Movement of Calcium Across Tips of Primary and Lateral Roots of Phaseolus vulgaris

Abstract

Calcium (Ca) movement across tips of primary and lateral roots of Phaseolus vulgaris was determined by applying 45Ca2+ to one side of the root and collecting radioactivity in an agar receiver block on the opposite side of the root. The ratios of cpm in receiver blocks on the bottom of primary roots: cpm in receiver blocks on the top of primary roots were 1.87 and 2.47 after 1 and 2 hr, respectively. This polar transport of Ca across tips of primary roots correlated positively with a gravicurvature of 43° after 2 hr. The ratio of cpm in receiver blocks on the bottom of lateral roots: cpm in receiver blocks on the top of lateral roots was 1.20 after 2 hr. The decreased polar movement of Ca across tips of lateral roots correlated positively with lateral roots being nongraviresponsive. These data 1 ) support the suggestion that gravistimulation induces polar movement of Ca toward the lower side of tips of primary roots, and 2) suggest that the reduced polar movement of Ca across tips of lateral roots may be involved in uncoupling gravistimulation from gravicurvature in lateral roots. SEVERAL STUDIES have suggested that calcium (Ca) plays an important role in plant gravitropisms. Evidence supporting this conclusion includes the following observations: 1) There is a gravity-induced accumulation of Ca on the upper side of horizontally oriented shoots (Arslan-Cerim,1966;GoswamiandAudus,1976; Slocum and Roux, 1983). 2) Inhibitors of calmodulin interfere with shoot gravitropism (Biro et al., 1982). 3) Ca-chelating agents applied to root tips cause a loss of gravitropic sensitivity that can be restored by replacing the chelating agent with CaCl2 (Lee, Mulkey and Evans, 1983b). 4) Unilateral application of Ca near the root tip induces curvature toward the Ca source (Lee et al., l 983b).5) Gravistimulation induces polar movement of Ca toward the lower side of caps of horizontally oriented primary roots (Lee et al., 1983a). Based on these observations, Lee et al. (1984) suggested that gravity-induced redistribution of Ca across the root cap may play an important role in the development of root gravicurvature. Primary roots are strongly graviresponsive, while lateral roots are only minimally graviresponsive (Ransom and Moore, 1983, 1985; Moore and Pasieniuk, 1984a, b; Moore, 1984b). X Received for publication 5 November 1984; revision accepted 21 March 1985. This research was supported by grants from the National Aeronautics and Space Administration and the University Research Committee of Baylor University. I thank David Herbert for his excellent technical assistance. Thus, comparisons of primary and lateral roots are an excellent means for distinguishing events critical to gravicurvature from those merely correlated with the process. In this study I have monitored the polar movement of 45Ca2+ across tips of primary and lateral roots of Phaseolus vulgaris (Fabaceae). Structural and functional studies of these roots are presented elsewhere (RansomandMoore, 1983, 1984, 1985;Moore,