Osmotic adjustment, an active plant response to stress, has been reported to maintain growth during drought. The impact of a 6 day stress followed by a 6 day recovery period on shoot and, particularly, root growth was assessed using cotton (Gossypium hirsutum L.CV. Stoneville 506). Vegetative cotton plants were grown in box rhizotrons (window angled at 15 degrees; 15 1 capacity; height 0.5 m) containing sand in a growth chamber. Measurements included daily leaf expansion and shoot growth, osmotic potentials of leaves, whole roots and root tips, and root extension. The effects of position (zone) in the root profile (upper half or lower half of window) and root size (small 0.3 mm and medium 0.6 mm mean diameter) were investigated. All visible root tips on the rhizotron windows were marked and extension during 2-day intervals recorded. We assumed that root growth against the window was representative of root growth within the soil profile. Osmotic potentials were measured for noon and pre-dawn harvests using thermocouple psychrometry and vapor pressure osmometry.

Although osmotic adjustment occurred in the shoot, leaf expansion of stressed plants declined after 2 days of drought, ceasing growth in the middle of the stress period. The root:shoot ratio increased from 0.23 to 0.41 and plant height decreased under stress. In the recovery period leaf osmotic adjustment persisted 6 days, leaf expansion rates were still lower than the control, but the root:shoot ratio returned to the control value. Mature roots and root tips from both the small-sized category of the upper zone and stressed plants tended to be thinner. Osmotic potentials of these tissues were less negative than the control, caused by hydration water adhering to samples. For medium-sized root tips, stress did not change osmotic potentials at peak stress, but tips in the upper zone differed from those in the lower zone (-0.43 and -0.68 MPa respectively). No differences in watering regime, zone or size of root were apparent for osmotic potentials of tips after 7 days recovery. Mature roots showed zonal differences at peak stress (-O.52 and -0.59 MPa for upper and lower zones respectively), which were not evident after recovery.

The total root growth was reduced by stress, and the rate of root growth increased in the recovery period to achieve the final control value, the rate of which had remained constant. The increase in the root:shoot ratio was achieved by increased root growth relative to the control, but root growth was below that of the control. The growth rate for medium roots increased dramatically in the recovery period in the upper and lower zones of stressed plants. Small roots produced more root length at the end of the experiment because a greater number of small-sized roots were present.Generally, more root growth occurred in the lower zone. The number of growing roots was reduced by stress to 40 % of all roots, but was restored to the control percentage of 80-90 in recovery. This return to the pre-stress percentage was due to regrowth upon rewatering of previously stalled roots, followed by initiation of new root growth against the window, especially roots of medium size 2 days into the recovery period. Medium-sized roots and roots in the lower zone were less susceptible to stress. The mean root length (MRL, adjusted for differing numbers of growing roots in time) declined during stress but recovered upon rehydration.Medium roots of stressed plants grew less than those of the control but more than small roots. The MRL was greater in the lower zone. Although in recovery the rate of growth for MRL for small roots in the stressed treatment exceeded the control when averaged across the profile, roots did not grow as much as the medium size. Root volume was reduced in stress but the rate of producing root volume was similar to the control during recovery for both root sizes. Volumes of small stressed roots were small (2 mmm³) compared to medium roots (12 mmm³). Roots of medium size and those in the lower zone contributed the greatest volumes. Small roots in both zones had high rates of recovery, but although there were more small roots growing, the volume attained by fewer medium-sized roots was 8 times greater.

These preliminary findings suggest that medium-sized roots are more important for continuing growth in stress and recovery, although the surface area presented by the greater number of small roots, especially in recovery, may warrant attention. Substantial changes in initiation of medium roots were seen in upper and lower zones during the recovery period.