Document: AST-3-31-6

Age dependent respiration of fine apple roots.

VOLDER, A.* and D.M.EISSENSTAT

Pennsylvania State University, University Park, PA 16802, USA ¹

Abstract:
The fine root system is a population of mixed-age individual roots that are responsible for the acquisition of water and nutrients. The efficiency (E) of a root to perform these tasks can be expressed as the ratio of nutrient gained per unit carbon expended. E changes with root age, due to changes in root physiology and soil nutrient depletion. We predict that the optimal lifespan of a root is that which maximizes E roots will be shed when their costs start outweighing their benefits. A model has been developed to predict E under various environmental conditions where both root respiration and nutrient uptake are thought to decrease with root age. Earlier experiments using split-pot systems indicated that median root lifespan was approximately 40 to 60 days for apple seedlings grown under homogeneous N supply. When plants were grown with a heterogeneous N supply, the median lifespan of roots in a low-N patch was decreased, while roots growing in a high-N patch exhibited increased median lifespan, suggesting that plants do adjust root lifespan when E changes. However, even though the median lifespan was 40-60 days, a sizeable portion (20%) did not die within 200 days. Why? We examined respiration rates of individual fine roots ranging in age from 1 to 244 days grown under high, low, and heterogeneous N supply, using both intact and excised roots. While there was a correlation between root age and respiration for roots less than 30 days old, we found no age effect thereafter. Moreover, root diameter and pigmentation had no measurable effect on respiration. Excised roots measured within minutes after excision were a reasonable predictor of intact root respiration, except for roots respiring at rates 35 nmol O₂ g^-1 s^-1, which were underestimated by excised roots. Our results suggest that the use of E as a predictor of root lifespan works well for the majority of apple roots (<50 days old), but at the present time, we cannot explain why some fine roots live such a long time.

Keywords: fine roots, root respiration

This abstract is being presented at: 3:30 PM in session:
Oral Session #12: Roots.