Imbibition Response of Winter Wheat to Water-Filled Pore Space

Reduced temperature and increased bulk density associated with conservation tillage systems cause lower seed germination, seedling emergence, and early growth rates resulting in reduced plant stands. Prediction of the influence of soil condition on seed imbibition through simple soil measurements would help make agronomic decisions such as planting date and/or density. Our objectives were to evaluate the influence of soil water-filled pore space on winter wheat (Triticum aestivum L.) seed imbibition and to assess the possibility of describing the relationship through simple mathematical models. We measured the rate of water uptake by heat-killed wheat seeds at three levels of water-filled pore space (WFPS: 0.35, 0.60, and 0.85) and temperature (T: 278, 283, 288 K) and two levels of bulk density (Pb: 1.25 and 1A0 Mg -3) i n a Sharpsburg silty clay loam topsoil. The model proposed in 1972 by Blacklow to estimate seed water content (0s) after imbibing water for time t, 0s~t) = (m + ot) (m 0s~0)) e-qt, was fitted to seed water content as a function of time and initial seed water content, 0s<0). This equation adequately described the process of water absorption (for 18 treatment combinations, Rz > 0.963). The model parameter o was related (Rz = 0.88) to WFPS and Ob, and q was related (Rz = 0.78) to T and WFPS. The third parameter, m, was significantly but weakly related (P < 0.01, ~ =0.26) to ini tial see d weight. We showed that eas ily mea sured soil properties and simple mathematical models can be used to predict wheat seed imbibition under a variety of soil conditions. p ROPER GERMINATION of planted seeds and establishment of seedlings are of general importance in agriculture because they determine density of stand, influence the degree of weed infestation, and in extreme situations limit yield (Hunter and Erickson, 1952; Hillel, 1972). The first phase of germination, imbibition, is characterized by rapid hydration of seed constituents and is associated with important changes in macroand microstructure of the seed (Bewley and Black, 1978; Hegarty, 1978; Simon, 1984). Imbibition is a physical process (not associated with seed viability) that is related to seed and substrate colloid properties (Mayer and Poljakoff-Mayber, 1982). The seed-soil system is complex, with two media (soil and seed) through which water must move. Water must also traverse the seed-soil interface during imbibition (Bewley and Black, 1978). The process of imbibition is determined by three factors: composition (chemical and structural) of the seed, characteristics of the tissues covering the seed, and availability of water in liquid or gaseous phase (Mayer and Poljakoff-Mayber, 1982). Thus, the rate at which water is taken up by seeds is a complicated function of both the soil microenvironment and intrinsic properties of the seed (Vertucci, 1989). The driving force of water flow from the soil to the seed is the difference in water potential between the two (Bewley and Black, 1978; Vertucci, 1989); however, soil hydraulic conductivity and seed-soil contact (Hadas and Russo, 1974; Shaykewich and Williams, 1971a,b; Ward and Shaykewich, 1972) also influence imbibition. Scientists have repeatedly reported that a reduction in water potential of the surrounding medium (Hadas and Russo, 1974; Hadas, 1976, 1977; Singh and Singh, 1982), hydraulic conductivity, or seed-medium contact (Hadas and Russo, 1974; Shaykewich and Williams, 1971a; Ward and Shaykewich, 1972; Williams and Shaykewich, 1971) decreases seed water uptake rate. Soil temperature is a factor of primary importance in determining the rates and directions of soil physical processes (Hillel, 1980b). In most seed types, the main effect of temperature is on the rate of water diffusion and uptake (Blacklow, 1972; Leopold, 1980; Vertucci and Leopold, 1983), manifest through changes in both water viscosity (Murphy and Noland, 1982; Vertucci and Leopold, 1983) and hydration pattern of cellular constituents (Crowe et al., 1989). All soil characteristics affecting soil matric potential, hydraulic conductivity, and temperature are affected, either directly or indirectly, by soil management practices (Hillel, 1980a), especially tillage system (Unger and McCalla, 1980; Griffith et al., 1986; Mielke et al., 1986). Prediction of tillage system effects on the process of seed imbibition would be an important tool to help make agronomic management decisions (planting date and/or density). Very complex models are required to account for all the factors involved in the process (Bruckler, 1983; Bouaziz and Bruckler, 1989) and are of doubtful usefulness for general field application if based on inputs that are difficult to measure. Such a tool becomes helpful only when accurate prediction can be made based on simple measurements. Water-filled pore space (also known as degree of saturation; Hillel, 1980b) is an easily measured soil parameter; it is the volume of water present in the soil relative to total volume of pores. Past research has demonstrated that soil WFPS adequately explained the variation in soil microbial activity under different tillage systems (Linn and Doran, 1984; Doran et al., 1988). We proposed that close relation also exists between seed imbibition and the combined effect of soil WFPS, temperature, and bulk density and that quantification of this effect can be used to predict seed imbibition under various soil conditions. Therefore, we designed an experiment to determine and evaluate the influence of the combined effects of WFPS, temperature, and bulk density on wheat seed imbibition and to assess the possibility of describing these relationships through simple mathematical models. G.A. Studdert, Facultad de Ciencias Agrarias, Univ. Nac. de Mar del Plata, C.C. 276, (7620) Balcarce, Buenos Aires, Argentina; and W.W. Wilhelm and J.E Power, USDA-ARS, Dep. of Agronomy, Univ. of Nebraska-Lincoln, Lincoln, NE 68583-0938. This paper is a joint contribution of USDA-ARS and Nebraska Agric. Res. Div. and is published as Journal Series no. 10214. Received 14 Dec. 1992. *Corresponding author (Email: [email protected]). Published in Agron. J. 86:995-1000 (1994). 995 Abbreviations and variables:f (t), the linear portion of the Blacklow (1972) equation, tn + ot; m, o, q, curve-fitting parameters; oe, estimated value of o; PVC, polyvinyl chloride; qe, estimated value of q; t, time; ti, time to occurrence of event i (completion of exponential phase); T, temperature; WFPS, water-filled pore space; 0s, seed water content; 0s(0), initial seed water content; 0s0), seed water content at time of event i (completion of exponential phase); 0sit), seed water content at time t; Oh, bulk density; ~s, seed .water potential. 99(5 AGRONOMY JOURNAL, VOL. 86, NOVEMBER-DECEMBER 1994 Table 1. Particle size analysis and organic carbon content for the Sharpsburg soil used in the experiment. The site of collection was AIvo, NE (40°51’ N, 96036’ W). Particle size analysis, Ixm <2 2-50 50-200 Organic C Depth Horizon kg kg -j cm 0.3.’;9 0.631 0.030 0.017 0-30 A MATERIALS AND METHODS Particle size distribution and organic C content of the topsoil (Sharpsburg series; fine, montmorillonitic, mesic Typic Argiudoll) used for this experiment are shown in Table 1. A soil conditioning step was performed before starting the experiment to stimulate microbial activity and improve aggregation. This step consisted of planting oat (Arena sativa L. cv. Ogle) seeds into soil at a high density (= 1400 seeds -2 soil). O at p lants were grown in a greenhouse until the reproductive stage, when plants were cut and removed. Three treatment factors were considered in this experiment: soil temperature, soil bulk density, and soil WFPS. Temperature levels (T) of 278, 283, and 288 K were achieved by storing the soil and carrying out the experiment in a controlled environment room with temperature and relative humidity controls. Variation in T was + 1 K for each factor level. Low (1.25 Mg m-3) and high (1.40 Mg -3) bulk densities ( Pb) were s elected as similar to conditions in seedbeds prepared using conventional tillage and to conditions achieved after several years of cropping without tillage, respectively (L.N. Mielke, personal communication, 1987). Three levels of WFPS were used: 0.35, 0.60, and 0.85 m3 H20 m-3 soil pores. The 0.60 m3 m-3 WFPS treatment was chosen on the assumption that the relationship between WFPS and seed imbibition would be similar to that between WFPS and soil microbial activity reported by Linn and Doran (1984) and Doran et al. (1988). The two other values were sen to provide conditions of low and high water availability to seeds. Calculated total porosity, gravimetric water content, volumetric water content, and water potential for each pb x WFPS treatment level combinations are shown in Table 2. Winter wheat (cv. Centurk 78) seeds were used for the experiment. Seed was sieved to ensure that seed of a uniform size was planted. Prior to planting, seed was heat treated (363 for 12 h in a forced-convection oven) to kill the embryos and prevent germination, so that weight increase due to water absorption could be recorded over time. Seed used in the experiment was stored at 277 K and 70% relative humidity. Initial seed water contents are reported in Table 3. The experiment was conducted as a split-plot design, with the 2 × 3 (0b × WFPS) factorial treatments (Steel and Torrie, 1980) assigned to the subunits. The main experimental unit was the constant temperature room to which levels of Twere assigned ow~.r time. The experiment was conducted at the lowest T level (278 K) first, followed by 283 and then 288 K. The experiment was repeated three times for each T level. Individual observation effects were nested with T effects (Steel and Torrie, 1980). Experimental subunits were PVC cylinders (100.0 mm high, 51.7 mm i.d.). Treatment level combinations and location within the main experimental unit were randomly assigned to each subunit. Three groups of 10 seeds each (one of heat-killed seeds and two of viable seeds, although the latter were not used in the part of the experiment reported here) were placed between layers of preweighed, premoistened soil (according to the information shown in Table 2, to achieve target WFPS for each treatment level combination). Soil and seed groups were introduced alternately into the cylinders, and then the contents of each cylinder were packed with a hydraulic press to obtain 75.0-ramhigh soil cores with the desired p~ and WFPS. Seed spatial disTable 2. Calculated total porosity, gravimetric and volumetric water contents to achieve target water-filled pore space (WFPS) predetermined bulk densities (Pb), and measured soil matric potentials.