205 Rubisco turnover and its relationship with herbage yield traits in perennial ryegrass (Lolium perenne L.) E.N. KHAEMBAH 1 , L.J. IRVING 1 , E.R. THOM 2 and C. MATTHEW 1 1 Institute of Natural Resources, Massey University, PB 11222, Palmerston North 2 DairyNZ, PB 3221, Hamilton e.n.khaembah@massey.ac.nz Abstract Leaf Rubisco content was monitored over the life span of the sixth leaf blades of hydroponically grown perennial ryegrass tillers of 18 genotypes, to determine protein turnover patterns and investigate possible links between protein turnover pattern and plant performance. Leaf Rubisco turnover was assumed to follow a log-normal distribution over time defined by three curve parameters; d – the peak height, g – the timing of the peak, and f – the retention time. Highly significant differences among genotypes were observed for all three Rubisco turnover curve parameters. Principal component analysis of Rubisco turnover data with morphological and plant dry weight (DW) data indicated that Rubisco retention time tended to increase with increased leaf appearance interval, and that high DW plants tended to have a lower, earlier Rubisco peak with longer retention. These results must be regarded as tentative until confirmed by further study. Keywords: Rubisco, perennial ryegrass, remobilisation, dynamic model, mapping population Introduction Use of inorganic nitrogen (N) fertiliser to overcome low soil N availability and to increase productivity has been linked to a range of environmental problems, in particular, eutrophication and damage to aquatic ecosystems (e.g. Raven & Taylor 2003). The need to reduce the use of N fertiliser is urgent, but also a challenge, given the rising global food demand from population growth and rapidly changing consumer preferences (Kasha 1999). Plant breeding efforts, therefore, are now being directed at improving nitrogen use efficiency (NUE). Internal recycling of N from senescing tissues is a major nutrient conservation mechanism, and represents the major N source for growth of new tissues (Lattanzi et al. 2005; Mae & Ohira 1981; Peoples & Dalling 1988). N taken up from the soil represents less than 40% of the N in new leaves, even when soil N is readily available. In most plant tissues, proteins represent the largest fraction of organic N which is potentially available for remobilisation during senescence. Rubisco (E.C.4.1.1.39), the photosynthetic enzyme, represents the major fraction of chloroplast N accounting for 15- 30% of total leaf N in C 3 plants (Evans 1989; Makino & Osmond 1991). Therefore, understanding the regulation of Rubisco turnover is a logical first step to understanding the regulation of N cycling within the plant. Rubisco concentration increases rapidly during leaf expansion, peaks around full leaf expansion and then declines slowly as the leaf ages (Mae et al. 1983). The products of its degradation during senescence are re- utilised as an N source for developing tissues (Mae et al. 1983; Makino et al. 1984). The amount of Rubisco in the leaf is the result of a balance between its synthesis and degradation. The dynamic model developed by Irving & Robinson (2006) describes the time course of leaf Rubisco content by a log-normal curve (Fig. 1). Rubisco concentration at any time is a result of the Figure 1 Log-normal model for time course of leaf Rubisco concentration proposed by Irving and Robinson (2006). d: maximum Rubisco content; g: time when d occurs; f: a measure of curve width. d f g Time (days after leaf emergence) Rubisco concentration