Growth, Phosphate Pools, and Phosphate Mobilization of Salt-stressed Sesame and Pepper

Abstract

The growth and phosphate mobilization of control and salt-stressed sesame (Sesmum hmgum L.) and pepper (Capsicwm anuum L.) plants were examined to ascertain whether or not translocation imits growth of salt-stressed plants. Plants were grown In a complete nutrient solution with and without excess salt. One-half of the control and salt-stressed plants were later transferred to phosphate-free culture solution ("-P" plants). Measurements of growth and phosphate pools in leaves indicated that with or without salnity "-P" plants utilzed their phosphate reserves to support growth for a time at rates equaling those of plants supplied with phosphate. The results indicate that mobilization was not limiting for growth of salt-stressed plants. Defoliation experinents were performed at a developmental stage when the import of assimilates by the youngest expanding leaves could be changed by removing certain source or sink leaves. These experiments also indicated that phloem transport was not limiting for leaf growth on salt-stressed plants. Salinity suppresses plant growth by mechanisms that are not understood. Since young growing tissues must import metabolites, it is of interest to know if the rate of translocation limits growth of salt-stressed plants as has been suggested (6, 8, 10, 11). We investigated this possibility by ascertaining the effects of salinity on growth, phosphate mobilization, and on the dependence of young leaves on source leaves. MATERIALS AND METHODS Plant Culture. Seeds of sesame (Sesamum indicum L. cv. Long pod) and bell pepper (Capsicum annuum L. cv. Yolo wonder) were germinated in Vermiculite moistened with dilute (1:10) Hoagland solution No. 1 (5). This solution was further modified by reducing the Pi concentration to 0.02 mm for pepper and 0.1 mm for sesame and by supplying iron as the chelate, diethylenetriamine penta-acetate (Sequestrene 330). Seedlings 1 or 2 weeks old, depending on the experiment, were transferred to 15-liter polyethylene pots containing a 1:2 dilution of Hoagland solution; the Pi concentration was 0.02 mm for pepper and 0.1 mm for sesame. All experiments except one were conducted in a growth chamber at 25 C and between 60 and 80% RH. A light intensity of 72 w/m2 at the plant tops was provided by a combination of fluorescent and incandescent lamps. The photoperiod was 16 hr/day. The defol-iation experiment in which young sesame leaves were removed was performed between December and February in a heated ' Permanent address: greenhouse with a day-night temperature range of 32 to 18 C and a natural light photoperiod of approximately 12 hr/day. The culture solutions were renewed weekly and/or adjustments were made to maintain the original concentrations. The pH was maintained between 5.5 to 6.5. The Pi concentration in the pepper cultures was closely controlled by circulating the solution from each set of four containers (replicate pots) through a 200-liter reservoir six times daily, each cycle lasting 0.5 hr, and periodically adding Pi to the reservoirs to avoid depletion of more than 30%o of the initial concentration. Culture solutions were salinized when plants were 2 to 4 weeks old. Salts were added to decrease the osmotic potential of culture solutions by 3 bars for pepper (2 bars NaCl + 1 bar CaCl2) and 2 bars for sesame (1 bar NaCl + 1 bar CaCl2 in the phosphate mobilization experiment and 2 bars of NaCl in defoliation experiments). The salts were added to the culture solution at the beginning of the dark period in increments of 1 bar/day. Growth and Phosphate Mobilization Experiments. To determine the efficiency of plants at mobilizing their reserve phosphate, Pi was removed from half of the cultures 10 to 16 days after salination. Plants were sampled randomly, separated into roots and shoots, and their dry weight (oven-dried at 70 C) measured. In one experiment with pepper, growth increments were measured by successive weighings of intact plants after allowing their roots to drain for 30 sec to remove most of the surface-adsorbed water. The acid-soluble phosphate fraction of lead samples previously frozen and stored in liquid N2 was extracted by grinding with cold, dilute perchloric acid (9). The Pi and total acid-soluble phosphate were measured by the Fiske and SubbaRow (4) procedure or by the Bartlett (1) modification of this procedure. In some samples from "-P" plants, the Pi concentration was too low to be determined, so only the total soluble phosphate (Pi + phosphate-esters) is reported. Defoliation Experiments. Defoliation experiments were performed 10 to 21 days after salination and at a developmental stage when it was expected that the supply of assimilates to the youngest unfolded leaf could be decreased (by removal of old source leaves) or increased (by removal of competing young leaves). AU leaves that attained two-thirds of their full size (as determined from separate but similarly treated plants) were regarded as exporting or source leaves and those less than two-thirds full size were considered as importing or sink leaves (3). In all experiments, the most recently unfolded leaf was retained and its rate of expansion measured. Leaf measurements and diagrams of representative patterns of defoliation and locations of the measured leaves are shown in Figures 4 and 5. RESULTS GROWTH AND PHOSPHATE MOBILIZATION Sesame. Removal of Pi from the culture solution did not change the growth (shoot or root) rates ofeither control or salinized plants 229