Tolerances of 20 Rice Cultivars to Excess Al and Mn

Manganese toxicity is a rare occurrence in flooded or paddy rice (Oryza saliva L.); however, this is not the case for upland rice grown on aerobic soils. Although acid upland soils may contain toxic levels of both Al and Mn, cultivar tolerances to excess Al and Mn do not necessarily coincide. These studies were to determine whether a procedure could be devised for rapid screening of rice cultivars for tolerances to excess Mn and to correlate tolerances between excess Al and excess Mn. Nine rice cultivars were grown in a greenhouse for 4 weeks in nutrient solutions containing 1 µg·cm⁻³ and 80 µg·cm⁻³ Mn. The relative tolerance was expressed as a ratio of the excess Mn treatment to the optimum or control treatment using weights of plant parts, height, root length, and tiller number. These expressions of relative tolerance to excess Mn were used to rank the nine cultivars. These rankings were then compared to a ranking based on previous results obtained at IRRI when the cultivars were grown on an acid soil. Significant Spearman's rank correlation coefficients were observed between rankings for tolerance to acid soil toxicity and relative root weight (r₈ = −0.93**, significant at the 0.01 level) and relative tiller number (r₈ = −0.68*, significant at the 0.05 level). Other rank correlations with acid soil toxicity were not significant. Subsequently, 20 cultivars were screened for tolerance to excess Mn by growing them in a greenhouse for 4 weeks in solutions containing 0.5 and 80 µg·cm⁻³ Mn. These cultivars were also screened for tolerance to excess Al by growing them for 3 weeks at 3 and 30 µg·cm⁻³ Al. The correlation between tolerances to excess Al and Mn was tested using relative root length for Al and relative shoot weight for Mn. The correlations were not significant. Large differences in tolerance to excess Mn were observed among rice cultivars; however, it was not established that screening trials using soils containing excess amounts of available Mn can be replaced by short-term screening in nutrient solutions. The results of this study clearly indicate that tolerance to excess Al is not necessarily correlated with tolerance to excess Mn.