Salinity Affects the Timing of Phasic Development in Spring Wheat

Understanding effects of environmental factors on crop phenological development is useful for predicting crop growth stages and scheduling management practices. We evaluated the effect of salinity on the rate of leaf appearance and the duration of critical stages of growth in wheat (Triticum aestivum L.) in terms of both thermal unit (TU; “C day) and phyllochron intervals. Two hard red spring wheat cultivars, Yecora Rojo and Anza, were grown two cropping years in greenhouse sand cultures and outdoor field lysimeters. In each case, two saline treatments were compared with a nonsaline control. The sand cultures were irrigated with complete nutrient solutions to which NaCl and CaCb were added. The electrical conductivities, to,,, were 2.0, 14.3, and 18.1 dS m-’ in 1989 and 1.7, 12.2, and 15.1 dS m-r in 1990. The ~4~ for the field lysimeters were 0.9, 10.7, and 17.2 dS m-r in 1989 and 0.8, 11.4, and 17.1 dS m-’ in 1990. Leaf appearance rate was determined by regressing the number of mainstem leaves against cumulative TU. In all treatments, the cultivars differed in both rate and duration of leaf appearance. The phyllochron increased with salinity. Leaves emerged more slowly in the greenhouse than in the field. Durations of the vegetative stages (“C day) from sowing to the initiation of the flag leaf and its subsequent appearance were relatively insensitive to salinity inasmuch as increases in leaf plastochron (TU between the initiation of successive leaves on a culm) and leaf phyllochron were balanced by decreases in leaf number. However, for both cultivars, salinity significantly reduced the thermal time between sowing and the reproductive phenological stages. C ROP GROWTH MODELS have become important tools for understanding and improving crop management. Several successful models of wheat canopy development are based on the timing of certain morphological events, specifically, the rate of appearance of successive leaves on the main culm (McMaster et al., 1991, 1992a,b; Rickman and Klepper, 1991). The usefulness of simulation models can be extended by evaluation and incorporaUSDA-ARS, U.S. Salinity Lab., 4500 Glenwood Drive, Riverside, CA 92501. Received 21 Dec. 1993. *Corresponding author. Published in Crop Sci. 34:1544-1549 (1994). tion of the effects of environmental factors, such as salinity, on leaf development. Numerous investigators have addressed the influence of environmental variables on leaf development in wheat. The final number of mainstem leaves is determined early in the life cycle of the crop and depends on the rate and duration of leaf initiation. For a given genotype, total mainstem leaf number can vary with sowing date (Kirby et al., 1985), light intensity (Simmons, 1987), water deficit (Oosterhuis and Cartwright, 1983), temperature (Amores-Vergara and Cartwright, 1984), plant population (Kirby and Appleyard, 1984), soil fertility (Longnecker et al., 1993), and salinity (Maas and Grieve, 1990). Leaf appearance on the main culm of wheat is a highly significant, linear function of TU expressed as “C day. Regression of leaf number, expressed in terms of Haun growth scale units (Haun, 1973), on TU gives leaf appearance rate. The reciprocal of this rate, the phyllochron, is the TU between appearance of successive leaves on a culm. This procedure for estimating the phyllochron is commonly, but not universally, used. Alternative methods of calculating the phyllochron often give different values even when the same data set is analyzed. Kirby (1988) found that the phyllochron value based on estimates of emerged leaves only (i.e., with the ligule emerged) was substantially shorter (82°C d) than the value obtained when both emerged and emerging leaves were considered (101°C d). McMaster et al. (1992b) determined the phyllochron by dividing the TU from seedling emergence by the Haun growth stage of 30-d-old plants. The phyllochron may also be calculated from the mean TU between growth stages of successive leaves as measured by the time when the youngest expanding blade Abbreviations: TU, thermal unit; C, nonsaline control treatment; M, medium salinity treatment; H, high salinity treatment; PPFD, photosynthetic photon flux density. GRIEVE ET AL.: SALINITY EFFECTS ON SPRING WHEAT PHASIC DEVELOPMENT 1545 can be seen emerging from the enclosing penultimate leaf (Krenzer et al., 1991). Temperature has a significant effect on leaf appearance rate, although plant response may be moderated by the change in photoperiod (Baker et al., 1980). Cao and Moss ( 1989) reported that the phyllochron of winter wheat varied from 56°C d leaf-’ at 7.5”C to 116°C d leaf-’ at 25” C. Leaf appearance is relatively unresponsive to nutritional deficiences unless they become severe. Longnecker et al. (1993) reported that severe N deficiency increased the phyllochron of spring wheat grown in sand cultures. Other investigators (Bauer et al., 1984; Frank and Bauer, 1982) found that, under field conditions, N status had no effect on the timing of leaf emergence. Reports on the effects of water deficit on leaf appearance of spring wheats are inconsistent. Bauer et al. (1984) found that, under field conditions, the phyllochron was insensitive to changes in the soil water regime. However, water stress, imposed under growth chamber conditions, slowed leaf appearance of several winter wheat cultivars, and the phyllochron increased from 87°C d leaf-’ under well-watered conditions to 102°C d leaf-’ in response to mild water deficit (Krenzer et al., 1991). Based on their finding that the phyllochron of water-stressed, nonirrigated wheat was significantly less than that of irrigated plants, Baker et al. (1986) speculated that higher canopy temperatures may have resulted in a faster rate of TU accumulation by the meristems of the nonirrigated plants. Leaf appearance rate also decreased in response to salinity. Compared with that of the control plants, the phyllochrons for a spring wheat and a durum cultivar salt stressed at -0.65 MPa increased 12 and 9%, respectively (Maas and Grieve, 1990). The objectives of the current study were to investigate the phasic patterns of two hard red spring wheat cultivars, Yecora Rojo and Anza, grown in the greenhouse and in field lysimeters and to quantify the effect of salt stress on the duration of discrete developmental periods. MATERIALS AND METHODS Greenhouse! Experiments The semidwarf hard red spring wheat cultivars, Yecora Rojo and Anza, were grown in sand tanks in a greenhouse at the U.S. Salinity Laboratory at Riverside, CA. Desdription of the tanks, nutrient solution composition, maintenance of the sand cultures, and irrigation scheduling are given in Grieve et al. (1993). Two salinity treatments were imposed by adding NaCl and CaC& (2: 1 molar ratio to simulate saline soil conditions) to the nutrient solutions. The base nutrient solution served as the nonsaline control treatment. In 1989, the electrical conductivities of the irrigation waters were 2.0, 14.3, and 18.1 dS m-’ to give osmotic potentials of -0.05, -0.65, and -0.85 MPa, respectively. In 1990, the electrical conductivities of the irrigation waters were 1.7, 12.2, and 15.1 dS m-’ with osmotic potentials of -0.05, -0.55, and -0.70 MPa, respectively, and are hereafter designated as C, M, and H salinity treatments. Salination began at seedling emergence, 3 d after planting. The osmotic potentials of the saline treatments were decreased to target levels by incremental additions of the salts for 3 consecutive d to avoid osmotic shock to the seedlings. The experimental design consisted of three salinity treatments replicated three times in a randomized completeblock, split-plot design, with salinity level as the main plots and cultivar as the subplots. During the 1989 experiment, daytime air temperatures ranged from 21 to 36°C (mean = 30°C) and nighttime temperatures, from 14 to 30°C (mean = 20°C). Relative humidity ranged from 40 to 99 % , with a mean of 60 % during the day and 81% during the night. In 1990, daytime air temperatures ranged from 23 to 36°C (mean = 28°C); nighttime from 6 to 19°C (mean = 15’C). Relative humidity ranged from 42 to 96%, with a mean of 59% during the day and 81% during the night. Field Lysimeter Experiments Description of the lysimeters and the cultural practices followed in the field plots are given in Grieve et al. (1993). Seeds of both cultivars were sown in the center 5.8-m2 area of the lysimeters on 11 January in both 1989 and 1990. Eight rows of each cultivar were planted per lysimeter each year. Rows were spaced 0.15 m apart, with the seeds placed 4 cm apart within the rows to give a sowing density of 167 seeds me2. Sowing depth was = 1.5 cm. In both years, the experimental design was identical to that given for the greenhouse study. To facilitate germination each year, 25 mm of low-salinity water (0.9 dS m-l) was applied to each lysimeter after sowing. When the coleoptiles had just emerged through the soil surface, differential salination was initiated by applying irrigation water containing equal weights of NaCl and CaC12. This represents a NaCl/CaC12 molar ratio of 1.9: 1, similar to the solution composition used for the greenhouse study. The average irrigation water salinities (K,,) were0.9, 11.3, and 17.5 dS m-’ in 1989 and0.8, 12.2, and 17.7 dS m-’ in 1990 to give the C, M, and H salinity treatments, respectively. A neutron probe and tensiometers were used to monitor soil matric potential and to guide irrigation frequency. Soil water contents were measured before and after most irrigations at depths of 25, 45, 75, and 105 cm and at two locations within each lysimeter. Soil water salinity was determined at these same depths by extracting soil solutions with porous ceramic suction tubes and measuring their electrical conductivity. Timeand depth-averaged salinities imposed on the cultivars from sowing in the field lysimeters to the end of various phenological growth stages are presented for 1989 and 1990 in Table 1. The following example illustrates the method for estimating the salinity stress encountered by the plants from sowing to the end of shoot primordium initiation. In 1990 Treatment M, terminal spikelet formation in Yecora Rojo occurred about 6 15 ‘C d (5 1 calendar d) after sowing. Rooting depth, based on neutron probe measurements of soil water extraction at this time, was =45 cm. The average salinity in the 0to 45-cm profile had ranged from 14.9 dS m-’ at sowing to 17.7 dS m-l on Calendar Day 50 with a mean of 16.3 dS m-l. The 1990 soil salinity values (Table 1) were higher than in 1989 because the soil profiles were still salinized from the previous year. Standard meteorological measurements were made with a Class I agrometeorological station adjacent to each experimental location. Soil-sand temperatures were measured at a depth of about 1 cm; air temperatures were measured about 2 m above the soil surface. Hourly temperatures were measured and integrated during the 24-h period; daily mean temperatures were summed to give cumulative thermal time (“C d). A base temperature of 0°C was used. At each location, PPFD was 1546 CROP SCIENCE, VOL. 34, NOVEMBER-DECEMBER 1994 Table 1. Timeand depth-averaged soil salinities from sowing to various phenological stages of Yecora Rojo and Anza spring wheat grown during two seasons in field lysimeters.