Quantitative Translocation of Photoassimilates from Nonsurviving Tillers in Barley

Many tillers produced in barley (Hordeum vulgure L.) crop communities senesce without bearing grain. To better understand the physiological and agronomic implications of this tiller mortality, partitioning of photoassimilate by nonsurviving tillers was monitored for field‐grown plants at St. Paul, MN on a Waukegan silt loam soil (fine‐silty over sandy or sandy‐skeletal, mixed Typic Hapludoll). Primary tillers formed in the axils of the first and second main shoot leaves (designated T1 and T2, respectively) were labeled with 14C02 on four occasions and the labeled plants sampled at 7‐ to 10‐day intervals. Some labeled tillers were immediately girdled using steam to restrict photoassimilate export and permit estimation of radioactivity losses due to tiller respiration. Knowing the respiratory losses as well as the quantity of radioactivity remaining in a labeled tiller following a given translocation period, the proportional 14C export from the tiller was calculated. These estimates were coupled with changes in tiller dry mass to calculate the photoassimilate dry matter transferred from a tiller between labeling dates, which were summed to give the cumulative dry matter translocated during the entire period of labeling. Nonsurviving T1 tillers translocated 45% of their photoassimilate to other plant parts, which equated to approximately 150 mg of dry matter. These tillers retained 17% and respired 38% of their fixed radioactivity. Nonsurviving T2 tillers exported 57% (approximately 40 mg), retained 20%, and respired 23%. Most photoassimilate exported from nonsurviving tillers was recovered in the main shoot stem, with lesser amounts in the main shoot spike and leaves. Nonsurviving primary tillers may contribute substantial amounts of dry matter for growth of other plant parts, particularly the main shoot. Thus, initiation of such tillers may not be inherently wasteful of plant photoassimilates.