COTTON ROOT-GROWTH AS AFFECTED BY CHANGES IN SOIL-WATER DISTRIBUTION AND THEIR IMPACT ON PLANT TOLERANCE TO DROUGHT

The capability of mature cotton plants (Gossypium hirsutum L.) to adju st to progressive drying of their root zone by promoting root growth t o adjacent wetted zones, and the implications of this process on irrig ation design were investigated. Field grown plants that developed shal low root systems in response to a drip irrigation management of daily, surface soil wettings were exposed 85 days after emergence (DAE), whi le in the flowering stage, to a sudden change in water distribution in the form of deep soil wetting (DSW) followed by termination of irriga tion. The shallow rooted plants (SRP) failed to respond to further sur face soil wetting and the progressive drying of the profile by rapid r oot growth to the deeper-wetted zones; consequently, the SRP suffered from water deficiency for at least two weeks, evidenced by a gradual d ecrease in their leaf water potential (Lpsi(w)). Potted plants respond ed similarly. Daily irrigations of the pot surface with water amounts similar to those lost by evapotranspiration led to the development of a system in which most of the roots and available water became concent rated at the pot's upper section. A transition to irrigation from the bottom of the pot led to a reversed soil-water content gradient and fa iled to promote rapid root spreading to the deeper-wetted layers, in s pite of the accelerated drying of the upper zone. The slow deepening o f the root system was accompanied by water-stress symptoms as indicate d by a considerable reduction in dry matter production. The root shoot ratio in these plants was not much greater than in non-stressed plant s in which the surface wetting was continued. This indicated that pref erential root growth relative to the shoot did not occur in response t o the progressive drying of the shallow root zone. Rewetting of the ro ot zone after a long period of soil water deficiency failed to promote rapid recovery of the root system in the form of root regrowth in thi s zone. It was concluded that the capability of mature cotton plant ro ots to adjust their growth to large changes in water distribution in t he soil, is slow and that this should be taken into account when deter mining an irrigation regime in which the depth at which water is appli ed is changed during the growing season.