Phoma medicaginis Isolate Differences Determine Disease Severity and Phytoestrogen Production in Annual Medicago spp.

Phoma black stem and leaf spot disease of annual Medicago spp., caused by Phoma medicaginis, not only can devastate forage and seed yield, but also reduces herbage quality by inducing production of phytoestrogens, particularly coumestrol and 4'-O-methylcoumestrol, that can reduce ovulation rates of animals grazing infected forage. We determined the consequent phytoestrogen levels on three different annual Medicago species/cultivars (M. truncatula cv. Cyprus, M. polymorpha var. brevispina cv. Serena and M. murex cv. Zodiac), following inoculation with 35 isolates of P. medicaginis. Across the isolate x cultivar combinations, leaf disease incidence (%LDI), petiole disease incidence (%PDI), leaf disease severity (%LDS), petiole disease severity (%PDS), and leaf yellowing severity (%LYS) ranged up to 100, 89.4, 100, 58.1 and 61.2%, respectively. Cultivars Cyprus and Serena were most susceptible and cv. Zodiac the most resistant to P. medicaginis. Isolates WAC3653, WAC3658 and WAC4252 produced the most severe disease. Levels of phytoestrogens in stems ranged from 25 to 1995 mg/kg for coumestrol and from 0 to 418 mg/kg for 4'-O-methylcoumestrol. There was a significant positive relationship of disease incidence and severity parameters with both coumestrol and 4'-O-methylcoumestrol contents, as noted across individual cultivars, and as across the three cultivars overall where r=0.39 and 0.37 for coumestrol and 4'-O-methylcoumestrol, respectively (P<0.05). Although, cv. Serena was most susceptible to P. medicaginis and produced the highest levels of phytoestrogens in the presence of P. medicaginis, cv. Zodiac contained the highest levels of phytoestrogens in comparison with other cultivars in the absence of P. medicaginis. There was a 15-fold increase in coumestrol in cv. Serena but only a 7-fold increase in cv. Zodiac from infection of P. medicaginis. The study highlights that the intrinsic ability of a particular cultivar to produce phytoestrogens in the absence of the pathogen, and its comparative ability to produce phytoestrogens in the presence of the P. medicaginis, are both important and highly relevant to developing new annual Medicago spp. cultivars that offer improved disease resistance and better animal reproductive outcomes.

A cytogenetic and pollen study of annual Medicago species from Soummam Valley (Northeastern of Algeria)

This paper reports a cytogenetic study of eight Medicago L. species sampled from the Soummam Valley (northeastern Algeria). Chromosome numbers and meiosis irregularities during microsporogenesis were explored. Pollen viability rate and pollen size were also examined. The studied taxa are diploid and display biva-lent pairing and regular chromosome segregation during meiosis. Although meiosis appears regular, some anomalies were detected in relatively high cumulated rates (14.66%–26.14%). The most common meiotic abnormality examined here is related to cytomixis (from 14.66% in M. littoralis to 25.83% in M. laciniata). Other anomalies were also detected, including chromatic bridges, asynchronous divisions, micronuclei and multipolar cells. Consequently, the species exhibited varying percentages of pollen viability (from 70.11% in M. laciniata to 99.14% in M. littoralis). Pollen viability was negatively correlated with meiotic abnormalities (Pearson correlation coefficient R = −0.72, p = 0.043). The pollen grains were also heterogeneous in size. Medicago truncatula Gaertn. and M. laciniata (L.) Miller presented the most variable pollen size (relative standard deviation exceeding 19%). Medicago littoralis is distinguished from other species by possessing homogeneous and large sized pollen (relative standard deviation RSD = 6.73 %). The cytogenetic and pollen data provided by this study are discussed in the context of species systematics and in the perspective of genetic improvement.

Diversity of Salt Tolerance During Germination in Medicago ciliaris (L.) and Medicago polymorpha (L.)

Variability of salt tolerance in eight wild populations of two annual Medicago species (Medicago ciliaris and Medicago polymorpha) was evaluated at germination stage using tree treatments of salinity: 50, 100 and 150 mM of NaCl. Results showed that germination is clearly affected by high salt stress; their germination rate doesn’t exceed 7% at 150 mM. The greatest variability in tolerance was observed at moderate salt stress (50 mM of NaCl) and the decrease in germination seems to be more accentuated in Medicago polymorpha than Medicago ciliaris. Whereas, the Tunisian population of Medicago ciliaris was the best tolerant in all ecotypes studied in this work. This population prospected on soils affected by salinity, exhibits a particular adaptability to salt environment, at least at this stage in the life cycle. This intra-specific variation in salt tolerance may be used to select genotypes particularly suitable for cultivation on lands relatively affected by salinity. On the other hand, when ungerminated seeds from NaCl treatments were transferred to distilled water, they recovered largely their germination at all the populations studied after only 2 days. This, indicate that the germination inhibition was related to osmotic stress rather than ion toxicity. In addition, seed germination in all populations tended to be extremely rapid than that observed in distilled water which indicate that this pretreatment raises dormancy.

Response of growth and nitrogen fixation to pod elimination of three annual Medicago species submitted to drought and subsequent recovery

This research examined the effect of pod elimination on growth and N2 fixation ability in three annual Medicago species during drought stress and subsequent recovery. This study was carried out as a factorial experiment based on a completely randomized design with four replications. The treatments included annual Medicago species, presence or removal of the pod, and water stress at four levels: 100% (as a control), 80%, 60%, and 40% field capacity (FC) during the period of stress. The results showed that annual Medicago species can be considered as semi-drought tolerant species and their recovery ability from mild and moderate drought stress is relatively good. M. polymorpha performed better and showed higher recovery ability in comparison with other tested species. The results of this study showed that elimination of pods from annual Medicago species before sowing enhanced seedling performance under drought stress. However, the effect of pod elimination showed higher significance on growth parameters than nitrogen fixation. The information from this research would be useful for better understanding the physiological basis of changes in drought resistance, as well as targeted breeding programs, to improve the performance of annual Medicago species under conditions of limited or scarce water supply.