Contribution of Individual Culms to Yield of Salt-Stressed Wheat

Grain yield in wheat (Triticum aestivum L. emend. Thell.) is highly dependent upon the number of spike-hearing tillers produced by each plant. Soil salinity can greatly decrease their number and productivity. Knowing the contribution of specific tillers is essential for breeding salt-tolerant genotypes and for developing wheat growth simulation models. Our objective was to determine the effects of soil salinity on the contribution of individual cuhns to total grain and dry matter yields of two spring wheat cultivars, Anza and Yecora Rojo. Plants were grown in Pacbappa fine sandy loam soil (mixed, thermic, Mollic Haploxeralf) in outdoor lysimeters for 2 yr. Three salinity treatments were imposed by irrigating with waters containing equal weights of NaCl and CaClr (electrical conductivities z 1, 12, or 18 dS m-r). Despite substantial losses in the number of tillers at moderate levels of salt stress, grain yields of the main stem (MS) and tillers Tl and T2 were as great or greater than those on nonstressed plants of both cultivars. The contribution of the MS to yield on a land area basis increased from about 25 to 35% in nonsaline treatments to over 80% with increasing salinity. The contribution of primary tillers (E 5 8 65% in nonsaline conditions) decreased substantially only at the highest salinity levels. Salinity stress significantly decreased the number of spikelets per spike but the number of kernels per spike either increased or was unaffected except at the highest level of stress. Increasing salinity decreased total straw yields primarily because of fewer tillers, but dry weights of the MSs and remaining tillers were also smaller. Results show that loss of spike-bearing tillers accounts for most of the yield reduction in salt-stressed wheat. I N WHEAT (Triticum spp.), grain yield per unit area is highly dependent upon the number of productive tiller spikes, particularly in multi-tillering cultivars (Power and Alessi, 1978; Nerson, 1980: McMaster et al. 1994). Maas and Grieve (1990) found that yield reductions of USDA-ARS, U. S. Salinity Laboratory, 450 W. Big Springs Rd., Riverside, CA, 92507. Contribution from the U. S. Salinity Laboratory, USDAARS, Riverside, CA. Received 11 July 1994. *Corresponding author (!a03lcrivussl). Published in Crop Sci. 36: 142149 (1996). salt-stressed wheat and durum (T. turgidum L.) grown in a greenhouse were due primarily to the decrease in tiller spikes per plant. The detrimental impact of tiller losses on yield were also observed in the field (Francois et al., 1994). However, under conditions where control plants averaged less than one tiller per plant, soil salinity, i.e., mean rootzone electrical conductivity of a saturatedsoil extract (K~) up to 14 dS m-’ (deciSiemens per meter = mmho cm-‘), did not reduce the number of tiller spikes per unit land area (Francois et al., 1986). Of course, since tillering is highly dependent upon plant population density (Puckridge and Donald, 1967; Darwinkel, 1978), the effects of salinity on tiller production could be expected to vary depending on the number of plants per unit area. It is generally accepted that the MS and the primary true-leaf tillers produce most of the grain (Rawson, 197 1; Ishag and Taha, 1974; Fraser and Dougherty, 1978; Power and Alessi, 1978; Thorne and Wood, 1988). The coleoptilar, prophyll, and higher-order tillers are the most susceptible to environmental stresses. Knowing the contribution of specific tillers to total yield and their vulnerability to salt stress could greatly improve efforts to breed higher-yielding genotypes that are more tolerant of salinity. This information is also essential to the development of realistic wheat growth simulation models (Roy and Gallagher, 198.5). The results reported here are part of a larger study designed to determine the effects of salt stress on the rate and extent of growth and development of all shoot organs of two semidwarf, hard red spring wheat (T. aestiAbbreviations: K,, electrical conductivity of a saturated-soil extract; K,,, electrical conductivity of irrigation water; K,,, electrical conductivity of soil water; X,,, depth-averaged electrical conductivity of soil water in the = root zone; K,,, depthand time-averaged electrical conductivity of soil water in the root zone during the growing season; MS, main stem; TO, T1 , Tn. tillers; LSDaos, least significant difference at the 5% level of probability; HI, harvest index. MAAS ET AL.: INDIVIDUAL CULM YIELD OF SALT-STRESSED WHEAT 143 vum L. emend. Thell.) cultivars, Anza and Yecora Rojo. Some results have been reported (Grieve et al., 1992, 1993; Lesch et al., 1992). Tiller development and abortion were addressed earlier (Maas et al., 1994). Anza is a more vigorous tillering cultivar than Yecora Rojo and appears to be slightly more salt tolerant (Francois et al., 1994). Kelman and Qualset (1991) reported that Anza was more tolerant than Cajeme 71, a sib cultivar of Yecora Rojo. The specific objective of this paper was to determine the effect of soil salinity on the contribution of individual culms to total grain and dry matter yields. MATERIALS AND METHODS The experiment was conducted in outdoor lysimeters at the U. S. Salinity Laboratory, Riverside, CA. Two cultivars, Anza and Yecora Rojo, were grown during two consecutive years, 1989 and 1990. Details of the experiment have been described (Maas et al., 1994). Briefly, seed were sown on 11 January in 1989 and 1990 in lysimeters (3.0 m by 3.0 m by 1.5 m deep) containing Pachappa fine sandy loam. A planting density of 167 plants mm2 was attained with seed placed 40 mm apart in 0.15-m rows. The experimental design consisted of three salinity treatments replicated three times in a randomized, split-plot design, with salinity as main plots and cultivars as subplots. Differential salination was initiated on 30 Jan. 1989 and 23 Jan. 1990 by irrigating with Riverside tap water (control) or one of two saline waters containing different amounts of a mixture of NaCl and CaCl, (1: 1 by wt.). The average electrical conductivities of the three irrigation waters (K,,) were 0.9, 11.3, and 17.5 dS m-’ in 1989 and 0.8, 12.2, and 17.7 dS mm’ in 1990. All lysimeters were irrigated every 7 to 10 d to keep the soil matric potential of the control treatment above -85 J kg-’ at the 0.25-m depth. The total amount of irrigation water applied to each lysimeter between sowing and harvest was 690 mm in 1989 and 580 mm in 1990. The electrical conductivity of the soil water (K,,) was determined on soil solutions extracted after most irrigations at 25-, 45-, 75-, and l05-cm soil depths. Since rooting depth increased throughout the season, K,, was averaged over progressively deeper depths in the root zone. The soil salinity data and the method of averaging were described previously (Maas et al., 1994). The mean rootzone z,W values for the period between 20 Jan. and 9 May 1989 were 2.3, 13.0, and 15.9 dS mm’ for the control, medium, and high salinity treatments, respectively, and between 17 Jan. and 9 May 1990 they were 3.3, 18.7, and 25.8 dS m-l. Plants within each treatment were harvested as they matured (kernel hard)-Anza between 26 May and 8 June in 1989 and 30 May and 12 June in 1990; Yecora Rojo between 18 and 23 May in 1989 and 18 May and 4 June in 1990. Salt-stressed plants always matured first. Shoot dry matter and grain yields were determined on 10 randomly selected plants of each cultivar in each lysimeter. The plants were chosen from the third, fourth and fifth rows in from the edge of each lysimeter to minimize border effects. The MS and each tiller were identified and tagged as they emerged in accordance with the nomenclature of Klepper et al. (1982). Grain and straw yields of individual culms were measured after drying at 70°C.