X-RAY-MICROANALYSIS OF FROZEN-HYDRATED, FREEZE-DRIED, AND CRITICAL-POINT DRIED LEAF SPECIMENS - DETERMINATION OF SOLUBLE AND INSOLUBLE CHEMICAL-ELEMENTS AT ERYSIPHE-GRAMINIS EPIDERMAL-CELL PAPILLA SITES IN BARLEY ISOLINES CONTAINING ML-O AND ML-O ALLELES

Three specimen preparation procedures were used in conjunction with sc anning electron microscopy and energy dispersive X-ray microanalysis t o determine, by comparison among preparation methods, both soluble and insoluble elements at Erysiphe graminis - barley leaf epidermal cell encounter site areas where attempted fungal penetration by appressoria failed. Near isogenic lines (RISO 5678-R arid RISO 5678-S) of barley differing by mutation at the Ml-o locus were used. The recessive ml-o allele conditions barley epidermal cells to respond with papilla-assoc iated resistance to E. graminis, while the dominant Ml-o allele allows for successful penetration of the majority of E. graminis germlings. Frozen-hydrated and freeze-dried specimens maintained soluble and inso luble elements, while specimens fixed by formalin - acetic acid - alco hol and critical point dried lost soluble elements. The effects of spe cimen preparation on electron-beam penetration and depth of X-ray exci tation were calculated and are illustrated. Mean elemental intensity v alues were corrected for X-ray absorption by nickel coating of specime ns (used for electrical conductivity) and for X-ray detector efficienc y. The elements Cl, K, Mn, Ca, and Mg were highly soluble both at rece ntly deposited (16 h) and at matured (24 h) papilla deposition sites. Elemental Si levels were elevated and in a partially soluble state in recently deposited papilla sites (16 h), but Si became bound or insolu ble in matured (24 h) papilla sites. Elemental P and S are insoluble. The physiological role of each element is briefly discussed relative t o its known function in healthy and diseased plants, with emphasis on E. graminis - barley epidermal cell encounter site penetration failure .