The Effects of Polyamines on Root Morphology and Arbuscular Mycorrhiza of Citrus Seedlings

Polyamines (PAs) are important plant bioregulators commonly used in fruit production systems. It has been well documented that PAs modify the reproductive and stress physiological process of plants. Here, we report the influence of PAs on root morphology and arbuscular mycorrhiza (AM) of citrus seedlings. Thin rhizobox was employed for better harvesting of the root system. One seeding was left in a rhizobox, irrigated with a PAs solution (putrescine, spermidine or spermine, 5x10(4) mol/L) or not. Seedlings grew for 11 weeks in a greenhouse. Upon harvest, leaf chlorophyll content was measured using a chlorophyll meter. Fresh weights of roots and shoots, as well as the 1(st) and 2(nd) lateral root numbers were recorded. Images of the root system was captured and root length, root surface area, average root diameter, root tip number and root volume were analyzed with the software WINRHIZO. Root colonization by indigenous AM fungi was also determined. Results indicated that PA application increased the leaf SPAD values, fresh weight and lateral root numbers. Root length, root surface area and root volume were also elevated by PA application, whereas average root diameter and R/S ratio was decreased. PAs mainly promoted the fine root length of diameter below 0.4 mm. Microscopic observations revealed that PAs increased the root colonization by indigenous AM fungi. It is suggested that PAs may benefit nutrient uptake by roots through modified root morphology and mycorrhizal development.