Allocation of Photoassimilate by Main Shoots and Nonsurviving Tillers in Barley

Abstract

Field‐grown barley (Hordeum vulgare L.) plants produce many tillers that do not survive to bear grain, especially under water‐stressed conditions. Such nonsurviving tillers have traditionally been regarded as wasteful of plant resources. To test this premise, a greenhouse study was conducted to evaluate the contribution of nonsurviving tillers to the C economy of a barley plant. The study quantified the reciprocal transfer of photoassimilate between the main shoot and nonsurviving tillers during the tillering phase of development under well‐watered and water‐deficient (stressed) conditions. Main shoots and two primary tillers (T1 and T2) were labeled by exposure to 14CO2. Radiocarbon export and retention by the labeled shoots were monitored at maturity. The radioactivity partitioning data were combined with changes in the dry mass of labeled shoots over the study period to estimate dry matter exchange between shoots. For nonstressed plants, 33% of the initially fixed radioactivity was lost prior to maturity, probably due to respiration. The percentage of radioactivity lost was higher under conditions of water stress. Nonsurviving tillers retained only 18% of their initially fixed radioactivity at maturity, regardless of watering regime. Under both well‐watered and later‐imposed water stress conditions, the calculated dry matter exchange between shoots showed that nonsurviving T1 tillers returned twice as much dry matter to the main shoot as was provided to them by the main shoot. Tiller T2 also exported more than it imported but less than T1. When water stress was imposed early, tillers allocated slightly less assimilate to the main shoot than they received. We conclude that nonsurviving tillers may contribute more dry matter to surviving shoots than has previously been recognized.