Short‐term soil quality response to forage species and pH

Forages are important throughout the south‐east USA for livestock production and wildlife habitat. However, little is known about how forage species commonly grown in this region influence soil‐quality indicators. The objectives of this research were to determine short‐term response of soil water‐stable aggregates (WSA) and density of fungal hyphae (DFH) to: (i) forage species or mixtures grown at identical soil pH levels and (ii) forage species or mixtures grown at field‐state versus adjusted soil pH levels. Nine warm‐season species (sole crop) and eleven cool‐season sole crop or legume–grass mixtures were grown under protected culture in southern coastal plain soil microcosms. Levels of WSA and DFH, and plant shoot and root biomass were evaluated after two 12‐week experimental growth periods in both field‐state and adjusted‐pH soil. Both warm‐ and cool‐season forage species significantly altered short‐term responses of WSA and DFH levels; these responses differed when the soil pH was adjusted from the field state with lime addition. The short‐term responses of both WSA and DFH to lime addition in coastal plain soil were negative only when certain forage species were grown, and no response was detected for the control. It appeared that differences in plant shoot and root developmental characteristics played a key role in soil WSA and DFH responses to the species studied. Further long‐term studies are needed to understand how these relationships are expressed in more variable environments over expanded time frames.     

Participatory farmers’ selection of common bean varieties (Phaseolus vulgaris L.) under different production constraints

On‐farm evaluation of 15 common bean varieties was undertaken with nine farmer groups under two fertilizer applications levels in four subcounties of Hoima and Rakai districts for two seasons to select farmers’ desired and undesired varieties. Farmers’ votes for acceptance and rejection of varieties at podding stage were converted to a preference index, and analysis of variance was conducted to examine differences in farmers’ preference indices among subcounties and combined across subcounties, seasons, management and gender. Management had no‐significant influence on the way farmers selected varieties in the subcounties. Choice of varieties varied significantly (p ≤ .001) between seasons and gender in the different subcounties. Variety Masindi Yellow Long and Farmers’ seed (Kaduli), and the introduced KATB1 were accepted by farmers due to their medium seed size, desired seed colour and potential of varietal adaptability to their farm conditions. Varieties NABE2, ROBA1 and RWR719 were deselected due to possession of traits less desired in the market. These results demonstrate the need for breeding programmes to involve diverse stakeholders in capturing the diverse traits preferences in varietal development process.     

Elevated carbon dioxide level along with phosphorus application and cyanobacterial inoculation enhances nitrogen fixation and uptake in cowpea crop

Climate change, as a result of increase in the concentration of greenhouse gases, influences growth and productivity of leguminous crops. A study was carried out to analyse the impacts of elevated carbon dioxide (CO₂) and cyanobacterial inoculation on growth, N₂ fixation and N availability and uptake in cowpea crop, under different doses of phosphorus. Cowpea crop was grown under ambient (400 µmol mol^?1) and elevated (550 ± 20 µmol mol^?1) CO₂ levels using Free-Air Carbon dioxide Enrichment facility. Elevated CO₂ level increased chlorophyll content in leaves, improved nodulation and nitrogen fixation by the crop. Increase in P dose up to 16 mg kg^?1 soil enhanced nodule development and N₂ fixation under high CO₂ condition. Cyanobacterial inoculation increased nodule weight, leghaemoglobin content in nodules and total nitrogenase activity. Although nitrogen concentration in cowpea seeds decreased in high CO₂ treatment, higher N uptake was recorded. Under elevated CO₂ condition, cyanobacterial inoculation and higher P doses led to enhanced root growth and N₂ fixation and availability of soil nitrogen. The study illustrated the synergistic effect of high CO₂ and cyanobacterial inoculation in enhancing crop growth and availability of soil N, mediated by biological N₂ fixation in cowpea under different levels of P.     

Effects of feeding clover silages on feed intake, milk production and digestion in dairy cows

Silages were made from pure crops of perennial ryegrass, red clover and white clover over 2 years. In all but one case the silage was stored as bales. A silage additive specially adapted for bales (Kofasil Ultra^TM) was used for all silages and they were all of good hygienic quality. The additive contained sodium nitrite, hexamethylene, tetraamine sodium bensoate and sodium propionate. The silages were offered ad libitum, either pure or mixed [grass/clover 0·50/0·50 on a dry‐matter (DM) basis] with a fixed amount (8 kg) of concentrate. Two experiments, one in each year, were performed with high‐yielding multiparous dairy cows in mid‐lactation, and both rumen‐cannulated and intact cows were used. The experiments were carried out using an incomplete changeover design with fifteen cows and five treatments each year. The cows consumed large quantities of these silages (12·7–16·3 kg DM per cow per day). The highest intakes were obtained when the red clover and the 0·50 red clover:0·50 perennial ryegrass silage diets were offered. However, there was a difference between years. In year 1, 0·50 red clover:0·50 perennial ryegrass and 0·50 white clover:0·50 perennial ryegrass silage diets showed the highest intakes while pure perennial ryegrass and white clover silage diets gave lower intakes. In year 2 the highest intake of silage was obtained when the diet containing silage from red clover from a second cut was offered, while the silage from red clover from a first cut gave the lowest intake. The voluntary intakes of silages from white clover and perennial ryegrass were intermediate. No cases of bloat or other digestive disturbances were observed. Milk yield was significantly lower for the perennial ryegrass silage diet compared with all other diets in year 1. In year 2 milk yield was highest for the white clover silage diets and lowest for the red clover silage diets from both cuts. In year 1 there were relatively small differences in milk composition while in year 2 milk fat content was significantly lower with white clover silage diet and milk protein content was significantly higher with the perennial ryegrass diet. The overall conclusion from these experiments was that cows were able to consume large quantities of pure legume silage without serious disturbance to their metabolism. Differences in measurements of rumen metabolism were found between diets and especially between years. Milk production differences appears to be coupled to both differences in rumen physical characteristics, such as passage rate and particle size as well as differences in volatile fatty acid production in the rumen.     

Role of host specificity in the speciation of <Emphasis Type="Italic">Ascochyta</Emphasis> pathogens of cool season food legumes

Ascochyta/legume interactions are attractive systems for addressing evolutionary questions about the role of host specificity in fungal speciation because many wild and cultivated cool season food legumes are infected by Ascochyta spp. and most of these fungi have described teleomorphs (Didymella spp.) that can be induced in the laboratory. Recent multilocus phylogenetic analyses of a worldwide sample of Ascochyta fungi causing ascochyta blights of chickpea (Cicer arietinum), faba bean (Vicia faba), lentil (Lens culinaris), and pea (Pisum sativum) have revealed that fungi causing disease on each host formed a monophyletic group. Host inoculations of these fungi demonstrated that they were host-specific, causing disease only on the host species from which they were isolated. In contrast to the strict association between monophyletic group and host observed for pathogens of cultivated legumes, Ascochyta fungi causing disease on wild bigflower vetch (Vicia grandiflora) were polyphyletic. Genetic crosses between several pairs of closely related, host-specific, and phylogenetically distinct Ascochyta fungi were fully sexually compatible. Progeny from these crosses had normal cultural morphology and segregation of molecular markers indicating a lack of intrinsic, post-zygotic mating barriers between the parental taxa. However, when progeny from a cross between a faba bean-adapted isolate (A. fabae) and a pea-adapted isolate (A. pisi) were assessed for their pathogenicity to the parental hosts, almost all progeny were non-pathogenic to either faba bean or pea. These results suggest that although these fungi have retained the ability to mate and produce progeny with normal saprophytic fitness, progeny are severely compromised in parasitic fitness. The host specificity of these fungi, coupled with the inability of hybrid progeny to colonize and reproduce on a host, may constitute strong extrinsic, pre-zygotic and post-zygotic mating barriers in these fungi and promote the genetic isolation and speciation of host-specific taxa. A phylogeny of the host plants is also being developed, and with more extensive sampling of pathogens and hosts from sympatric populations in the centre of origin, the hypothesis of cospeciation of pathogens and hosts will be tested. The objectives of this review are: (1) to summarize recent phylogenetic, host specificity and speciation studies of Ascochyta fungi, and (2) to suggest how current and future research using these pathosystems may lead to a better understanding of the role of host specificity in the speciation of plant-pathogenic fungi and the cospeciation of pathogens and their hosts.     

Differential heat sensitivity of two cool-season legumes,chickpea and lentil,at the reproductive stage,is associated with responses in pollen function,photosynthetic ability and oxidative damage

Increasing temperatures are adversely affecting various food crops, including legumes, and this issue requires attention. The growth of two cool-season food legumes, chickpea and lentil, is inhibited by high temperatures but their relative sensitivity to heat stress and the underlying reasons have not been investigated. Moreover, the high-temperature thresholds for these two legumes have not been well-characterised. In the present study, three chickpea (ICCVO7110, ICC5912 and ICCV92944) and two lentil (LL699 and LL931) genotypes, having nearly similar phenology with respect to flowering, were grown at 30/20°C (day/night; control) until the onset of flowering and subsequently exposed to varying high temperatures (35/25, 38/28, 40/30 and 42/32°C; day/night) in a controlled environment (growth chamber; 12 hr/12 hr; light intensity 750 µmol m⁻² s⁻¹; RH-70%) at 108 days after sowing for both the species. Phenology (podding, maturity) was accelerated in both the species; the days to podding declined more in lentil at 35/25 (2.8 days) and 38/28°C (11.3 days) than in chickpea (1.7 and 7.1 days, respectively). Heat stress decreased flowering–podding and podding–maturity intervals considerably in both the species. At higher temperatures, no podding was observed in lentil, while chickpea showed reduction of 14.9 and 16.1 days at 40/30 and 42/32°C, respectively. Maturity was accelerated on 15.3 and 12.5 days at 38/28°C, 33.6 and 34 days at 40/30°C and 45.6 and 47 days at 42/32°C, in chickpea and lentil, respectively. Consequently, biomass decreased considerably at 38/28°C in both the species to limit the yield-related traits. Lentil was significantly more sensitive to heat stress, with the damage—assessed as reduction in biomass, reproductive function-related traits (pollen viability, germination, pollen tube growth and stigma receptivity), leaf traits such as membrane injury, leaf water status, photochemical efficiency, chlorophyll concentration, carbon fixation and assimilation, and oxidative stress, appearing even at 35/25°C, compared with 38/28°C, in chickpea. The expression of enzymatic antioxidants such as superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase and non-enzymatic antioxidants declined remarkably with heat stress, more so in lentil than in chickpea. Carbon fixation (assessed as Rubisco activity) and assimilation (assessed as sucrose concentration, sucrose synthase activity) were also reduced more in lentil than in chickpea, at all the stressful temperatures, resulting in more inhibition of plant biomass (shoot + roots), damage to reproductive function and severe reduction in pods and seeds. At 38/28°C, lentil showed 43% reduction in biomass, while it declined by 17.2% in chickpea at the same time, over the control temperature (30/20°C). At this temperature, lentil showed 53% and 46% reduction in pods and seed yield, compared to 13.4% and 22% decrease in chickpea at the same temperature. At 40/30°C, lentil did not produce any pods, while chickpea was able to produce few pods at this temperature. This study identified that lentil is considerably more sensitive to heat stress than chickpea, as a result of more damage to leaves (photosynthetic ability; oxidative injury) and reproductive components (pollen function, etc.) at 35/25°C and above, at controlled conditions.     

Nitrogen-fixing potential of Acacia mangium and Acacia auriculiformis seedlings inoculated with Bradyrhizobium and Rhizobium spp.

Summary Two Australian Acacia species, A. mangium and A. auriculiformis were inoculated in vitro with eight strains of Bradyrhizobium spp. and two strains of Rhizobium spp. On the two plant species, only Bradyrhizobium spp. strains formed effective N₂-fixing nodules. A. mangium, which nodulates effectively with a restricted range of Bradyrhizobium spp. strains, is a specific host compared to A. auriculiformis. A. auriculiformis is assumed to be a promiscuous host because it nodulates effectively with a wide range of Bradyrhizobium spp. strains. Nodule efficiency as expressed by the ratio of N₂ fixed to nodule dry weight appeared to be higher in A. auriculiformis (0.44–0.81) than in A. mangium (0.23–0.55).     

Radiation interception and biomass and nitrogen accumulation in different cereal and grain legume species

The increasing interest in the sustainability of agricultural systems has emphasised the importance of incorporating legumes into cereal production, in spite of their lower and less reliable grain yields. The basis of the poor performance of legumes has been analyzed in a 2-year comparison between varieties of pea, faba bean, durum wheat and triticale, in terms of resource capture and use. The cereals developed a full canopy 350 °Cd earlier than did the grain legumes, and the triticale more rapidly than the durum wheat. This difference, and the 11-day longer duration of the growing cycle of cereals allowed them to intercept more photosynthetically active radiation (PAR) than grain legumes. This, combined with their higher radiation use efficiency (2.35 ± 0.07 vs 2.10 ± 0.05 g MJ⁻¹), resulted in a biomass greater, on average, by about 500 g m⁻². Within the cereals, triticale accumulated 34% more biomass than durum wheat. Radiation interception and nitrogen uptake are closely tied in both cereals and grain legumes. There was no difference between cereals and legumes in the relationship between the amount of nitrogen assimilated and the fraction of intercepted PAR (FIPAR), but there were differences in the form and in the parameters of the relationship between nitrogen assimilated and PAR intercepted. Below a FIPAR of 0.8, the relationship between FIPAR and N uptake is crop independent, underlining the influence of FIPAR on N uptake. The significance of this FIPAR level is that by the time it has been achieved, the plants will have accumulated most of the N present in their biomass at maturity.     

Long‐term storage of forage legumes greatly alters the hard seed breakdown pattern in situ

Knowledge of the hard seed content of annual legumes, and its pattern of breakdown, is critical to the understanding of their ecology and management within farming and natural ecosystems. For logistical reasons, seed that has been stored for varying lengths of time is often used for pasture establishment and agronomy experiments. However, the implications of storage on hard seed physiology are unknown. The aim of this study was to explore the impact of seed storage on its subsequent pattern of hard seed breakdown when exposed to field conditions. Experiments examined seed from six different annual legume cultivars that had been either produced the prior year or stored under ambient conditions for 11–22 years. Comparisons were then made between the two seed sources on hard seed breakdown patterns in situ. Although initial hard seed levels were mostly unaffected by storage (seed of five cultivars remained over 93% hard after more than 11 years of storage), the patterns of release from dormancy during exposure to hard seed breakdown conditions of stored seed differed greatly (p < .05) from freshly produced seed. Hard seed in the stored seed of most cultivars was reduced from >90% to <10% within 68 days over autumn, with shallow burial, whereas fresh seed remained >90% hard during the same period. Given this large and consistent impact, it is recommended that studies of the patterns of hard seed breakdown in legumes compare seed stored under consistent and well‐defined conditions.