Differential root morphology response to no versus high phosphorus, in three hydroponically grown forage chicory cultivars

Forage chicory is a productive forage resource for eastern North America; however, many soils in the region are acidic and deficient in P and might restrict the widespread use of forage chicory. There is no published information on response of forage chicory to P, or P acquisition strategies for morphologically different chicory cultivars. The literature suggests the following null hypothesis: "specific root length (SRL) will increase with P deficiency". We conducted controlled environment experiments using nutrient culture to determine plant mass, mineral composition, and root morphology of three forage chicory cultivars (Grasslands Puna (GP), LaCerta (LC) and Forage Feast (FF)) as a function of P supply, and test the null hypothesis with chicory. Phosphorus increased chicory growth irrespective of cultivar. Root morphology differed among cultivars independent of P supply with FF producing about twice the taproot mass of GP or LC. Root morphology was also impacted by P supply and the specific interactions between P and cultivar. Total root length and surface area of GP increased, and did not change in LC or FF under -P conditions. Thus, the null hypothesis must be rejected. Results suggest at least two different plant responses to -P conditions in chicory that seem to be attributes of specific cultivars: (a) increase in root length of the 0.28 mm root class (GP); (b) decrease in non-taproot mass density with -P and no change in root length or ratios between diameter classes (LC). The change in root length of small diameter class roots, as observed for GP, is typical of the responses to -P described in the literature. The decrease in root density seen with LC is probably an anatomical response that is not coupled with any observable morphological response. We conclude that use of the above null hypothesis as a paradigm for plant root response to P deficiency must be rejected. The routine use of specific root length as an indicator of environmentally induced changes in root system function is precluded by the presence of anatomical and physiological changes (adaptations) that have no concomitant gross morphological changes. (c) 2005 Elsevier B.V. All rights reserved.