Effects of decreasing soil water content on seminal and lateral roots of young maize plants

Soil micropores that contain water at or below field capacity cannot be invaded by seminal or first-order lateral roots of maize plants because their root diameters are larger than 10 Pm. Hence, at soil-water levels below field capacity plant roots must establish a new pore system by displacement of soil particles in order to access soil water. We investigated how decreasing soil water content (SWC) influences growth and morphology of the root system of young maize plants. Plants were grown in rhizotrons 40cm wide, 50cm high, and approximately 0.7cm thick. Five SWC treatments were established by addition of increasing amounts of water to soil and thorough mixing before filling the rhizotrons. No water was added to treatments 1-4 throughout the experiment. Treatment 5 was watered frequently throughout the experiment to serve as a control. Seminal-root length and SWC in soil layers 0-10, 10-20, 20-30, 30-40, and 40-50cm were measured at intervals of 2-3 d on scanner images by image analysis. At 15 d after planting, for treatments 1-4 shoot dry weight and total root length were directly related to the amount of water added to the soil, and for treatments 4 and 5, total root length and shoot dry weights were similar. Length of seminal roots visible at the transparent surface of the rhizotron for all treatments was highest in the uppermost soil layer and decreased with distance from the soil surface. For all layers, seminal-root elongation rate was at maximum above a SWC of 0.17cm(3)cm(-3), corresponding to a matric potential of -30 kPa. With decreasing SWC, elongation rate decreased, and 20% of maximum seminal root elongation rate was observed below SWC of 0.05cm(3)cm(-3). After destructive harvest for treatment 1-4, number of (root-) tips per unit length of seminal root was found uninfluenced over the range of initial SWC from 0.10 to 0.26 cm(3) cm(-3). However, initial SWC close to the permanent wilting point strongly increased number of tips. Average root length of first-order lateral (FOL) roots increased as initial SWC increased, and the highest length was found for the frequently watered treatment 5. The results of the study suggest that the ability to produce new FOL roots across a wide range of SWC may give maize an adaptive advantage, because FOL root growth can rapidly adapt to changing soil moisture conditions.