SEVERITY OF NET BLOTCH (Pyrenophora teres) IN BARLEY: PHYTOHORMONE SIGNALING UNDER ALUMINIUM TOXICITY AND WATER DEFICIENCY

Abstract

Aluminium toxicity, drought and net blotch coupled with their interactions, biochemical reactions and induction of hormonal signaling are responsible for low barley yields in Kenya. The studies aimed at identifying genotypes exhibiting multiple tolerances to the three stresses, assess interaction effect of drought and Al toxicity on net blotch severity; and assess hormonal signaling effect in the management of these three stress factors. Baseline screening for Al toxicity, drought and net blotch response was performed to group genotypes. Al tolerance was assessed on Magnavaca solution with 0 and 148 μM Al under 14 hour light intensity (340 μmoles m-2s-1) at 30 oC and 70% relative humidity alternated with dark conditions at 22 oC and 90% relative humidity inside a growth chamber. Data on net root length, relative net root growth, percent response and hematoxylin staining were recorded. For drought, electrical conductivities of leaf leachates were measured at 40 oC for 30 as C1 and at 100 oC for 15 minutes as C2 then used to determine the membrane stability index. Same genotypes were maintained at 20% (stressed) and 80% field capacities and height, tillering ability, number of grains per spike and 1000 seed weight were recorded. Net blotch screening was done under hot-spot field conditions in RCBD and scored on a 0 – 9 scale. Secondly, trait specific genotypes were inoculated with 5 x 103 spore concentration of P. teres at Zadoks growth stage 15 in a split – plot in CRD with 148 μM Al, 20% FC and 80% FC conditions as main plots and genotypes as sub plots. 0 – 7 severity scales was used. Thirdly, hormonal treatments with 50 μM SA, 20 μM ABA, SAxABA and a control were applied on barley affected by drought, Al toxicity and net blotch infection. Data were analyzed on Genstat statistical package version 14.1 and means separated using contrast, comparison. Genotypes differed significantly (p < 0.05) and only 4 out of 32 showed multiple tolerances to three stresses. The rest exhibited double and single tolerance. Also, initial exposure to drought and Al toxicity significantly reduced net blotch severity compared to unstressed set of the same genotypes. SA induced tolerance to disease while ABA signaled tolerance to drought and Al toxicity. Synergistic effect of SAxABA was minimal on net blotch but more significant for abiotic stresses. In conclusion, tolerance to stress in barley involves a complex system of inherent traits, interaction of stress factors and activation of signal pathways induced by phytohormones.