THE CELLULAR PATHWAY OF PHOTOSYNTHATE TRANSFER IN THE DEVELOPING WHEAT-GRAIN .2. A STRUCTURAL-ANALYSIS AND HISTOCHEMICAL-STUDIES OF THE PATHWAY FROM THE CREASE PHLOEM TO THE ENDOSPERM CAVITY

In the developing wheat grain, photosynthate is transferred longitudin ally along the crease phloem and then laterally into the endosperm cav ity through the crease vascular parenchyma, pigment strand and nucella r projection. In order to clarify this cellular pathway of photosyntha te unloading, and hence the controlling mechanism of grain filling, th e potential for symplastic and apoplastic transfer was examined throug h structural and histochemical studies on these tissue types, It was f ound that cells in the crease region from the phloem to the nucellar p rojection are interconnected by numerous plasmodesmata and have dense cytoplasm with abundant mitochondria, Histochemical studies confirmed that, at the stage of grain development studied, an apoplastic barrier exists in the cell walls of the pigment strand. This barrier is compo sed of lignin, phenolics and suberin. The potential capacity for sympl astic transfer, determined by measuring plasmodesmatal frequencies and computing potential sucrose fluxes through these plasmodesmata, indic ated that there is sufficient plasmodesmatal cross-sectional area to s upport symplastic unloading of photosynthate at the rate required for normal grain growth. The potential capacity for membrane transport of sucrose to the apoplast was assessed by measuring plasma membrane surf ace areas of the various cell types and computing potential plasma mem brane fluxes of sucrose. These fluxes indicated that the combined plas ma membrane surface areas of the sieve element-companion cell (se-cc) complexes, vascular parenchyma and pigment strand are not sufficient t o allow sucrose transfer to the apoplast at the observed rates. In con trast, the wall ingrowths of the transfer cells in the nucellar projec tion amplify the membrane surface area up to 22-fold, supporting the o bserved rates of sucrose transfer into the endosperm cavity. We conclu de that photosynthate moves via the symplast from the se-cc complexes to the nucellar projection transfer cells, from where it is transferre d across the plasma membrane into the endosperm cavity. The apoplastic barrier in the pigment strand is considered to restrict solute moveme nt to the symplast and block apoplastic solute exchange between matern al and embryonic tissues. The implications of this cellular pathway in relation to the control of photosynthate transfer in the developing g rain are discussed.