A. Carmi et al., COTTON ROOT-GROWTH AS AFFECTED BY CHANGES IN SOIL-WATER DISTRIBUTION AND THEIR IMPACT ON PLANT TOLERANCE TO DROUGHT, Irrigation science, 13(4), 1993, pp. 177-182
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.