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Salinity generally reduces shoot growth of crops more than root growth, based on dry weight rather than length measurements. The objective of this study was to compare changes in root length to changes in shoot height under conditions of salinity, using the minirhizotron technique. The experiment was conducted in soil containers (55 cm long, 15 cm diam.), using greenhouse‐grown soybean [Glycine max (L.) Merr.] and field‐grown maize (Zea mays L.) as test crops. Segmented, nonlinear regression analysis showed changes in root length as viewed through minirhizotron tubes to be more sensitive to salinity than changes in shoot height, while there was no response to salinity treatment for final root dry weight. The regression coefficients of changes in relative shoot and root length vs. the electrical conductivity of the soil saturation extract (ECe) were 5.3 ± 4.0% per dS m−1 for soybean shoots and 6.6 ± 4.3% per dS m−1 for soybean roots. For maize, the values were 6.9 ± 1.6% per dS m−1 for shoots and 9.0 ± 4.9% per dS m−1 for roots. The regression coefficients of relative shoot height vs. relative root length of soybean and maize were 0.61 ± 0.27 and 0.66 ± 0.20, both significantly smaller than one. The discrepancy between the response in root length as measured by the minirhizotron technique and the response as measured in final root dry weights was attributed to the fact that changes in root length, as viewed in the minirhizotron video frames, depend more on new lateral root initiation than on extension of individual existing roots. Root extension is a turgor‐dependent process, as is shoot elongation, while lateral root initiation is not turgor dependent, and thus may be more sensitive to salinity. One might also speculate that the young, thin lateral roots are more sensitive to salt stress than the older, thicker main root system. Additionally, final root weight is insensitive to the addition of lateral roots if salinity treatments are initiated after the main root system is established, as was the case in this experiment.

The effect of salinity on hydraulic conductance of intact roots of tomato (Lycopersicon esculentum Mill.) and sunflower (Helianthus annuus L.) was determined in split‐root experiments using salinized nutrient solutions. Experiments were conducted in controlled climate chambers under two or three relative humidity levels and four solution osmotic potential levels. The relationship between water flux through roots (Jv) and total water potential difference between the leaves and the root medium (Δψ) was linear, usually with a small intercept. Thus, the root hydraulic conductance (L) was not affected by salinity within the range of fluxes obtained in these experiments, with L= 0.036 mm h−1 bar−1 for tomato and L= 0.0167 mm h−1 bar−1 for sunflower. Our results agreed with theoretical analysis of coupled water and ion uptake. From Cl− and Na+ uptake data, the reflection coefficient (o) for tomato roots was calculated as 0.956, which was compatible with the near‐zero intercept. A large intercept for sunflower could not be readily explained. Relative humidity strongly affected root growth, with more rapid growth under low humidity conditions. Transpiration of sunflower plants was reduced by 20% when the relative humidity was increased from 34% to 84%, whereas transpiration in tomato was reduced 50%.