EFFECTS OF CA AND SR ON ZEA MAYS SEEDLING PRIMARY ROOT GROWTH

Either Ca or Sr in a mineral nutrient medium prevented toxic effects of other ntutrient ions on aerated primary roots of maize; other monovalent or divalent cations substituted for Ca did not. Calcium in the complete nutrient solution, or as CaCl2 alone, stimulated the division of apical meristem cells in these roots, as compared to division in water aloine, but Sr did not. However, the replacement of Ca in the nutrient medium by Sr resulted in the development of additional secondary roots, probably in part by decreasing the rate of cell division in the apical meristem of the primary root and thereby diminishing apical dominance. IN EARLIER STUDIES under greenhouse conditions maize seedlings died when their primary roots were immersed in a non-aerated nutrient solution with Na replacing Ca, but they did not die when the Ca was replaced with Sr (Queen, Fleming, and O'Kelley, 1963). It is common knowledge that among several functions in plants Ca prevents toxicity by other nutrient ions; for example, Mg ions have been described as toxic in the absence of Ca (I\/feyer, Anderson, and B6hning, 1960). This paper describes additional growth experiments on aerated maize roots which indicate that Sr exerts a "detoxifying" effect on maize seedlings similar to Ca, but that it may have no other nutritional value. MATERIALS AND METHODS Seeds of 'Truckers' Favorite' variety of Zea imays L. were germinated at 22 i 1 C in darkness on wet filter paper. Water used here and subsequently was first distilled and then deionized. At age 3 days 10 or more seedlings carefully selected for uniformity were transferred to polyethylene bottles, three seedlings per bottle, with the primary roots (25 mm long) immersed in 1 liter of water or nutrient solution. After transfer to the bottles seedlings were grown with constant root aeration, at 22 i 1 C and 12-12-hr light-dark periods. Illumination was approximately 300 ft-c from cool-white fluorescent tubes (Sylvania). The basal Ca-containing nutrient solution was a modified Knop's solution (Table 1). M\/Iodifications of this solution and of other solutions used are described with the results obtained. Branches of the primary roots wvere counted under a binocular microscope. Whole roots were squashed and exarnined microscopically. For each I Received for publication 4 Juiie 1968. Supported in part by the University of Alabama Research Committee, Project 469. 2 Present address: Biology Department, Tennessee Wesleyan College, Athens, Tennessee. root the length of the region anid the iurumber of cells in a file were determined (Fig. 1) from the apex of the vascular cylinder to the first xylem element possessing a recognizable secondary cell wall. Similarly, the length of the region from this xylem element to the base of the primary root was determined. For convenience in this study these portions are designated as apical region and basal region, respectivelv. In each root four xylem elements withl secondary walls selected at random were measured and a mean element length was determined from these for each ulutrient solution treatment. For each treatrnent, the mean nurmber of vessel elements with secondary walls in file (Fig. 1) in the root was calculated as the length of the basal region divided by' the mean length of the xylem elements in this region. The statistical significance of differences between means was determined using the "t" test, with the 95 % or higher probability level described as significant and the 99 % or higher probability level described as highly significant (Lacey, 1953). RESULTS-Primary root elonrgation At the time the seedlings were 8 days old root length in the complete Ca-containing nutrietnt solution was compared with that in solution modifications, mostly substitutions of other nitrates for Ca(NO3)2 (Fig. 2). Growth was greatest when nitrate was suipplied as Ca, next with Sr(N03)2, and less with otlher nitrates used. Addition of NaCl (4.24 miu) had no significant effect on primary root growth if the solution contained Ca or Sr nitrate. Similarly, in solutions lacking Ca and Sr, it made no difference by the end of the 8-day period whether nitrate was supplied as KN03 or NaNO3, or if all nitrate Awas excluded from the nutrient medium. At the end of the 8-day period where M\Ig(NO3)2 replaced Ca(NO3)2, primary root length of seedlings was less than it