Defoliation Effects on Alfalfa Winter Survival , Root Physiology , and Gene Expression

before a killing freeze was imperative for winter survival and plant persistence. Cutting during cold hardening Harvesting alfalfa (Medicago sativa L.) after mid-September in lowered both plant persistence and root total nonstructhe North-Central USA often reduces plant winter survival, but the physiological mechanisms associated with poor winter survival are not tural carbohydrate (TNC) concentrations. However, understood. Our objective was to determine how autumn harvesting other research has shown that cutting during this critical affects alfalfa root physiology, gene expression, and plant winter surfall rest period may not impact persistence or subsevival. In Exp. 1, seven fall harvest dates were used to identify 1 to quent yield (Tesar and Yager, 1985; Sheaffer et al., 1986; 15 October as a critical interval where significant changes in alfalfa Edmisten et al., 1988; Bélanger et al., 1992). In Virginia, winter survival and root physiology occur in Indiana. In Exp. 2, rows harvesting in late rather than early fall generally inof six alfalfa cultivars possessing contrasting fall dormancy (FD) were creased shoot growth the following spring (Edmisten et established in May. Plants in one-half of each row were defoliated al., 1988). Bélanger et al. (1992) used growing degree in mid-October, and roots were sampled at this defoliation and again days instead of calendar dates to show that winter injury in December. Winter injury was determined in mid-April. Shoot rewas reduced if the interval between the final harvest in moval in mid-October increased winter injury and reduced plant vigor in spring. As expected, the October defoliation reduced root protein late autumn and the previous harvest was 500 growingand starch concentrations in December, but unexpectedly increased degree-days or greater. Improved plant persistence obroot sugar concentrations. In addition, defoliation did not reduce the tained by delaying the final fall harvest is generally besteady state transcript levels of several cold-acclimation responsive lieved to result from increased root TNC levels (Tesar (car) genes that are associated with genetic variation in winter survival. and Yager, 1985; Sheaffer et al., 1986), but this hypotheAlthough positively associated with genetic differences in winter harsis has not been rigorously tested. Edmisten and Wolf diness, factors other than root sugar accumulation and expression of (1988) speculated that an extended period of slow these car genes regulate defoliation-induced changes in winter survival growth, low dark respiration rates, and high rates of of these alfalfa cultivars. photosynthesis during autumn months stimulates TNC accumulation and improves winter hardiness. Recently, Dhont et al. (2002) reported that mass of TNC in roots P in the North-Central USA sow fall dorin autumn had a stronger correlation with shoot growth mant alfalfa cultivars because of the positive associain spring than does the root TNC concentration. tion of FD with winter survival. One negative conseThe physiological mechanisms for reduced persisquence of growing fall dormant cultivars is their reduced tence and vigor in spring resulting from autumn defoliashoot elongation and leaf area expansion rates after tion are not clearly understood. In addition, it is not forage harvest in summer (Volenec, 1985). This limits known how autumn defoliation alters the expression of the number of annual cuttings, and subsequently respecific cold hardiness genes whose expression has been duces seasonal forage yield potential. When compared consistently associated with genetic differences in alfalfa with dormant cultivars, those with intermediate FD [5 winter hardiness (Cunningham et al., 2001). The goal to 7, a relative measure as described by Teuber et al. of this research was to determine the effects of autumn (1998)] may provide enough forage for additional hardefoliation on root physiology and winter survival of vests each year. If less fall dormant cultivars were made alfalfa. This objective was achieved through two experiavailable to northern production areas, benefits could ments: (i) assessment of several autumn cutting dates include increased yield potential. However, a potential on the accumulation of reserves in alfalfa roots; (ii) risk of raising cultivars possessing intermediate FD is evaluating the effects of autumn defoliation on root their lower winter hardiness, a problem exacerbated by physiology and winter survival of six alfalfa cultivars improper fall harvest management. possessing contrasting FD and winter hardiness. We Past research indicates that autumn defoliation may expected that autumn defoliation would prevent accudecrease winter survival and stand persistence (Silkett mulation of sugars, starch, and protein in roots, and et al., 1937; Grandfield, 1943). Smith (1972) reported be accompanied by reduced winter survival. We also that a period of 4 to 6 weeks of uninterrupted growth expected untimely autumn defoliation to reduce the expression of specific car genes whose expression is Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907-1150 USA. Contribution from the Purdue Univ. Agric. Exp. Stn. , Journal Abbreviations: car, cold-acclimation response gene; FD, fall dorSeries No. 16820. Received 4 July 2002. *Corresponding author mancy; HPLC, high performance liquid chromatography; RFO, raffi(jvolenec@purdue.edu). nose family oligosaccharides; TNC, total nonstructural carbohydrate; VSP, vegetative storage protein. Published in Crop Sci. 43:1340–1348 (2003).