The Physiology of Flowering Plants - KHAM PHA MOI

PARTITIONING OF TRANSLOCATE BETWEEN SINKS 147mathematical precision (see allometric growth, Chapter 6). Thisimplies that partitioning of organic nutrients, too, is precisely controlled.There must be signalling between the sinks and the sources.Partitioning control is a complex process. Farrar (1996) lists thesteps involved in C flow to a sink: ‘phloem loading in a source leaf,phloem transport into the sink, unloading and short distance transportwithin the sink and then metabolism and storage in the sink.’Just about everything that affects the physiology of a plant has thepotential to affect partitioning, and control of partitioning is far frombeing fully understood. One important signalling molecule issucrose itself (Fig. 5.5). Sucrose acts in growing regions as a promoterof the expression of genes involved in growth and respiration (whichprovides the energy for growth); in the photosynthetic leaves,sucrose represses the expression of genes involved in photosynthesis,including the gene coding for the small subunit of Rubisco.Leaf growth leads to an increased production of sucrose and its translocationto the roots, where the sugar would stimulate the activity ofgenes which promote growth of more root biomass – corresponding tothe growth of the leaves. A fall in the sink demand has been shown todecreasetherateofloadingatthesource,althoughitisnotclearhowthemessageistransmitted.Turgorhasbeensuggested,achangeinturgoratFig: 5:5 Sucrose as aninformation molecule integratingactivity between source and sink.