CONICET | Buscador de Institutos y Recursos Humanos

Defoliation, that impacts plant growth, AM fungal symbiosis and nutrient mobilization is an important factor because the capacity of a forage plant to regrowth after clipping reflects the ability of this species to growth after grazing at field condition. Unfortunately, the available reports regarding the effect of defoliation in Lotus species have not included information of the influence on AM fungi and how the mycorrhizal symbiosis and nutrient mobilization can be associated in defoliated plants. Hence, we investigated how different defoliation intensities affect the ability of Lotus tenuis plants to recover the aerial biomass, mobilizing nutrients within the plant and to maintain the symbiosis with native AM fungi and Rhizobium bacterias in a saline-sodic soil of Depresion del Salado (Buenos Aires). Five L. tenuis cv. Chaja plants were grown in pots in a Typic Natraqualf saline-sodic soil (pH 9.4; electrical conductivity 5.07 dS/m, exchangeable sodium 60%) in a greenhouse. After 70 days of growth, five pots were harvested (initial time) and the remaining pots were subjected to defoliation treatments replicated five times. Defoliation treatment comprised removing the 25, 50, 75 and 100% of total aerial biomass with respect to the pots harvested at the initial time. For 100% treatment, the aerial biomass was clipped off 0.5 cm above the soil surface. Plants of five pots were not defoliated and used as control (0% treatment). Defoliated and non-defoliated plants were then grown for an additional 35-day period (recovery period) and harvested (final time). The effect of defoliation intensities during the recovery period was evaluated by measuring dry yields, P, N and Na+ concentrations, total chlorophyll, morphology of mycorrhizal colonization and Rhizobium nodules in roots and relative growth rates (RGR) for shoot and root dry mass from control and defoliated plants. Low or moderate defoliations did not affect the shoot regrowth with respect to non-defoliated plants. The RGR of shoot consistently increased after clipping with increasing defoliation and explains why the plants were able to compensate the clipped biomass during the regrowing period. However, the 100% defoliation treatment affected the shoot regrowth and plants were not able to compensate the growth reached by the non-defoliated plants. The roots were more affected by defoliation than the shoots. Both the biomass and the RGR of the roots drastically decreased with increasing defoliation intensities. This effect was also showed by the increase of the shoot:root ratio that was 1.8, 2.1, 2.9, 3.6, and 12.6 for the control and the 25-100% defoliation intensities respectively. These results suggest that the plants transfer most of the resources from the roots to the shoots to compensate the demands generated by the shoot regrowth. Lotus tenuis plants increased the concentration of P and N in shoot and root tissues with increasing defoliation intensities. This was because of the balance between the decrease in shoot biomass after clipping and the changes of nutrient uptake with defoliation. The concentration of Na+ in roots was always higher than in shoots from non-defoliated to moderate defoliated plants. The results are consistent with the salinity tolerant mechanism that control increases of Na+ in shoot tissue. However, at high defoliation intensity the Na+ concentration in roots and shoots markedly increased. We hypothesized that from non-defoliated to moderate defoliation, L. tenuis prevents the increase of Na+ concentration in shoots by reducing the Na+ uptake from the soil solution and then the subsequence Na+ transport from roots to shoots by increasing the amount of Na+ accumulated in the root system. At high defoliation intensity, the salinity tolerance mechanism is altered and the Na+ concentration in shoots increased and was higher than in roots. The increase of Na+ concentration in shoot tissue at high defoliation intensity was associated with a decrease in chlorophyll concentration. The leaves of high defoliation intensity plants were chlorotic compared to those of non-defoliated or low to moderate defoliated plants. This suggests that salt interferes with chlorophyll synthesis, and may affect the capacity of Lotus plants to recover the defoliated biomass. Defoliation intensity did not affect the fractions of root colonized by AM fungi, arbuscules and hyphae. The high proportion of roots colonized by arbuscules recorded in this work (ranging from 66% to 77%) suggests that Lotus plants and AM fungi may establish a functional symbiosis even when the plants have suffered intensive defoliations in a saline-sodic soil. However, the vesicular colonization significantly increased from 42% to 73% for the 25%-100% defoliated plants respectively compared with the non-defoliated plants. In addition, both the number of entry points per unit of root length and the spore density in soil did not change during the experiment, but the length of the extraradical hyphae increased with defoliation intensity. Our hypothesis suggests that the strategy of the AM fungal symbiont consists in investing more of the C resources to maintain preferentially the arbuscular colonization and the inoculum by exporting C compounds to keep up extraradical structures such as spores and hyphal network. Rhizobium nodules per unit of root weight did not change with defoliation intensities. Conclusions: Lotus tenuis plants growing in a saline-sodic soil were able to compensate the removed shoot biomass during a 35-day recovering period after being subjected to defoliation intensities till 75%. The regrowth was associated with the ability of the plants to satisfy the nutrient demands generated by the shoot growth and to control the Na+ uptake from the soil and the subsequent distribution of Na+ within the plant tissue. In addition, defoliated plants could maintain symbiotic relationships with native AM fungi and Rhizobium bacterias and the symbiosis may not have an additional C cost for the plant.