Starch metabolism in the fructan-grasses: Patterns of starch accumulation in excised leaves of Lolium temulentum L.

Excised and illuminated leaves of temperate gramineae have previously provided a powerful system for the analysis of the regulation of partitioning of photosynthate between sucrose and fructan. This paper complements previous data by describing the concomitant accumulation of transitory starch, an assimilate which has been ignored in this system. Leaves of Lolium temulentum were depleted to near-zero reserve carbohydrate content. In this condition, the tissue contained no detectable fructan or starch and only a trace of sucrose. When subsequently excised and illuminated continuously, the accumulation of sucrose and starch commenced immediately and occurred simultaneously. Starch anabolism was constitutive and the polymer was apparently synthesised de novo. The maximal rate of starch accumulation, at 0.6 mg g−1 fresh mass, was 5-10-fold lower than that of sucrose. Fructan was also synthesised de novo and was induced after a lag of 8 h, after which rates of net sucrose and starch accumulation slowed, ceasing completely by 14-16 h. Whilst total non-structural carbohydrate concentration increased continuously, reaching 56 mg g−1 at 30 h, starch concentration was asymptotic and was limited to a maximum of only 7 mg g−1 reached at 14-16 h. Hence, net starch accumulation was not restricted by the continued production of photosynthate. The illumination period spanned 1.25 diurnal cycles. Despite continued photoassimilation, there was no resumption of net starch synthesis at the beginning of the second cycle and hence the limitation of net starch accumulation was not imposed by an endogenous circadian rhythmicity. When assimilate partitioning into fructan was abolished by transpirational feeding of cycloheximide, sucrose concentration increased 2.7-fold to 49 mg g−1. This abnormally high concentration did not feedback to enhance starch accumulation. Transpirational feeding of 10 mmol/L mannose inhibited sucrose accumulation by c.90 %, but this inhibition was not accompanied by an increase net starch accumulation, rather, starch accumulation was inhibited by 60 %. Mannose caused the formation of maltose in the tissue. Starch synthesis, in common with sucrose and fructan syntheses, was shown to be localised predominantly in mesophyll cells where presumably the control of partitioning between these three assimilates is exercised. The low rates, restricted tissue concentration, sucrose-insensitivity and mannose-inhibition of starch synthesis differ from many species where starch is the primary reserve. Although the available information is limited, some of these features are in common with other fructan grasses. The low capacity for starch accumulation provides a possible explanation for the evolution of fructan synthesis in these species.