Studies of Calcium and Other Storage Minerals in Embryos of Cucurbita Maxima, Cucurbita Andreana and their Reciprocal Hybrids

Abstract

<p>Calcium is an essential element in plant and often it is the first nutrient to become limiting during seedling growth. Despite this requirement for CA, embryos have been found to contain low amounts of Ca compared to other macro-nutrients that they store. Cucurbita maxima (squash) and C. andreana (bitter gourd) were selected for the study of calcium storage within embryo tissue because it has already been shown that these two species have a 10-fold difference in see size, a 3-fold difference in Ca level per gram embryo tissue and the reciprocal crosses between them produce visible hybrids. Calcium was analyzed by atomic absorption spectroscopy and the four storage minerals (Mg, K, Ca and P) were analyzed by neutron activations analysis. Atomic absorptions analysis required the destruction of organic matrix of the samples. Dry ashing of cucurbit embryo tissues resulted in the discovery of some unique characteristics of these tissues. When ground embryo samples of less than 0.05 g weight were ashed in porcelain crucibles, the measured Ca levels were abnormally high due to sample-crucible interactions. The apparent enhancement of Ca level occurred because of changes in the K and perhaps Na levels of the samples. Although cucurbit embryo tissues were resistant to complete ashing at temperatures below 550°C, Ca was more easily extracted from the black ash obtained at these temperatures than from carbon-free ash obtained with higher temperature ashing. The Ca binding at the higher ashing temperatures, which occurred because of a low cation-to-P ratio within the cucurbit ash, was not present in other seed tissues with higher cation-to-P ratios. The problems were overcome by ashing samples of at least 0.01 g and by acid digestion of the ash to convert Ca to more soluble forms. Cucurbit embryos contained Mg, K, Ca and P but Ca made up less than 5% of the total mineral reserve while P made up 50%. All four minerals were present in high concentrations in the root-shoot axis than in the cotyledons, but the percent of the total Ca in the axis was higher than that of Mg, K and P. The embryos of C. maxima, the species with the larger sees, stores higher amounts of Ca in the axis than the smaller embryos of C. andreana. The Ca levels were more variable in the large-seeded species and were negatively correlated with embryo size. No correlation between embryo size and Ca level was found for C. andreana. The parent and hybrid embryos differed less in their Mg, K and P levels than in their Ca levels. The Ca levels in the hybrid embryos (C. Maxima X C. andreana and C. Andreana X C. Maxima) were significantly different from the parental Ca levels indicating that the hybrid embryos differed in Ca level from each other and that each hybrid approached the Ca levels of its female parent showed that maternal influences on Ca level were also prominent. In both species mobilization of minerals from cotyledons to the growing axis occurred only in seedlings grown in distilled water. In normal growth conditions with the expanding cotyledons, which became the first photosynthetic leaves, assimilated nutrients from the medium. The assimilation was particularly marked for Ca. This study has contributed toward the understanding of mineral storage in seeds of a group that has not been widely studied to date. The study has also emphasized the difference between the store of Ca and the storage of Mg, K and P.</p>