Influence of mycorrhizae and Rhizobium on cytokinin content in drought-stressed alfalfa

Abstract

The objective of this research was to study the growth response to drought of arbuscular mycorrhizal and non-mycorrhizal alfalfa (Medicago sativacv. Aragón) in relation to leaf cytokinin levels. In the experiment, four treatments were used: (a) plants inoculated with Clomus fasciculatum (Taxter sensu Gerd.) Gerdemann and Trappe and Rhizobium meliloti 102 F51 strain, (MR); (b) plants inoculated with only Rhizobium (RP); (c) plants inoculated with only mycorrhizae (MN); and (d) plants non-inoculated (NP). Non-mycorrhizal plants were supplemented with phosphorus and nonnodulated ones with nitrogen to achieve similar size in all treatments. Plants were subjected to drought by withholding irrigation in a cyclic way. The effects of drought on growth, number of stems, degree of senescence, and leaf cytokinin levels were measured. Results of identification of cytokinins showed that dihydrozeatin riboside (dHZR) and ortno-topolin riboside (oTR) were predominant in alfalfa leaves. Nonsymbiotic plants (NP) showed higher total cytokinin concentrations (dHZR and oTR). Under drought, NP plants showed the largest percentage drop in cytokinins and lower number of stems as well as increased degree of senescent leaf tissue relative to control values. By contrast, stressed symbiotic plants (RP, MN and MR) showed higher green leaf weight than nonsymbiotic ones (NP) due to delay of leaf senescence and maintenance (RP) or increase (MN, MR) of stem leaf cytokinin levels during drought. The relationships between growth and the different cytokinins are discussed, suggesting an important role of mycorrhizal symbiosis in maintaining cytokinin levels under drought.