Y. Cheng

Yield-limiting potential of necrotic and non-necrotic strains of bean yellow mosaic virus in narrow-leafed lupin (Lupinus angustifolius).

The yield losses caused by necrotic and non-necrotic strains of Bean yellow mosaic virus (BYMV) in narrow-leafed lupin (Lupinus angustifolius) were quantified in field experiments. Clover plants infected with either were introduced into plots to provide infection sources, and aphids spread infection to the lupin plants. When the effects of virus infection were examined in individual lupin plants infected with necrotic BYMV, they were killed by early infection so there was no seed production. With late infection, shoot dry wt, seed yield, and seed number were decreased by at least 55%, 80%, and 74%, respectively. With non-necrotic BYMV, shoot dry wt, seed yield, and seed number diminished with increasing duration of plant infection, these decreases ranging over 27-88%, 48-99%, and 35-98% for late to early infection, respectively. In partially infected stands in which both necrotic and non-necrotic BYMV were spreading, an additional incidence of 28% in plots with introduced non-necrotic strain foci over that in plots without introduced foci was sufficient to decrease overall seed yield significantly. However, an additional incidence of 10% was insufficient to do so in plots with introduced necrotic strain foci. In plots into which different numbers of clover plants infected with non-necrotic BYMV were introduced, subsequent incidence of infection depended on the magnitude of the initial virus source, and yield was decreased by 21-24%, 31-43%, and 64-66% with 4, 8, or 16 foci/plot, respectively. With both types of strain, yield loss in infected plants was mainly due to failure to produce any seed or to fewer seeds being produced, but smaller seed size also contributed. These results show that non-necrotic strains of BYMV have considerable yield-limiting potential in narrow-leafed lupin crops despite causing milder symptoms than necrotic strains. No evidence was obtained of seed-transmission of non-necrotic BYMV in narrow-leafed lupin, but a 0.2% seed transmission rate was detected in yellow lupin (Lupinus luteus).

Medicago sativa and Medicago murex differ in the nodulation response to soil acidity

Nodulation of Medicago sativa by Sinorhizobium meliloti is challenged by acidity, but the ability of M. murex to nodulate in acid soils provided the opportunity to compare the symbiotic development between the two species in an acid sandy soil. Soil acidity had different effects on the nodulation of Medicago spp. In soil of pH 4.3, M. murex produced fewer nodules than plants grown in soil of pH 7.0, but these nodules developed at a similar rate to those on plants grown in soil of neutral pH. The uppermost nodule on M. murex formed lower down the tap-root of plants grown in soil of pH 4.3. In identical soils, M. sativa produced fewer nodules when grown in the acidic soil and nodules appeared later compared to those on plants grown in soil of pH 7.0. However, the location of the uppermost nodule was the same in plants grown in soils of pH 4.3 and 7.0. It is suggested that M. murex formed the first nodules near the actively growing root tip, while M. sativa formed nodules later at the more mature region of the root, above the root tip.

Root and root hair mechanisms that confer symbiotic competence for nodulation in acidic soils within Medicago species: a holistic model

Three experiments were undertaken to investigate the mechanisms used by the annual medic species, Medicago murex (Murex medic) to achieve nodulation more rapidly in acidic soils than the perennial species Medicago sativa (lucerne). In experiment 1, numbers and locations of root hairs on the primary roots of medic and lucerne were determined from plants grown in soil of pH 4.3 and 7.0. Experiment 2 enumerated the numbers of Sinorhizobium medicae (rhizobia) associated with the roots of medic and lucerne when grown in an acidic soil. Experiment 3 used a GFP-marked strain of rhizobia to determine the localised distribution of rhizobia along various zones of the primary roots of medic and lucerne in soil of pH 4.3 and 7.0. When grown in an acidic soil, medic produced 60% more root hairs/mm root along the primary root axis compared with lucerne, from 7 days after sowing. In soil of low pH, medic also had higher numbers of rhizobia associated with its entire root system than lucerne, with about 103 cfu/cm root throughout the 24-day period of the experiment, compared with lucerne for which rhizobial numbers decreased from 102 to 11 cfu/cm root over the same time period. When the intensity of fluorescence emitted by a GFP-marked transconjugant of rhizobia was measured at localised zones along the primary root of medic and lucerne grown in the acidic soil, it was 1.8-fold higher from the roots of medic than lucerne at 7 days after sowing, indicating higher numbers of rhizobia along medic roots. Hence, the numbers of rhizobia associated with the entire root system, and those colonised on localised zones of the primary root of medic were both higher than that of lucerne. These fundamental differences between the 2 species, combined with the earlier finding that lucerne tends to acidify its rhizosphere more than medic, allowed us to construct conceptual models that attempt to explain the increased symbiotic competency of medic compared with lucerne, and the different nodulation responses between medic and lucerne at low pH.