Horizontal root growth: Water uptake and stomatal resistance under microgravity

With the increased interest of plants in space, gravity cannot be ignored in studies of plant-water relations. Although photosynthesis has been measured on the International Space Station, apparently no published information exists concerning the stomatal resistance of plants in space. Growth is directly related to stomatal opening, which is controlled by hormones produced in the roots. The objective of this study was to determine the effect of gravity on plant water take and stomatal resistance. To simulate gravity-free conditions, wheat plants (Triticum aestivum L. 'Jagger') were grown in columns oriented horizontally. Two experiments, each with a different depth of planting ( 3 cm or 3 mm), were performed in a growth room. Four columns ( 7 cm diam., 40 cm long), filled with a commercial potting mix were used in each experiment: two were placed horizontally and two were placed vertically. Stomatal resistance was measured with a steady-state porometer. In the first experiment (3-cm planting depth), plants in horizontal columns emerged poorly. In the second experiment (3-mm planting depth), plants in horizontal columns emerged, but leaves often were plastered on the soil surface, making it difficult to determine stomatal resistance. In both experiments, roots in columns placed on their sides did not grow into the columns. Because the plants grown horizontally could not take up water, they wilted and died, and stomatal resistance readings were high (> 50 s cm(-1)).