Soil surrounding a growing root must be displaced to accommodate the increased root volume. To ease soil penetration, root caps produce border cells and mucilage that lubricate the root surface, decreasing friction at the root-soil interface. Rhizosphere deformations caused by roots with or without a functional root cap were compared to determine the effects of the root cap on sand displacement and penetration. Intact (KYS wild type) and decapped (agt1 dec mutant) primary maize roots were grown in observation chambers filled with sand. Non-destructive time-lapse micro-imaging combined with particle image velocimetry was used to visualize and quantify sand displacements as small as 0.5 μm caused by growing roots. Decapped (agt1 dec ) roots displayed typical responses of mechanically impeded roots at sand densities that did not affect intact KYS roots. Sand displacement decreased exponentially with distance from the root and extended four to eight root radii into the sand. The calculated mean sand density increase and the compressed sand area were doubled by decapping. Maximum density often occurred in front of the apex of decapped roots whereas it occurred along the sides of intact roots. Periodic variation in sand deformation was observed, probably associated with root circumnutation, which may also facilitate soil penetration. Sand particles moved alongside KYS roots more easily than they did alongside agt1 dec roots. A functional exuding cap was therefore essential for efficient rhizosphere deformation and penetration by roots. Manipulating root tip, and specifically root cap, properties is a possible target for improving root penetration in hard soil.

Tropospheric ozone is a major air pollutant affecting plants worldwide. Plants in northern regions can display more ozone injury than plants at lower latitudes despite lower ozone levels. Larger ozone influx and shorter nights have been suggested as possible causes. However, the effects of the dim light present during northern summer nights have not been investigated. Young Trifolium subterraneum plants kept in environmentally controlled growth rooms under long day (10 h bright light, 14 h dim light) or short day (10 h bright light, 14 h darkness) conditions were exposed to 6 h of 70 ppb ozone during daytime for three consecutive days. Leaves were visually inspected and imaged in vivo using thermal imaging before and after the daily exposure. In long-day-treated plants, visible foliar injury within 1 week after exposure was more severe. Multivariate statistical analyses showed that the leaves of ozone-exposed long-day-treated plants were also warmer with more homogeneous temperature distributions than exposed short day and control plants, suggesting reduced transpiration. Temperature disruptions were not restricted to areas displaying visible damage and occurred even in leaves with only slight visible injury. Ozone did not affect the leaf temperature of short-day-treated plants. As all factors influencing ozone influx were the same for long- and short-day-treated plants, only the dim nocturnal light could account for the different ozone sensitivities. Thus, the twilight summer nights at high latitudes may have a negative effect on repair and defence processes activated after ozone exposure, thereby enhancing sensitivity.

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Plants in Nordic regions can be more ozone sensitive at a given ozone concentration than plants at lower latitudes. A recent study shows that the Nordic summer photoperiod, particularly the dim nighttime light, can increase visible foliar injury and alter leaf transpiration in subterranean clover. Effects of photoperiod on the ozone sensitivity of white and red clover cultivars adapted to Nordic conditions were investigated. Although ozone induced visible foliar injury and leaf transpirational changes in white clover, the effects were independent of photoperiod. In red clover, ozone combined with a long photoperiod with dim nights (8 nights) induced more severe visible injuries than with a short photoperiod. Furthermore, transpirational changes in red clover depended on photoperiod. Thus, a long photoperiod can increase ozone sensitivity differently in clover cultivars with different degrees of adaptation to northern conditions, suggesting that ozone indices used in risk analysis should take this effect into account.