Aluminum‐sensitive ‘Monon’ wheat (Triticum aestivum, spp vulgare (Vill. Host) Mac Key and ‘Kearney barley (Hordeum vulgare) (L. emend. Lam) varieties had higher root cation exchange capacities and induced lower pH levels in nutrient solutions than did Al‐tolerant ‘Atlas 66’ wheat and ‘Dayton’ barley varieties. Sensitive varieties contained higher concentrations of Al (and usually P) in their roots and lower concentrations of Ca in their tops than tolerant varieties, when grown in solutions containing Al. Furthermore, roots of Al‐sensitive varieties made less growth and contained higher concentrations of Al than those of tolerant varieties, even when the two were grown in the same container of vigorously aerated nutrient solution. This suggests either that zones of differential pH still exist around the roots of different varieties or that sensitive varieties absorb more Al at the same pH or both. Whether the lower Al tolerance of certain varieties is simply the result of greater accumulation of Al by roots, greater sensitivity to the same concentration of Al already absorbed, or both, cannot be determined from the present study. Differential Al tolerance of varieties was not closely related to differences in the Al or P contents of plant tops.

Differential Al tolerance of Atlas 66 and Menon wheat varieties in nutrient solutions and in acid Bladen soil was attributed, at least in part, to plant-induced differential pH changes around their roots.In nutrient solutions the Al-sensitive Monon variety lowered the pH and the Al-tolerant Atlas variety raised the pH, resulting in differences as large as 0.7 pH unit in the nutrient media.In acid Bladen soil this difference was much smaller, and a high root-to-soil ratio was necessary to produce observed pH differences that were significant. The factor responsible for higher Al-tolerance of Atlas was not transferred to Monon in mixed cultures in Bladen soil. The evidence indicates that the layer of plant-altered soil surrounding the roots was extremely thin.

No abstract

Differential aluminum tolerance of ‘Perry’ and ‘Chief’ soybean varieties, determined previously from growth on acid Bladen soil, was confirmed in nutrient solutions containing Al as the known growth‐limiting factor. Differences in Al tolerance between the two varieties were steadily increased as the Ca level of the nutrient solution was reduced from 50 to 8 to 2 ppm. Aluminum toxicity in soybeans was associated with decreases in concentrations of Ca in the tops and roots of both varieties, but this effect of Al was much more pronounced in the Al‐sensitive Chief variety than in the more tolerant Perry. Greater Al sensitivity of the Chief variety was associated with greater susceptibility to a petiole collapse symptom. This symptom was related to a lower Ca concentration in the leaves and petioles, and specifically, related to a lower Ca concentration in the small petiole zone actually showing the collapse. Soil and solution studies indicated that the Ca deficiency observed in acid Bladen soil was Al induced. Aluminum appears to interfere to different degrees in the uptake and use of Ca by these two soybean varieties. The petiole collapse appeared to be a secondary effect of Al injury, and the presence of the symptom was not required for yield reduction by Al. The fact that soybean varieties differ in Al tolerance suggests that plant breeders may be able to develop varieties that can root more effectively in acid, Al‐toxic subsoils. Varieties differing in Al tolerance also provide valuable tools for fundamental studies on the physiological nature of Al toxicity in plants. The petiole collapse symptom associated with Al sensitivity may be useful to plant breeders in screening genetic populations of soybeans for Al tolerance.

Previous studies have indicated that wheat (Triticum aestivum L.) cultivars absorbed nutrients more efficiently and induced a higher pH in the root zone than Al sensitive cultivars. In many of these studies, plants were screened on acid soils fertilized with NH4NO3 or in nutrient solutions containing up to 15% NH+4‐N. The research reported in this paper was conducted to determine the impact of NH+4‐N nutrition on the response of wheat varieties to Al and to define the relationship between plant‐induced pH changes in nutrient solutions and the utilization of NH+4‐N. ‘Atlas’ and ‘Redcoat’ wheat were grown in nutrient solutions that contained variable ratios of NO‐3‐N and NH+4‐N (100/0, 90/10, 80/20, and 70/30) with or without Al. In the absence of NH+4‐N and Al, both Atlas 66 (Al‐tolerant) and Redcoat (Al‐sensitive) wheats increased the pH of the nutrient solution at equal rates. With NH+4‐N (10 to 30% of the total N), Atlas 66 achieved a higher equilibrium pH than Redcoat. The Al treatments with NH+4‐N inhibited the uptake of NO‐3‐N, reduced the rate of pH increase and injured Redcoat more than Atlas. Aluminum injury was intensified by increasing the concentration of NH+4‐N in the solution. It was concluded that NH+4‐N uptake by Redcoat, with the coincident pH decrease and interference with NO‐3‐N metabolism, was a major determinent of sensitivity to Al.