PHYSIOLOGICAL INTEGRATION AMONG TILLERS OF HOLCUS-LANATUS - AGE-DEPENDENCE AND RESPONSES TO CLIPPING AND COMPETITION

The ecological consequences of physiological integration among tillers were examined in a glasshouse experiment on the clonal grass Holcus l anatus L. We measured the effects of severing the internode connection between a tiller and its parent on the growth and survival of this ma rked tiller. The effects of three factors on the response to severing were determined using this procedure: competition with the parent plan t, by comparing tillers repotted in isolation with tillers remaining i n the neighbourhood of the parent; clipping treatment, comprising no c lipping, clipping only the marked tiller and its daughter tillers or c lipping the whole parent plant, including the marked tiller; and the c hange in the response with tiller age at severing, using four tiller a ges (1, 2, 4 and 8 wk). These age, clipping and severing treatments we re applied factorially. After 8 wk of growth the responses to severing of the marked tillers were dependent on the age and clipping treatmen ts. Severing always decreased survival and growth (tiller production, biomass and tiller extension) of the youngest tillers (ages 1 and 2 wk ), indicating that they were dependent on the parent to support their early growth. Age 4 tillers were able to support their own growth and grew best in isolation; but when grown in the parent's neighbourhood c ompetition with the parent reduced growth, although parental support a meliorated these effects. Some of the oldest tillers (age 8 wk) showed decreased growth when unsevered. This indicated an outflow of resourc es to the parent and suggested that integration allowed the control an d coordination of tiller growth. The pattern of the severing effects w as similar in all clipping treatments, varying only in degree. There w as little evidence of increased support for clipped tillers or for a c hange in the pattern of integration when the whole plant was dipped, e xcept that age 8 tillers showed a continued benefit of the connection, in contrast to the positive effect of severing in the other two clipp ing treatments. The extension rate of the marked tiller showed complex responses to clipping and severing treatments, including effects of i ntegration on regrowth after clipping. This experiment has shown that the growth of tillers in H. lanatus is highly integrated but that this integration is extremely plastic in response to tiller age and, to a lesser extent, clipping treatment.