Experimental setup 1: Microtensile tests in water saturated air

For the experiments two seed batches (1 and 2, Col-O) were used to account for individual genetic variation. The seed batches were pooled from a variety of individual parent plants and seed batch 1 and 2 were grown at different times. Each seed batch was divided into 2 experimental series (a and b). The hypocotyls were tested at 4, 5, 6 or 7 days +/- 1.5 hours after sowing with a microtensile testing device which has been described in [17, 18]. The setup allowed a reasonably fast determination of mechanical properties (stiffness and ultimate tensile stress) along the longitudinal axis of the hypocotyl. After the length and diameter of a hypocotyl were measured by light microscopy, it was glued onto a foliar frame by using a combination of cyanacrylate glue and dental cement [7], Fig 1, horizontal hypocotyl test). A 2 mm long region in the lower part of the hypocotyl with fully elongated cells was chosen as the region to be tested to exclude the influence of different elongation states in the growth zone at the top of the hypocotyl [5, 15, 16] on the mechanical properties. After hardening of the glue (~10 min) the prepared samples were mounted with a pin-hole-assembly onto the tensile tester. The pin hole-assembly allows slight rotation of the sample in 2 dimensions (red arrow in Fig 1), allowing for sample alignment. Black lines close to the glue spots attaching the hypocotyl to the foliar frame served as video extensometry markers for strain determination. The samples were strained with a speed of 10 μm/s, force was recorded with a load cell with a maximum capacity of 500 mN (Honeywell Sensotec, Model 31E). To avoid drying during the test, the hypocotyls were rinsed with water vapour applied by a 1 mm diameter pipe from a commercial air humidifier.