Plant Determinants of Mycorrhizal Dependency in Soybean

Even though the mycorrhizal relationship is thought to be important in nutrient and water uptake by plants, earlier work showed that soybean cultivars differ in responsiveness. The reasons for cultivar differences are unclear. The objective of this growth-chamber study was to relate plant-growth parameters and rooting characteristics of three soybean cultivars to mycorrhizal dependency (MD). The soybean cultivars were chosen from a preliminary study to represent a range in Ml) values from high (Glycine soja Sieb. & Zucc. PI 468916; referred to hereafter as Soja), to intermediate [G. max (L.) Merr. cv. Mandarin] and low (G. max cv. Swift). Shoot weight, P percentage in shoot tissue, total shoot P uptake, P use efficiency, rate of growth, and root acid phosphatase activity were measured at three harvests when plants were grown at three relative P levels, each with and without mycorrhizal fungal (Gigaspora margarita Becker & Hall) colonization. Soja developed an active symbiosis more quickly than Swift or Mandarin, and mycorrhizae clearly aided in P uptake. When averaged across harvests at low P, mycorrhizal Soja had 7.8 times greater total shoot P than nonmycorrhizal Soja. Comparable values were 2.4 for Mandarin and 1.5 for Swift. Soja roots showed higher phosphatase activity and a higher percentage increase in phosphatase activity with mycorrhizal colonization than did the other two cultivars. Nonmycorrhizal Soja root lengths (avg. 1401 cm) at 5-wk growth were significantly shorter than Mandarin (2303 cm) and Swift (2236 cm) root lengths. The mean diameter of lateral roots of Soja (0.38 mm) was greater than that of Mandarin or Swift (≈0.26 mm). Root lengths and root surface areas were negatively correlated (P < 0.01) with MD. Contribution from the Dep. of Agronomy, Iowa State Univ., Journal Paper no. J-16644 of the Iowa Agric. Home Econ. Exp. Stn., Ames, IA. Project no. 3155.