Nitrogen accumulation, partitioning, and nitrogen harvest index increase during seed fill of field pea

Nitrogen (N) accumulation by plants and partitioning to seeds are critical processes in yield formation. No consistent description of these processes has been developed for crops, including field pea (Pisum sativum I,.). Data from a set of 31 field pea experiments were obtained to calculate N accumu lation by plants and partitioning to the seed during seed fill, and the cha nge in nitrogen harvest index (NHI). Three distinct patterns of plant N acc umulation were found: (1) no accumulation during seed fill, (2) accumulatio n only in early seed fill, and (3) constant accumulation throughout seed fi ll. N transfer rate to the seeds from vegetative tissues was stable within an early stage and a late stage of seed fill, but the rates between the two stages usually differed. In most cases the N transfer rate from the vegeta tive tissue in the late stage was greater than the early stage. The combina tion of current plant N accumulation and N transfer from vegetative tissue in the early stage was generally greater than the late stage N transfer alo ne. No quantitative association was identified, however, among rate of plan t N accumulation, early stage N transfer, and late stage N transfer. Altern atively, NHI increased linearly across all experiments with r(2) exceeding 0.92, whether calculated as a function of days or thermal units (TU). There was, however, variability in the slope of NHI increase such that the mean across experiments as a function of days was 36.7 mg g(-1) per day with a C V of 20% and as a function of TU was 1.7 mg g(-1) per TU with a CV of 17%. The expression of NHI as a function of days was found to increase slightly with increasing temperature, but no such variation was found when expressed as a function of TU. (C) 2001 Elsevier Science B.V. All rights reserved.