Plant determinants of mycorrhizal dependency in soybean

Even though the mycorrhizal relationship is thought to be important in nutr ient 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 par ameters and rooting characteristics of three soybean cultivars to mycorrhiz al dependency (MD), The soybean cultivars were chosen from a preliminary st udy to represent a range in MD values from high (Glycine soja Sieb, & Zucc. PI 468916; referred to hereafter as Soja), to intermediate [G, max (L.) Me rr. 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 fu ngal (Gigaspora margarita Becker & Hall) colonization, Soja developed an ac tive symbiosis more quickly than Swift or Mandarin, and mycorrhizae clearly aided in P uptake, When averaged across harvests at low P, mycorrhizal Soj a 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 ph osphatase activity and a higher percentage increase in phosphatase activity with mycorrhizal colonization than did the other two cultivars, Nonmycorrh izal Soja root lengths (avg. 1401 cm) at 5-wk growth were significantly sho rter than Mandarin (2303 cm) and Swift (2236 cm) root lengths. The mean dia meter of lateral roots of Soja (0.38 mm) was greater than that of Mandarin or Swift (approximate to 0.26 mm), Root lengths and root surface areas were negatively correlated (P < 0.01) with MD.