Organic acids and cellular changes in the endosperm of Euphorbia lambii seedlings

Abstract

Seedling of Euphorbia lambii Svent. were grown in the dark, and the levels of organic acids in the endosperm were monitored during the 6–7 day period in which all the reserves were depleted. Glycolate, glyoxylate, succinate, fumarate and 2‐oxoglutarate occurred in traces only, the citrate concentration remained rather constant (0.4 μmol endosperm−1), malate varied from 0.2 to 0.4 μmol endosperm−1, but malonate appeared to be the major organic acid in the endosperm ranging from 0.75 to 1.25 μmol endosperm−1. Radioactive malonate was easily taken up by the cotyledons of growing seedlings, and up to 11.2% of the label proceeded to the sterols, the triterpenes and triterpene esters in a 48 h incorporation period. No label from [14C]‐malonate was built into the triacylglycerols in the seedling. Maximum uptake values of 0.6 μmol malonate seedling−1 day−1 were measured, and this value was not altered by a simultaneous uptake of sucrose. Conversely, the uptake of labeled sucrose and its subsequent conversion into sterols and (latex) triterpenes was not altered by a simultaneous uptake of low concentrations of malonate. Increased amounts (from 0.25 μmol malonate seedling−1 and up) caused a 75–90% reduction of both uptake and conversion of sucrose into neutral lipids. To maintain a daily uptake of 4 μmol of sucrose by the cotyledons (required to maintain seedling growth) the simultaneous in vivo uptake of malonate from the endosperm was supposed not to exceed 0.2 μmol seedling−1 day−1.Thin sections of the endosperm revealed the morphology of the process of reserve depletion. The occurrence of vacuoles 2 days after germination, coincided with the increase in the malonate level. The protein bodies first disappeared completely from the outer layers, whereas the triacylglycerols gradually disappeared from the entire endosperm. About 80% of the endosperm cells contained a large vacuole until the stage of complete depletion, probably serving as the major site of malonate storage.