COTTON ROOT AND SHOOT RESPONSES TO SUBSURFACE DRIP IRRIGATION AND PARTIAL WETTING OF THE UPPER SOIL-PROFILE

The ability of cotton roots to grow downwards through a partially-wett ed soil (Calcic Haploxeralf) profile toward a water source located ben eath them was investigated. Plants were grown in 60-cm-high soil colum ms (diameter 10 cm), the bottom 15 cm of which was kept wet by frequen t drip irrigation, while the upper 45 cm was wetted three times per we ek up to 20, 40, 60, 80 or 100% of pot capacity. Pot capacity was defi ned as the water content which gave uniform distribution within the po t and was at a soil matric potential (psi(m)) of -0.01 MPa. Plants wer e harvested 42 and 70 days after emergence (DAE). Root length density was reduced by decreased soil moisture content. At 42 DAE, density was reduced in the soil profile down to 36 cm. The density in the middle segment of the cylinder (24-36 cm) increased at the second harvest, fr om 0.1 to 0.35 cm . cm(-3) at 40% and from 0.2 to 0.5 cm . cm(-1) at 6 0% of pot capacity, respectively. A significant rise in root length de nsity was found at all moisture contents above 20% in the two deepest soil segments. It was most marked at 40% where the rise was from 0.2 t o 0.8 cm . cm(-3), due to the development of secondary roots at the we tted bottom of the column. When only 20% of pot capacity was maintaine d in the top 45 cm of the profile, almost no roots reached the wetted soil volume, and root length density was very low. Hydrotropism, namel y root growth through dry soil layers toward a wet soil layer was thus not apparent. Root dry weight per unit length decreased with increasi ng depth in the column at all moisture levels. However, the only signi ficant decrease was, found between the top and the second soil segment s and was due to thicker primary roots in the top segment. There was n o clear relationship between length and dry weight of roots. Total pla nt dry weight and transpiration were reduced significantly only at 20% of pot capacity. Dry matter production by roots was less severely inh ibited than that by shoots, under decreased moisture content in the so il profile. Leaf water potential decreased when the soil moisture cont ent of the top 45 cm of the profile was reduced below 60% of pot capac ity. It was concluded that even at soil moisture content equivalent to a psi(m) of 0.1 MPa, the rate of root growth was sufficient to reach a wetted soil layer at the bottom of the soil column, where the plant roots then proliferated. This implies that as long as the soil above t he subsurface dripper is not very dry there is no real need for early surface irrigation.