Aluminum (Al) partitioning in intact roots of wheat (Triticum aestivum L.) cultivars that differ in sensitivity to Al was investigated. Roots of intact seedlings were exposed to Al for up to 24 hours and distribution of Al was assessed visually by hematoxylin staining or by direct measurement of concentration of Al by atomic absorption spectrophotometry or ion chromatography. Major differences in Al accumulation between Al-tolerant (Atlas 66) and Al-sensitive (Tam 105) cultivars were found in the growing regions 0 to 2 and 2 to 5 millimeters from the root apex. Al content was 9 to 13 times greater in the 0 to 2 millimeters root tips of cv Tam 105 than in the tips of cv Atlas 66 when exposed to 50 micromolar Al for 19 to 24 hours. The oxidative phosphorylation inhibitor carbonyl cyanide m-chlorophenylhydrazone and the protein synthesis inhibitor cycloheximide increased Al uptake by intact root tips of cv Atlas 66. Also, loss of Al from the roots of both cultivars was measured after the roots were "pulsed" with 50 micromolar Al for 2 hours and then placed in an Al-free nutrient solution for 6 hours. The 0 to 2 millimeter root tips of cv Tam 105 lost 30% of the absorbed Al, whereas the tips of cv Atlas 66 lost 60%. In light of these results, we conclude that the differential Al sensitivity in wheat correlates with the concentration of Al in the root meristems. The data support the hypothesis that part of the mechanism for Al tolerance in wheat is based on a metabolism-dependent exclusion of Al from the sensitive meristems.

Passive influx of 45Ca2+ into non‐growing corn root tissue (Zea mays L.) was increased as a result of actions (cutting, rubbing, chilling, heating, acidifying) or agents (cyanide, uncouplers) known to depolarize the cell membrane, and was decreased by actions (washing) or agents (fusicoccin) known to hyperpolarize it. These responses indicate the presence of Ca2+ channels which are voltage controlled. If the injuries were extensive, however, voltage control was lost and hyperpolarization with fusicoccin was expressed by increased 45Ca2+ influx. Control could be regained by tissue washing, and millimolar levels of external Ca2+ would protect against loss of control. Influx of Ca2+ was strongly inhibited by La3+, but only weakly by verapamil. Intact roots showed greater cold shock sensitivity in maturing cells than in growing cells. We conclude that corn roots normally restrict Ca2+ influx by a mechanism linked to hyper‐polarization of the plasmalemma. Calcium ions which enter cold‐shocked tissue are partially extruded during the early phase of recovery by a process stimulated by fusicoccin and subject to uncoupling.

A study was conducted to examine aluminum (A1) exclusion by roots of two differentially tolerant soybean (Glycine max L, Merr.) lines, P1-416937 (Al-tolerant) and Essex (Al-sensitive). Following exposure to 80/~M A1 for up to 2 h, roots were rinsed with a 10 mM potassium citrate solution and rapidly dissected to allow estimation of intracellular A1 accumulation in morphologically distinct root regions. Using 10 min exposures to 300 ~M 15NO~-and dissection, accompanying effects on NO 3 uptake were measured. With AI exposures of 20 min or 2 h, there was greater A1 accumulation in all root regions of Essex than in those of P1-416937. The genotypic difference in AI accumulation was particularly apparent at the root apex, both in the tip and in the adjacent root cap and mucilage. Exposure of roots to A1 inhibited the uptake of 15NO~-to a similar extent in all root regions. The results are consistent with AI exclusion from cells in the root apical region being an important mechanism of A1 tolerance.

(2,14,25). IP3 may be phosphorylated to form other inositol phosphates (19) or hydrolyzed in a 3-step dephosphorylation into inositol and Pi by specific phosphatases (3).Although very little is known about signal transduction in higher plant cells, calcium has become recognized as a key regulator of plant metabolism (11,12,18,23,26). It activates protein kinases (13), and plays a critical role in cell division (12), secretion (7), protoplast fusion (10), and tropic responses (22,23). All of these physiological responses appear to be preceded by an increase of cytosolic free Ca2' concentration, but it is not clear whether the increase in cytosolic Ca24 concentration is due to an increase in Ca2`influx across the plasma membrane or to an increase in Ca2' release from internal stores (i.e. ER, vacuole, or mitochondria) or both.There is evidence that the biosynthesis and metabolism of polyphosphoinositides occur in higher plant cells (6,20,24 MATERIALS AND METHODS Protoplast Isolation. Wild carrot cells (Daucus carota L.) which yield fusogenic protoplasts, were grown in suspension culture as described previously (4). Protoplasts were isolated by placing 0.3 g of cells in 20 ml solution containing 2% Driselase (Plenum Scientific Co., Hackensack, NJ), 0.4 molal sorbitol, 2 mM EGTA, and 1 mm Mes (pH 4.8) at 25°C on a rotary shaker (125 rpm) for 1.5 h for the 45Ca24 efflux experiments and for 2 h for the 45Ca2+ influx experiments. The protoplasts were centrifuged (40g) and the supernatant discarded. The recovered protoplasts were washed twice with an osmoticum solution consisting of 0.45 molal sorbitol and 0.5 mM Mes (pH 6).4"Ca2 Influx. For calcium influx experiments, protoplasts (1-2 mg protein/ml) were incubated in 8 ml osmoticum containing 45Ca2" (0.4 MCi/ml) to a final Ca2" concentration of approximately 1 uM. The protoplast suspension was placed on the rotary shaker (90 rpm) at 25°C. At intervals, 3 aliquots of 200 ,l were taken and protoplasts spun in a Beckman Microfuge B through a supporting and separating gradient formed, in order from the bottom, by 50 Ml of 5% dextran (mol wt = 17,900) in osmoticum, 50 ,ul silicone oil (p = 1.03), 50 ,l 2 mM EGTA in osmoticum, and 50 gl silicone oil (p = 1.01). The protoplast pellet was collected at the bottom of the microfuge tubes. The microfuge tips were cut and the protoplast pellet resuspended in 300 Ml deionized H20. Aliquots of 100,l ofthe resuspended protoplasts were dissolved in 7 ml Scintiverse-Il (Fisher