Changes in Post-anthesis Assimilates in Stem and Spike Components of Italian Ryegrass (Lolium multiflorum Lam.). I. Water Soluble Carbohydrates*

Abstract

Stem carbohydrate reserves, in ryegrass grown for seed, may play a vital role in maintaining seed growth, especially under conditions of limited photosynthesis. Little is known concerning the processes controlling stem carbohydrate utilization and partitioning in ryegrass with respect to seed growth. The objective of this investigation was to determine detailed post-anthesis changes in stem and spikelet carbohydrates as affected by modification of source and sink strength. Source-sink relations were altered by imposing detillering or detillering-defoliation treatments at anthesis. Patterns of carbohydrate distribution of the ryegrass stem were different, both among positions within the stem and with age. Stem carbohydrates accumulated during early stages of seed growth and then declined as seeds matured. Reducing sugars comprised only a small fraction of the stem's total water soluble carbohydrates. Detillering induced the formation of new tiller sinks, thus increasing sink strength and reversing the carbohydrate gradient from spikelet (seed) sinks to new tiller sinks. Defoliation, combined with detillering, decreased source strength by reducing total stem carbohydrate. In control plants, carbohydrate levels appeared adequate to support maximum seed set, whereas conditions for reduced carbohydrate levels, resulting from detillering or detillering plus defoliation, lowered seed set. Results suggest that under conditions of limited source strength (e.g. reduced photosynthetic capacity), the stem plays a major role in partitioning assimilates to compensate for sink demand. New tiller growth during the period of seed development may out-compete seeds for available carbohydrate and thus reduce seed set