EFFECTS OF SALINITY AND CALCIUM ON SEEDLING EMERGENCE , GROWTH , AND SODIUM AND CHLORIDE CONCENTRATIONS OF CITRUS ROOTSTOCKS

Salt tolerance of several citrus rootstocks during ger mination, emergence, and early seedling stage was studied under greenhouse conditions. Salinity delayed and depressed seedling emergence, reduced seedling biomass, and altered mineral status of young citrus seedlings. However, magnitude of these parameters varied among rootstocks. The addition of 50 mol m-3 NaCI to a nutrient solution delayed seedling emer gence by 3 to 5 days with the exception of Troyer citrange (TC) which emerged the soonest beginning at day 10 after sowing. Final emergence was reduced by less than 30% in Carrizo citrange (CC), TC, and Swingle citrumelo (SC), and by more than 65% in Ridge pineapple (RP), Cleopatra mandarin (CM), and rough lemon (RL). Total seedling biomass was re duced by over 50%. The addition of NaCI increased Na and Cl in the shoots and roots of all rootstocks. The addition of 5 mol m-3 CaS04 to the saline solution enhanced the emergence of the first seedling in CC, CM, and RL, improved final emer gence in SO, SC, RP, CM, and RL, and improved seedling growth of SC and RL. For citrus, no uniform trend was found in the relationship between salt tolerance during emergence and during seedling growth. Salt tolerance at emergence may not be a useful indicator for rapid screening of citrus cultivars. Citrus rootstocks are propagated mostly from seeds. Although modern techniques place an increasing emphasis on plant uniformity and the importance of seeds in the overall citrus nursery operation (Castle, 1981), there are only 2 reported studies of water stress involving the effect of polyethylene glycol (PEG) on citrus seedling emergence (Chilembwe et al., 1992; Kaufmann, 1969) and only one Florida Agricultural Experiment Station Journal Series No. N-00657. ■Present address: c/o Jack M. Berry, Inc., P. O. Box 459, LaBelle, FL 33935. study dealing with the effect of PEG, NaCI, and Na2SO4 on citrus germination (Mobayen and Milthorpe, 1978). Soil salinity may reduce seed germination by reducing water uptake by seeds or by allowing seeds to absorb exces sive ions that are potentially toxic (Ayers and Hay ward, 1948). These physico-chemical effects may reduce germi nation and the emergence of seeds. Salt stress may have different effects on seed germination and young seedling growth. Some crops, such as alfalfa and sugar beets, are relatively tolerant to salinity during the later stages of growth but are sensitive to salinity during germination (Ayers and Hay ward, 1948). Other crops, such as rice, are much more salt sensitive during the young-seedling stage of development than during germination (Pearson et al., 1966). In citrus, it is not known whether salt tolerance dur ing germination or seedling emergence is related to toler ance during later growth stages. The effect of salinity on citrus seedling emergence has not been well investigated, and information on the toler ance of citrus seedlings at early stages is lacking. There fore, a study was initiated to evaluate the effect of salinity on seedling emergence and early stages of seedling devel opment of various citrus rootstocks. Another objective of this work was to investigate the potential improvement in seedling emergence under saline conditions due to supple mental Ca because CaSO4 has been found to alleviate the adverse effects of NaCI on citrus tree growth (Zekri and Parsons, 1990). Materials and Methods The experiment was carried out in a greenhouse in which the temperature and relative humidity ranged from 16 to 34°C and from 50 to 100%, respectively. The tested rootstocks were sour orange (Citrus aurantium) (SO), Volkamer lemon (C. volkameriana) (VL), Ridge Pineapple sweet orange (C. sinensis) (RP), Cleopatra mandarin (C. reshni) (CM), rough lemon (C. jambhiri) (RL), Carrizo (CC) and Troyer citranges (TC) (C. sinensis x Poncirus trifoliata), and Swingle citrumelo (C. paradisi x P. trifoliata) (SC). 18 Proc. Fla. State Hort. Soc. 106: 1993. All seeds were about 1 yr old with the exception of those of RL which were 2 yr old. The seeds were sown in plastic trays composed of 96 individual cells. Each cell was 4 cm in diameter at the top and 12 cm deep having a conical shape at the bottom with one hole for water drainage. Two-thirds of each cell were filled with a commercial grow ing medium (Terra-lite Metro-mix 500), then one intact seed per cell was placed horizontally on the growing me dium and covered with 1 cm layer of coarse vermiculite. The seeds were watered to excess immediately after plant ing and every other day thereafter with corresponding treatment solution to prevent any build-up of salts. The initial seed water content on a dry weight basis was meas ured by drying 10 g of intact unsorted seeds for each rootstock in an oven at 105°C for 48 h. The experiment consisted of 4 salinity treatments: no salt in one-tenth strength Hoagland's solution (—0.01 MPa, 100 ppm), 50 mol nr3 NaCl (-0.23 MPa, 3000 ppm), 100 mol m-3 NaCl (-0.46 MPa, 6000 ppm), and 50 mol nr3 NaCl plus 5 mol m-3 CaSO4 (-0.24 MPa, 3500 ppm) dissolved in one-tenth strength Hoagland's solution. The experimental design was a split-plot with 4 main plots (treatments) and 8 subplots (rootstocks) and with 12 seeds as the experimen tal unit replicated 4 times. The appearance of the shoot at the surface of the grow ing medium was considered emergence. Emerged seedlings were counted daily for 2 months after the first seedling appeared. Number of days to emergence of the first seed ling, number of days between the emergence of the first and last seedling (emergence spread), and final percent emergence were calculated from the daily counts. Ten weeks after sowing, roots were washed free of Metro-mix and were separated from the shoots. Roots and shoots were oven-dried for at least 2 d at 65°C, weighed, ground in a Wiley mill, and stored for mineral analysis. Nitrogen was determined by the micro-Kjeldahl method using Buchi digestion, control, and distillation units. Chloride was measured using a Buchler-Cotlove chloridometer after extracting the samples with a nitric-acetic acid solution. Samples were analyzed for Na after dry ashing in a muffle furnace for 5 h at 500°C and disolving in IN HC1 solution. The concentrations of Na were determined using an inductively coupled argon plasma emission spectrome ter (Perkin Elmer, Plasma 40). Analysis of variance was used to determine significant differences among treatments and among rootstocks, and Duncan's multiple range test was used for comparison when the F test was significant at P < 0.05.