Changes in Root and Shoot Growth and Biomass Partition Resulting from Different Irrigation Intervals for Ligustrum japonicum Thunb.

Abstract

Although effects of irrigation frequency and volumes on overall plant establishment and growth have been reported, previous research has not examined how intermittent exposure to substrate water limitation affects partitioning of growth between root tips and buds and how this influences episodic growth patterns. The research presented here examines these effects on Ligustrum japonicum during the establishment period. Plants were exposed to two irrigation treatments: short wetting and drying cycles (SC, 2 days) and long wetting and drying cycles (LC, 7 days). Intermittent water limitations (LC) resulted in new shoot dry mass reductions of ≈28% compared with well-irrigated counterparts, particularly diminishing leaf growth. Water limitation effects on root-to-shoot ratio were dependent on plant growth stage. LC increased root-to-shoot ratios only when plants were at shoot flush, resulting in poor correlations (r = 0.53) between this ratio and differential percent volumetric water content, which was directly influenced by irrigation frequency. Patterns of shoot and root growth varied considerably between these clonal plants, which may be an important consideration on analyses of populations of woody plants. Large periods of episodic growth were not observed for most of the experimental period, but only after plant establishment. Root growth was similar in both treatments and there was no clear arresting of root growth during the experimental period. SC plants started bud expansion earlier than LC and had more shoot flushes and cumulative shoot growing points. A 7-day irrigation cycle was sufficient to establish two-year-old L. japonicum plants; however, shoot growth was less pronounced than root growth compared with plants irrigated on a 2-day cycle.