The fate of surface roots of citrus seedlings in dry soil

The top portions of the root system of deeply rooted plantsare frequently in dry soil while deeper roots still have accessto water. We expected that many surface roots would be shedwhen subject to localized soil drying. We further hypothesizedthat the cost of fine root construction per unit root lengthwould be negatively correlated with the rate at which root lengthis shed. Seedlings of four citrus root- stocks that varied widelyin specific root length (cm g^–1 root) were used to testthese hypotheses. Plants were grown for 4 months in a split-potsystem divided into a top and bottom pot. After roots were wellestablished in the bottom pot, water was withheld from the toppots of half of the plants; plants were harvested every 2 weeksthereafter. Sufficient water was supplied to the bottom potto prevent shoots of droughted seedlings from experiencing significantwater stress. All plants were labelled with ¹⁴CO₂ 48 h beforeharvesting, and autoradiographs made of the fine roots harvestedfrom the droughted compartment. Comparisons of the autoradiographswith digitized images of the root system allowed us to assessroot mortality and root sink activity. As expected, the proportionof ¹⁴C-labelled photosyrithate allocated to fine roots in thetop pot declined with soil drying in all four genotypes; however,there was no genotypic effect on this decline. Contrary to ourexpectations, extensive root mortality was not apparent forany genotype, even after 60 d of localized soil drying. Apparently,selection for rapid shedding of roots in response to soil dryinghas not occurred in these Citrus species. Key words: Carbon allocation, drought, root death, split root, root autoradiography