of a common mechanism. Whenever an increase in primordia frequency was observed, it was associated with an increase in the ethanol-soluble sugar content of the tissue. However, the reduction in frequency in NO3−-deprived roots was also accompanied by an increase in sugar content. There was no consistent relationship between total N content of the tissue and primordia frequency, but there was between primordia frequency and the rate of net NO3−uptake. The possible mechanisms controlling lateral root initiation are discussed.