Leaf and Tiller Development in Soft Red Winter As Influenced By Seeding Rate and Timing of Nitrogen Application.

Vegetative growth in the form of leaves and tillers is crucial in the formation and development of wheat (Triticum aestivum L.).  Several studies have been conducted to characterize the development of leaves and tillers under certain environmental conditions (Klepper et al., 1983; Baker et al., 1986; Krenzer and Nip, 1991; Krenzer et al., 1991).  These studies found that leaf number on the main stem and tillers increased at similar rates based on accumulated heat units and that tiller appearance is synchronous with main stem leaf development (Klepper et al. 1982).  However, these studies only examined early tiller development.  Few have examined the phenomena of tiller mortality, particularly in field conditions under modern production practices.  This study was done to examine the influence of management practices, seeding rates and nitrogen application timing, on leaf number and both tiller development and decline.

This study was conducted at three sites over two years using a split plot design with two seeding rates as the main plots and with either a single or split application of N as the subplot treatments.   Leaves on the main stem and each emerging tiller were marked at regular intervals during the season to help determine phyllochron intervals (PIs), maximum leaf number (MLN) and to measure leaf appearance and tiller mortality.  Destructive samples were taken to determine changes in tiller number.  As noted in other research studies there was a linear increase in leaf number when measured across growing degree days up to the point that the flag leaf emerged.  However, in contrast to the findings of Klepper et al. (1982) the leaves on the tillers appeared at a slower rate than those on the main stem.  This was confirmed by comparing PI from the main stem with those found on tillers T1 and T2. Changes in seeding rate did not influence the PI of the main stem (119.71 and 125.98 GDD leaf^-1 for the low and high seeding rate, respectively) but did influence the PI of tillers T1(289.59 vs 342.97 GDD leaf^-1) and T2 (403.81 vs 537.54 GDD leaf^-1).  At the higher seeding rate the PIs for the tillers were greater than those for the same tillers at the lower seeding rate.   Similar differences in MLNs were found between the tillers in the seeding rate treatments. The use of a split application of N in late winter had significant effects in two site-years.  At the higher seeding rate a split application of N resulted in significantly more leaves on the main stem and early tillers (T1 and T2).  At the lower seeding rate a split application of N increased leaf number on the higher order tillers (T5 to T8).   Tiller numbers from the destructive samples taken over the growing season indicated that tiller mortality differed between seeding rates.  The decline in tillers started earlier and was greater at the higher seeding rate when compared to the lower seeding rate.  The influence of N timing on tiller mortality was less consistent.  A split application of N only increased tiller number at one sampling period at one site-year.  Overall, this study found that leaf appearance on the main stem and particularly on each succeeding tiller is influenced by competition among plants for light, water, and nutrients and that seeding rate is an important factor influencing leaf development, MLN, and tiller mortality.