Submitted to: Australian Journal of Experimental Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/2000
Publication Date: N/A
Interpretive Summary: It is difficult to understand how grass plants respond to removal of top growth, partly because of the difficulty in measuring plant root responses. These responses to top removal must be defined to describe whole plant response. However, roots are buried in the soil and there is no way to measure the whole root system without digging up the plant. New techniques smust be tested to determine if they allow accurate measurement of root and whole plant responses. A study was undertaken to compare techniques that could allow measurement of responses by native Australian bunchgrasses to defoliation. Over a 93-day period (28 days for plant establishment; 65 days of top removal), groups of black speargrass plants were grown in pots within a hydroponics system (HS) that delivered nutrients and air to the pots on a regular schedule, and in pots filled with sand. Within each group of plants, two sets of plants were hand clipped at different time intervals, and a third group was untreated. Size of root systems and plan weights of HS plants were measured at regular intervals over 65 days, and all HS and sand-grown plants were harvested after 65 days and separated into roots and top growth. Root volumes and lengths of different size classes of roots were measured on all harvested root systems. Total lengths and volumes of roots of the sand and HS-grown plants were not different, nor were proportions of root lengths and volumes of the different size classes, except small (<0.10 inch diameter) roots. Unclipped plants produced top and root growth during the entire study, while clipped plants only produced top growth, but at lower levels than unclipped plants. This study showed that hydroponics systems could be used to help understand whole plant responses of clipped plants.
Technical Abstract: Understanding the effects of defoliation at the whole plant level is important to explaining responses of grasses to grazing systems. Difficulties in measuring root dynamics have impeded the understanding of plant responses. A study was undertaken to evaluate techniques to measure short-term whole-plant responses of native Australian tussock grasses to defoliation. A hydroponics system (HS) was developed that allowed precise and regular delivery of nutrients and air to plant roots, and could be modified to examine effects of different fertility levels. Black speargrass (Heteropogon contortus) seedlings were germinated and grown in vermiculite for 22 days, transplanted into hydroponic (n=24) and sand (n=15) growing media in 4L pots, and allowed 28 days to become established. Sets of plants (n=8 and n=5 for HS and sand-grown plants, respectively) were then clipped (defoliated to 7.5 cm lengths) after accumulating either 30 cm or 7.5 cm of regrowth, or left unclipped (controls). Root volumes and whole plant weight of HS plants were measured at regular intervals over a 65-day period. End of treatment root lengths and volumes of plants grown in sand and HS were not significantly different (P>0.05) at the end of treatments. Proportions of root lengths and volumes of different diameter size classes were also not different, except for fine roots (<0.25 mm diameter) of moderate and severely defoliated plants. Unclipped plants continued to accumulate root volume, and root and shoot mass during the study, while the clipped plants mostly accumulated shoot biomass at a reduced rate. Results suggest that hydroponics-based techniques may be a valid and efficient way to measure whole-plant responses of Australian tussock grasses to defoliation.