Nitrogenase-dependent acetylene reduction, nodule function, and nodule regrowth were studied during vegetative regrowth of harvested (detopped) alfalfa (Medicago sativa L.) seedlings grown in the glasshouse. Compared with controls, harvesting caused an 88% decline in acetylene reduction capacity of detached root systems within 24 hours. Acetylene reduction in harvested plants remained low for 13 days, then increased to a level comparable to the controls by day 18.Protease activity increased in nodules from harvested plants, reached a maximum at day 7 after harvest, and then declined to a level almost equal to the control by day 22 after harvest. Soluble protein and leghemoglobin decreased in nodules from harvested plants in an inverse relationship to protease activity.Nitrate reductase activity of nodules from harvested plants increased significantly within 24 hours and was inversely associated with acetylene reduction. The difference in nitrate reductase between nodules from harvested plants and control plants became less evident as shoot regrowth occurred and as acetylene reduction increased in the harvested plants.No massive loss of nodules occurred after harvest as evidenced by little net change in nodule fresh weight. There was, however, a rapid localized senescence which occurred in nodules of harvested plants. Histology of nodules from harvested plants showed that they degenerated at the proximal end after harvest. Starch in the nodule was depleted by 10 days after harvest. The meristem and vascular bundles of nodules from harvested plants remained intact. The senescent nodules began to regrow and fix nitrogen after shoot growth resumed.Harvesting the shoots of legumes, such as is periodically done in alfalfa and other forages, removes a primary source of energy for maintaining N2 fixation (15), nodule structure and function (3,25), and for the initiation of new nodules. The ability of nodules to remain functional after harvesting of the shoot may depend upon plant species, microsymbiont, nodule morphology, rate of shoot regrowth, nutrient availability, and competition between plant parts for energy. The interrelationships among some of these factors have been reported for several annual grain legumes (7,8 Roots from a portion of each replicate were shaken gently to remove sand and then washed in buckets containing cold water.Nodules were removed and placed in 20-ml beakers on ice. On each sampling day there were three replicates. At least two subsamples from each replicate were assayed for N2(C2H2) reduction3 2 Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by either the U.S. Department of Agriculture or University of Minnesota and does not imply its approval to the exclusion of other products that may also be suitable.3Abbreviations: N2(C2H2) reduction; nitrogenase-dependent acetylene reduction.

Nondormant alfalfa (Medicago sativa L.) germplasms have been selected for increased storage of reduced N in roots and crowns. It is important to learn how this improved germplasm will affect yield of a subsequent nonlegume crop in a cropping sequence. Our objectives were to (i) compare effects of two harvest systems on the forage yield, quality, and N incorporated for the nondormant experimental 'MN ROOT N' population and the moderately dormant cultivar 'Saranac AR'; and (ii) evaluate effects of previous cropping and fallow treatments on grain yield and N response of a subsequent corn crop. Field studies were conducted in 1982 and 1983 at four Minnesota locations on a Hubbard loamy sand (Udorthentic Haploboroll), Webster silt loam (Typic Haplaquoll), Normania clay loam (Aquic Haplustoll), and Webster clay loam (Typic Haplaquoll). Firstyear rotation components included seeding year alfalfa, corn (Zea

The winter hardiness of alfalfa cultivars (Medicago saliva L.) affects stand persistence in northern climates. Fall growth, a measure of fall dormancy, has been associated with alfalfa winter hardiness in some areas of North America, including Minnesota. This study evaluated 251 North American alfalfa cultivars for fall growth and winter injury and determined the relationship between fall growth and winter injury scores under Minnesota winter conditions. All cultivars were established by transplanting 9‐wk‐old plants into space‐planted field plots in June 1991, 1992, and 1993. Plants were clipped in mid July and early September each yr and overwintered in the field. Fall growth was measured as individual plant height in mid‐October 1992 and 1993. Winter injury was evaluated in May each year. Entries differed for fall growth and winter injury score in all years. The 2‐yr‐mean fall growth score was related to the 3‐yr‐mean winter injury score (r2 = 0.85). Only seven of 251 entries fell outside the 95% confidence interval of the linear regression of winter injury score on fall growth score. Fall growth scores from 1992 and 1993 were correlated (r2 = 0.88). Winter injury scores from 1992, 1993, and 1994 were also correlated (r2 = 0.69‐0.96). Although concerns exist about the use of fall growth score to predict winter hardiness in less severe climates, it remains a useful predictor of alfalfa winter hardiness in Minnesota when winter injury data are not available.

Specific leaf weight (SLW = leaf dry weight per unit leaf area) and net photosynthesis (Pn) of 13 alfalfa (Medicago sativa L.) clones were positively correlated (r = 0.790). Pn increased from 20 ± 7 to 50 ± 8 mg CO2, dm−2 hr−1 when SLW increased from 1.9 to 5.3 mg cm−2. The SLW‐Pn relationship was the same whether SLW was influenced by genetics or environment.Selections from four alfalfa populations (MSA‐C4, MSB‐11, ‘Vernal,’ and ‘DuPuits’) were grown in high (37.7 klux) and low (12.9 klux) light growth chambers. Except for DuPuits, they showed a SLW‐Pn relationship similar to the 13 clones mentioned above. At high light, the Pn of DuPuits did not exceed 33 ± 6 mg CO2 dm−2 hr−1 even when SLW was greater than 4 mg cm−2.SLW‐Pn relationships were similar for attached and detached leaves. Leaves selected for high SLW had the highest Pn at several light intensities between 6.3 and 64.0 klux. The possibility of increasing Pn by selecting for SLW is discussed.

However, more than one cycle of selection are needed in a brEeding frogturn to obtain populations significantly different from the original population.