Stalk rot resistant sorghum genotypes are resilient to pathogen-mediated photosystem II quantum yield retardation

Abstract

Globally, stalk rots are among the highest priority grain sorghum (Sorghum bicolor (L.) Moench) diseases. This study examined the variation of leaf chlorophyll index and photosystem II quantum yield (Fv/Fm ratio) of sorghum genotypes after inoculation with two stalk rot fungi. In 2014, 2015, two resistant and two susceptible sorghum genotypes were evaluated in the field. Fusarium thapsinum (FT) and Macrophomina phaseolina (MP) are fungal pathogens that cause Fusarium stalk rot and charcoal rot, respectively. Plant stalks were inoculated with FT, MP, or phosphate-buffered saline (control). Flag leaves were measured for chlorophyll index (using soil and plant development meter: SPAD) and Fv/Fm ratio at three developmental stages (soft and hard dough, and physiological maturity). Both pathogens significantly reduced Fv/Fm ratio (P < 0.016) of the susceptible genotypes across environments and developmental growth stages. Neither pathogen affected the Fv/Fm ratio of the resistant genotypes. FT and MP inoculations significantly reduced chlorophyll index compared to the mock-inoculated control (P < 0.0001) across genotypes and developmental stages. For the susceptible genotypes, there was a significant and positive correlation between mean chlorophyll index and Fv/Fm ratio across developmental stages (r = +0.67, P = 0.018) while the relationship for resistant genotypes was significant and negative (r = −0.59, P = 0.043). The ability of certain sorghum genotypes to retain higher photosynthetic competence during development regardless of their foliar chlorophyll decline under disease pressure appeared to be a possible mechanism of physiological resistance against stalk rot diseases.