Site, scale and time-course for adjustments in lateral root initiation in wheat following changes in C and N supply

The pattern of lateral root initiation in seminal roots of wheat (Triticum aestivum L. 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 0-10 and 10-20 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.