Abstract
Phosphorus (P)-deficiency is a major abiotic stress that limits legume growth in many types of soils. The relationship between Medicago and Sinorhizobium, is known to be affected by different environmental conditions. Recent reports have shown that, in combination with S. meliloti 2011, Medicago truncatula had a lower symbiotic efficiency than Medicago sativa. However, little is known about how Medicago–Sinorhizobium is affected by P-deficiency at the whole-plant level. The objective of the present study was to compare and characterize the symbiotic efficiency of N2 fixation of M. truncatula and M. sativa grown in sand under P-limitation. Under this condition, M. truncatula exhibited a significantly higher rate of N2 fixation. The specific activity of the nodules was much higher in M. truncatula in comparison to M. sativa, partially as a result of an increase in electron allocation to N2versus H+. Although the main organic acid, succinate, exhibited a strong tendency to decrease under P-deficiency, the more efficient symbiotic ability observed in M. truncatula coincided with an apparent increase in the content of malate in its nodules. Our results indicate that the higher efficiency of the M. truncatula symbiotic system is related to the ability to increase malate content under limited P-conditions.
Highlights
Barrel medic (Medicago truncatula Gaertn.) has emerged as a model plant for studying the general biology of legumes and for exploring the genetic and molecular aspects of N2-fixing symbiosis in leguminous plants [1,2,3,4]
For comparing the symbiotic efficiency between M. truncatula and M. sativa, both tested plant species were grown in sand cultures and provided with either 5% or 100% of their optimum P-requirements
The deficient and optimum P-levels were adjusted according to the daily P-requirement for each species, an important prerequisite that enables the full growth for each species without a possible underestimation for their potential growth
Summary
Barrel medic (Medicago truncatula Gaertn.) has emerged as a model plant for studying the general biology of legumes and for exploring the genetic and molecular aspects of N2-fixing symbiosis in leguminous plants [1,2,3,4]. This is due to its relatively small diploid (2n = 16) genome M. sativa is a perennial crop which is widely cultivated throughout the world and has been a focal point of N2 fixation research for many decades [14]. Both M. truncatula and M. sativa are readily nodulated by the soil bacterium
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