CROP PHYSIOLOGY & METABOLISM Root Protein and Vegetative Storage Protein Are Key Organic Nutrients for Alfalfa Shoot Regrowth J. C. Avice, A. Ourry,* G. Lemaire, J. J. Volenec, and J. Boucaud ABSTRACT Organic reserves in below-ground storage organs are generally recognized as indicators of regrowth and persistence potential of for- ages. The objective of this experiment was to examine the causal relationships betweenregrowth potential of alfalfa (Medicagosativa L.) and root organic reserves, i.e., nonstructural carbohydrates or N reserves as N, soluble protein, andvegetative storage protein (VSP). Variations in reserve level induced by cultivar differences (cv. Lodi or Europe), length of the previous regrowth period (30 or 45 d), or by competitive position for light within a dense canopy were studied during 35 d of regrowth. Field grown plants were harvested at weekly intervals, and separated as dominant, intermediate, and suppressed plants. Taprootstarch and N contents were influenced by the length of the previous regrowth period, but not by the position of the plant within the canopy. In contrast, soluble protein andVSP concentrations increased when the previous regrowth lengthened from 30 to 45 d and were greater in dominantplants within the canopy. Starch, N, soluble protein, and VSPcontents were greater in taproots of Lodi plants. Shoot regrowth waslinearly related to taproot soluble protein and VSP contents on the day of defoliation, whereasno relationships wasdetected between shoot regrowth and initial starch or N contents. These results suggest that root protein and VSPare key organic nutrient for alfalfa shoot regrowth after harvest. p I~IYS~OLOaICAI~ adaptations of plants recovering from defoliation have recently been reviewed (Richards, 1993; Volenec et al., 1996). Shoot removal caused instan- taneous reduction of C translocation to the roots and decreased Nz fixation, mineral N uptake, or both. Al- though it has been known for many years that initial shoot recovery after defoliation involves carbohydrate mobilization from the remaining source tissues to re- growing sink tissues, recent studies have shown that regrowth may be more dependent on organic N avail- ability rather than on C reserves. In M. sativa, most (61%) of the stored C was used for root respiration during 30 d of regrowth while only a small proportion (5%) was recovered in regrowing shoots (Avice et al., 1996b).Studies with alfalfa plants differing in initial root starch or N contents after defoliation showed greater shoot yield was associated with greater N reserves even when the root starch content was initially low (Ourry et al., 1994). Similarly, for Lolium temulentum L. (Ourry J.C. Avice, A. Ourry, and J. Boucaud, U.A. INRA Physiologic et Biochimie V6g6tales, Institut de Recherche en Biologic Appliqu6e, Universit6,14032 Caen Cedex, France; G. Lemaire, Station d’Ecophy- siologie des Plantes Fourrag6res, INRA, 86600 Lusignan, France; J.J. Volenec, Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907- 1150. Received 5 June 1996. *Corresponding author (E-mail: Ourry@ criuc.unicaen.fr). Published in Crop Sci. 37:1187-1193 (1997). et al., 1996), greater storage of N prior to defoliation significantly increased leaf regrowth rate. In most forage species, N reserves are mainly in or- ganic forms with protein-N constituting the largest pool and amino acid-N as the most readily mobilized from source to sink tissues (Ourry et al., 1989; Hendershot and Volenec, 1993a,b; Volenec et al., 1996). In alfalfa, three taproot polypeptides of molecular masses of 15, 19, and 32 kDa act as VSP (Hendershot and Volenec, 1993a,b). These polypeptides meet several criteria use- ful for defining VSP(Staswick, 1994). They represent a large proportion of the soluble protein pool. They also exhibit a cyclic pattern of synthesis-mobilization that is induced by defoliation (Hendershot and Volenec, 1993b) or spring growth (Hendershot and Volenec, 1993a). These VSPs are localized primarily in vacuoles, with smaller amounts associated with amyloplasts (Avice et al., 1996b). Moreover, VSPdegradation dur- ing early shoot regrowth coincides with N mobilization from source to sink tissues, as estimated withlSN labeling (Kim et al., 1991; 1993). Most of the previously cited studies were conducted with isolated plants, i.e., under biotic and abiotic condi- tions that did not limit resource (light, water, nutrients, etc.) availability. Competition for such resources in dense canopy affects growth of individual plants, and consequently plant survival. Irradiance within an alfalfa canopy decreases exponentially with depth, resulting in a decline in leaf N content and photosynthetic capacity (Evans, 1993). This pattern suggests a reduction in content of individual stems which leads to poor regrowth rates and even death and which could result from com- petition for light (Lemaire et al., 1991). Gosse et al. (1988) hypothesized that plants not possessing a suffi- cient number of dominant, rapidly growing stems were unable to reach an adequate reserve level prior to defoli- ation to exhibit rapid regrowth after cutting. It would be useful to knowwhether differential C and N storage during light competition is a factor contributing to dif- ferences in shoot yield and plant mortality. In recent work (Avice et al., 1997), we examined the effects defoliation interval (30 or 45 d) in twocontrasting alfalfa cultivars (cv. Europeand Lodi) on shoot yield, dry mat- ter, starch, and N contents of taproots. Increasing the length of the previous regrowth period had a positive effect on reserve accumulation (larger roots with higher N and starch contents), apparent mobilization, and shoot regrowth. The cultivar Lodi accumulated larger Abbreviations: kDa, kilodalton; PVDF, polyvinyldene difluoride; SDS-PAGE, sodium dodecyl sulfate polyacrylamide gel electrophore- sis; VSP, vegetative storage protein. 1187