Growth of Tiller Buds in Barley: Effects of Shade Treatment and Mineral Nutrition

Examination of the stem apex of Proctor barley showed that thebud of the coleoptile tiller, Tc, is probably present in thedry grain and that the bud, TI, carried in the axil of the firstleaf is present at or soon after 24 h from planting. Subsequentlytiller buds are initiated with a plastochron of about 4 days,this being rather longer than that for the foliar primordia.During the initial phase of bud growth vascular connectionsare established with the leaf above, but not to the subtendingleaf. At some time after these vascular connections are formedand when it has a dry weight of 4–7 µg the bud entersa phase of rapid, exponential growth in dry weight. Shading the first leaf delays the onset of rapid growth forboth Tc and Ti, but after a lag period rapid growth commences;this is coincident with development of the second leaf as anorgan exporting assimilated carbon. The phase of rapid growth of tiller buds is delayed when applicationof either nitrogenous or nonnitrogenous minerals is delayed.Ammonium was found to be less satisfactory as a nitrogen sourcethan nitrate, probably because of toxicity effects. Slight growthof Tc and T1 occurs in presence of nonnitrogenous minerals andabsence of nitrogen but growth is greater when nitrogen is suppliedin absence of the other minerals, although such growth is substantiallyless than that found when all nutrients are supplied. The interactionbetween nitrogen and non-nitrogenous minerals which controlsbud growth was not found to affect growth of the parent plantwhich is, as previously shown, controlled by timing of the nitrogensupply. AnotheT distinction is that higher concentrations ofnitrogen and the other minerals are required for maximum growthof the bud than for that of the plant. Tiller bud growth is interpreted as occurring in two phases.In the first, initiation, phase there is a close associationwith the subtending leaf, and nutritionally bud and leaf arelinked. This phase is followed by one in which the bud is directlyconnected by vascular traces to the leaf above, which becauseof this controls bud growth by modulating supply of assimilatedcarbon and nitrogen, and other minerals to it.