Tree Physiology 00, 1–11 doi:10.1093/treephys/tpaa092 Research paper Enhanced root exudation of mature broadleaf and conifer trees in a Mediterranean forest during the dry season Gilad Jakoby 1 , Ido Rog 1 , Shacham Megidish 1 and Tamir Klein 1,2 1 Department of Plant and Environmental Sciences, Weizmann Institute of Science, Rehovot 76100, Israel; 2 Corresponding author (tamir.klein@weizmann.ac.il) Received February 15, 2020; accepted July 6, 2020; handling Editor Michael Ryan Root exudates are part of the rhizodeposition process, which is the major source of soil organic carbon (C) released by plant roots. This fux of C is believed to have profound efects on C and nutrient cycling in ecosystems. The quantity of root exudates depends on the plant species, the period throughout the year, and external biotic and abiotic factors. Since root exudates of mature trees are difcult to collect in feld conditions, very little is known about their fux, especially in water-limited ecosystems, such as the seasonally hot and dry Mediterranean maquis. Here, we collected exudates from DNA-identifed roots in the forest from the gymnosperm Cupressus sempervirens L. and the evergreen angiosperm Pistacia lentiscus L. by 48-h incubations on a monthly temporal resolution throughout the year. We examined relationships of the root exudate C fux to abiotic parameters of the soil (water content, water potential, temperature) and atmosphere (vapor pressure defcit, temperature). We also studied relationships to C fuxes through the leaves as indicators of tree C balance. Root exudation rates varied signifcantly along the year, increasing from 6 μg C cm −2 root day −1 in both species in the wet season to 4- and 11-fold rates in Pistacia and Cupressus, respectively, in the dry season. A stepwise linear mixed-efects model showed that the three soil parameters were the most infuential on exudation rates. Among biotic factors, there was a signifcant negative correlation of exudation rate with leaf assimilation in Cupressus and a signifcant negative correlation with leaf respiration in Pistacia. Our observation of enhanced exudation fux during the dry season indicates that exudation dynamics in the feld are less sensitive to the low tree C availability in the dry season. The two key Mediterranean forest species seem to respond to seasonal changes in the rhizosphere such as drying and warming, and therefore invest C in the rhizosphere under seasonal drought. Keywords: carbon balance, rhizosphere, root exudates, root secretion, total organic carbon. Introduction The plant root system has major essential functions, such as water and nutrient uptake, and anchorage in soils. In order to succeed in these tasks, the root interacts with its immediate soil environment, the rhizosphere (Neumann and Römheld 2007). One of the major processes facilitating interaction in the rhi- zosphere is rhizodeposition—the release of a variety of com- pounds into the rhizosphere by plant roots. These compounds originate from lysates of sloughed-of root cap and border cells, as well as compounds secreted as root exudates in passive and active ways from intact root tissues (van Dam and Bouwmeester 2016). Through root exudation, a part of the carbon (C) that a plant assimilates by photosynthesis is transferred to the soil. This fux of C is believed to have profound efects on C and nutrient cycling in ecosystems. The estimated amount of C that is being released by root exudates is 5–21% of all C allocated into fne roots (Haichar et al. 2014), and can comprise up to 10% of net primary productivity in forests (Kannenberg and Phillips 2017). This fux is infuenced by many factors, including plant species and age, and environmental conditions such as light © The Author(s) 2020. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com Downloaded from https://academic.oup.com/treephys/advance-article/doi/10.1093/treephys/tpaa092/5875777 by Weizmann Institute of Science Library user on 02 September 2020