Root respiration and belowground carbon allocation respond to drought stress in a perennial grass (Bothriochloa ischaemum)

Abstract

In order to accurately estimate the carbon (C) balance in arid and semi-arid grasslands, it is necessary to first understand how the demands of photosynthates and root carbon (C) are allocated in response to drought. To accomplish this goal, we used a 13C pulse labeling technique to assess correlations between root morphology and the amount of 13C in the root, which is a measure of respiration. We compared respiration changes via the excised root 13C of Bothriochloa ischaemum at various root excision times (i.e., 0, 6, 24, 48, 216, and 360 h after labeling). Our results showed that 13C and root respiration continued to increase until 216 h after labeling, and then they decreased after 360 h. Root respiration correlated significantly with the carbon labeled as being from the fine roots that were undergoing drought stress treatments. This indicates that later-photosynthates are the primary C source for root respiration. Fine root 13C positively correlated with fine root biomass and specific root length under serious drought conditions. This suggests that fine root growth is based primarily on recently derived photosynthates. Serious drought was shown to inflate the root/shoot ratio by increasing both the 13C and the biomass, which led to greater C allocation in the root system. Finally, the fine/coarse root ratios of 13C and biomass were greatest in the serious drought treatment. This suggests that fine roots have a higher C demand than coarse roots under drought stress. Fine root respiration was also found to stimulate new C demand in roots because the amount of 13C created during root respiration was positively correlated with tissue N concentration of fine roots.