Abstract: In semi-arid area,root under drought stress should absorb as much water as it can for its needs.The fights for water among individals come along.In ordinary view,the developed root system is favorable for plant absorbing water.But crop production is a process of population.Its objective is to raise drought resistance of population,thus to acquire stable and high yield under drought conditions.In the semi-arid area,the connection between the behavior of roots and the yield of a water-limited crop can be expressed as follows:yield=water used×water-use efficiency×harvest index.Considering the water acquired during all growing period being limited,drought resistance breeding must focus on the rise of water-use efficiency and harvest index.Crop yield in drought condition may largely depend on the amount of usable water contained in the soil at about the time of flowering.Passioura(1972,1976) pointed out that when the wheat is growing on a limited supply of stored soil water,small roots can increase the resistance to water flowing and therefore they would conserve some of water for the critical period during flowering and grain filling.Spring wheat typically relies on a very few seminal axes and attendant laterals to collect water in the subsoil,so the activity of seminal root and diameter of the main xylem vessel directly affect the ability of water absorption.Small diameter of the xylem in the upper part of the seminal root like a "bottleneck" is resistant to the flowing of water,which restricts the absorption and transpiration of water and decreases the water use in the former growing period,and therefore it saves some water for later.Through the cryostat microtome method,the anatomical structure of the seminal root of six spring wheat varieties had been investigated.Under different water conditions,the diameter of the main xylem vessel in the seminal axes of the same variety have great differences,and show different change trends with the development of water stress,but concentrate on the same area at the end.This area may be the best diameter of the main xylem vessel to adapt to water deficit.The diameter of seminal root was not in a simple line relation with the diameter of main xylem vessel.The thick cortex has the function of water reserve.

Key words: spring wheat, seminal root, main xylem vessel, water stress