Factors responsible for genotypic manganese tolerance in cowpea (Vigna unguiculata)

Abstract

In experiments with 29 cowpea genotypes considerable variation in Mn tolerance could be found. Ranking according to Mn tolerance was almost the same in sand and water culture. Mn tolerance is not related to greater vigour or exclusion of Mn from uptake and translocation, but depends mainly on the internal tolerance to excess Mn especially in the leaf tissue.

Growth depression by Mn excess is characterized by local accumulation of Mn, deposition of Mn oxides, and typical macro-symptoms on the older leaves (brown spots→clorosis→shedding of the leaves). Autoradiographic studies with ⁵⁴Mn and extraction of the leaves with methanol and H₂O indicate a causal relationship between Mn tolerance and the more homogenous distribution of Mn in the tissue. In tolerant genotypes local accumulation and deposition of Mn is inhibited or retarded.

Mn applied to the petioles of fully expanded leaves induces the same toxicity symptoms on the leaf blades as Mn absorbed by the roots. There is a good agreement between the rankings of the different genotypes for Mn tolerance according to the depression of shoot dry matter production by Mn excess in long term pot experiments and the appearance of toxicity symptoms after application of Mn to the petioles.

The regulation of Mn tolerance at the leaf tissue level allows a quick and non-destructive screening of large numbers of genotypes for Mn tolerance.