COTTON ROOT-SHOOT RELATIONSHIPS UNDER DIFFERENT SOIL WATER REGIMES

Cultivated cotton (Gossypium hirsutum) requires irrigation. Cotton responds to irrigation in different ways depending on timing. Two field experiments in 1983 and 1984 with cotton (Gossypium hirsutum var. Paymaster 792) were established with three irrigation treatments: dry, cut off, and wet. Root distribution, water uptake and resistances in the water pathway were compared among treatments. Relations between root and shoot parameters were studied to evaluate their change with respect to different soil water regimes. Water uptake was estimated assuming negligible water percolation and evaporation. Soil resistances in presence of roots were calculated using Acevedo's model, and plant resistances in the water flow throughout the plant were calculated using the Van der Honert equation.;Experiments established in a growth chamber to assess the magnitude of the osmotic adjustment in roots in response to water stress (using PEG as osmotic agent) indicated that osmotic potential in roots decreased somewhat as water potential of the medium decreased, but solute potential in the leaves decreased more than in the roots. Pressure-volume isotherms were obtained to estimate the solute potential at full and zero turgor in roots. Preliminary results indicated that high elasticity of roots was maintained as water content decreased which tended to maintain turgor pressure in the cells.;Root length density (Lv) was greater in lower soil layers late in the season in the dry treatment. Root diameter of fine roots increased with plant age in all treatments. Average soil resistance was low in both treatments until the soil water potential reached -2.5 bars. At high soil water potential (> -1 bar) root resistance was higher than soil resistance. Shoot resistance in the wet treatment was larger than that of the dry treatment after 60 DAE. Root length increased at faster rate as percent light interception increased in all treatments. The ratio root length/% light interception remained constant after 50 DAE. This indicates that during all but early stages of the grand growth period root length kept pace with canopy development. Leaf elongation rate decreased as leaf water potential decreased, and intercepted photosynthetically active radiation (PAR) decreased. Net photosynthetic rate decreased later in the season in water-stressed plants. As a result of both responses less biomass was produced.