Effects of arbuscular mycorrhizal fungi on lettuce Lactuca sativa L. ‘Butterhead’ and rhizosphere microbes

Abstract

Recently, organic crop production areas are increasing due to health conscious consumers. Lettuce (Lactuca sativa L.) is the most important vegetable which has high demand all year round. Therefore, in order to improve quality and quantity of lettuce in organic farming system, natural microbe, arbuscular mycorrhizal fungi (AMF), were studied to select the potentially appropriate isolate and amount for increasing yield of lettuce var. ‘Butterhead’. Twenty-three AMF isolates were obtained from three organic farms planted lettuce (Nakhon Ratchasima; N1-N7, Ratchaburi; R1-R12, Pathum Thani; P1-P4). Potting experiments were conducted by inoculating 20 spores of each isolate into butterhead comparing to uninoculated plant used as a control. Sixty days after planting, lettuces were harvesteded and growth was measured. The result showed that isolate N3 was the best isolate that could significantly raise yield of lettuce comparing to control treatment, followed by N2, N4, N5 and R5, respectively. These five isolates were selected and number of spore of each isolate was varied for 50, 100, and 200 spores per plant as inoculum then their effects on butterhead growth were evaluated. Growth of butterhead inoculated with isolate N2, N4 and N5 were greater corresponding with increased number of spore while isolate N3 and N5 were inversed. The best isolates that could improve butterhead growth were 50 spores of N5 and 200 spores of R5. For cost-effective use, 50 spores of isolate N5 identified as Glomus mosseae was chosen for applying in trial field. Growth induction of 60 days-old butterhead in field experiment was presented in G. mosseae treatment by significant increase leaf number and leaf fresh weight. Chlorophyll a content of G. mosseae inoculation showed significantly higher while chlorophyll b content of the two treatments were not difference. The result of chlorophyll a content indicated correlation with net photosynthesis and intercellular CO2 in which net photosynthesis of 60 days-old G. mosseae inoculated butterhead was significantly higher than uninoculation while intercellular CO2 was lower. Organic matters, available phosphorous and exchangeable potassium were the highest in G. mosseae inoculated rhizosphere soil. Calcium and magnesium in rhizosphere soil decreased after butterhead cultivation. Total nitrogen, phosphorous and potassium tended to be higher in leaf and root of butterhead but were not significant difference. Catalase and Ascorbate peroxidase were not significant difference between treatments as well as vitamin C and phenolic content. Interestingly, scavenging of DPPH free radical revealed two folds higher in G. mosseae treatment than uninoculated treatment. Bacterial and fungal communities in G. mosseae inoculated soil were changed in diversity and abundance of some specific species. Proteobacteria was found to be increased and Acidobacteria was decreased in G. mosseae inoculated soil. Both of them were among the highest bacterial phyla abundance. Highest relative abundance fungal phylum was Ascomycota and the highest genera were Thielavia and Cochliobolus which exhibited dramatically decrease in G. mosseae inoculation soil. These results presented the practical application of AMF in field and they can be used to contribute many more experiments for finding other relevance microbes and methods which can improve plant cultivation in an organic farming system.