Leaf Emergence, Tiller Growth, and Apical Development of Nitrogen-Deficient Spring Wheat Nancy Longnecker,* E. J. M. Kirby, and Alan Robson ABSTRACT Conflicting reports exist about the effect of N supply on the rate of leaf emergence. We examined effects of N deficiency on leaf and tiller emergence, tiller initiation and apical development in ’Aroona’ and ’Gamenya’ spring wheat (Triticum aestivum L.). Fourlevels of N(e.g., 50 pMN = Nso) were supplied by hourly irrigation with complete nutrient solution of plants growing in sand. The control plants in Exp. 1 (N~oo) had 64 g -~ N in theshoots at t he two-leaf stage, compared with 33 in N~o, 50 in N3oo, and 58 in Nsoo. Compared with control plants, dry matter of Nso plants was 10%, N3oo50~, and Nsoo 80%. Results in Exp. 2 were similar. The rate of leaf emergence was de- creased in all Nso-treated and some N2oo-treated plants, but not or Nsoo-treated plants. Tiller bud initiation was decreased in the treatment. The number of tiller buds was correlated with total number of leaves; if a leaf emerged, Ndeficiency did not affect tiller initiation. Nitrogen treatment did not alter the sequence of tiller emergence, but tiller emergence was delayed or did not occur in Nso, N20o, and plants. Nitrogen treatmenthad little effect on the rate of apical de- velopment. The double-ridge stage of development was delayed ~2 d for both cultivars at the two lowest N treatments. Terminal spikelet productionwas also delayed by ~2 d at these N treatments in Aroona, but not in Gamenya spring wheat. The rate of primordia initiation was decreased in N~oand N~oplants, resulting in fewer spikelet pri- mordia. The level of Ndeficiency affected plant responseto the stress. T HE EMERGENCE of leaves and tillers of cereals is orderly and follows a predictable sequence (Friend, 1965). The rate of leaf emergence is a critical component of crop growth. In addition to its effect on canopy de- velopment, an important effect of the rate of leaf emer- gence on crop yield is its effect on the rate of tiller initiation, because early-formed tillers such as T1 and T2 are more likely to produce heads (Kirby and Riggs, 1978; Rawson, 1971). There are conflicting reports in the literature about the effects of N supply on the rate of leaf emergence of cereals. In a field study, Bauer et al. (1984) found effect on the phyllochron (degree-days required to pro- duce a leaf) of spring wheat. In sand culture, Dale and Wilson (1978) observed decreased rates of leaf emer- gence in N-deficient barley (Hordeum vulgare L.). Numerous studies have examined specific effects of N supply on tillering in cereals (e.g., Birch and Long, 1990; Bremner, 1969; Fischer and Kohn, 1966; Power and Alessi, 1978). However, the coordination of the processes of leaf and tiller emergence and apical devel- opment is incompletely understood. Tiller appearance can be measured with respect to the number of leaves that emerge in the interval between the emergence of a reference leaf and of the tiller that appears in the axil of that leaf. This can be expressed in terms of phyllochrons (Kirby et al., 1985) and the value has been reported N. Longnecker and A.D. Robson, Soil Science and Plant Nutri- tion, the Univ. of Western Australia, Nedlands, Perth 6009, WA, Australia; and E.J.M. Kirby, 147 Shelford Road, Cambridge CB2 2ND England. Received 20 June 1991. *Corresponding author. Published in Crop Sci. 33:154-160 (1993). approximately two to three (Klepper et al., 1982; Masle- Meynard and S6billotte, 1981). We use the term tillering interval to refer to the interval measured in phyllochrons betweenthe emergence of a leaf and the tiller in the axil of that leaf. In the relatively few studies that examine the effects of N deficiency on both leaf and tiller emer- gence and the relationship betweenthe two (Bauer et al., 1984; Dale and Wilson, 1978; Frank and Bauer, 1982; Frank and Bauer, 1984; Masle-Meynardand S6billotte, 1981), two-three levels of N were used and the results of high and low N treatments were compared. Except for the Dale and Wilson (1978) study, the N supply all of these studies is described in terms of the amount of N fertilizer addedrather than soil-available N. Our objective was to use controlled conditions to ex- amine the effects of a broad range of available N on leaf emergence, tiller emergence, tiller bud initiation, and apical development of the main stem of wheat. We com- pared two cultivars of spring wheat, Aroona and Ga- menya, since Aroonahas a larger increase in grain yield in response to N fertilizer additions (Anderson et al., 1991). MATERIALS AND METHODS Seed of two cultivars of spring wheat, Aroonaand Ga- menya, was sieved and the same size seed (meanweight of 47 mg seed -1) of each cultivar was sown in four fiberglass- lined circular concrete tanks (1 m diam., 1 mtall) containing washed white sand. The tanks were outside at ShentonPark, Western Australia (32 ° Slat) and were irrigated hourly with nutrient solution of the following concentration (/a2kO: 100 K2HPO4, 1000 K2SO4, 330 MgCI2"6H20,2000 CaCI2, 50 FeEDTA, 4.6 H3BO3, 0.45 MnCIE-4H20 , 0.097 CuSO4"5H20, 0.102 ZnSO4-7H20, 0.0103 (NH4)6Mo7024"4H20. The solu- tion was recireulated throughreservoir tanks containing200 L of solution. Solutions were changed weekly and adjusted to pH6.0. Four N treatments (one in each tank) were applied NH4NO 3 in each of two experiments. Nitrogen concentrations are given below. Plants were grown on an evenly spaced grid at 100 plants m -2. The tanks were divided into quadrants and Aroonaand Gamenya were grown in two of the quarters of each tank, providing duplicates of the cultivars at each N treatment. The tanks used for each N treatment were different in Exp. 1 and Exp. 2. Accumulated thermal time (growing degree-days) was cal- culated from daily maximum and minimum temperatures ob- tained from the Perth metropolitanmeteorological station. As all temperatures exceeded 6 °C, there was no need to subtract a base temperature. Regression analysis was done with Minitab(MinitabRelease 8, Minitab, State College, PA). The followingparameters were linearly regressed against accumulated thermal time using a general linear model:leaf emergence (up to emergence of the flag leaf), tiller budinitiation, and the ratio of tiller buds to total leaves. Statistical comparisons were made among N treat- ments and cultivars using the mean and variance of the regres- sion coefficients. The natural logarithm of dry weight was linearly regressed against days after sowing and the slope of the curve is reported as the relative growth rate. The number Abbreviations: DAS, days after sowing; EDTA, ethylenediami- netetraacetic acid; Tc, coleoptile tiller; Tn, nth tiller. 154 Published January, 1993