Premise of the study: In semi-arid regions, decreasing rainfall presents a challenge to perennial seedlings that must reach sufficient size to survive the first year’s seasonal drought. Attaining a large storage organ size has been hypothesized to enhance drought resilience in geophytes, but building larger storage organs requires greater growth rates, and paradoxically, some traits conferring faster growth are highly sensitive to drought. We examine if tuber size confers greater drought resilience in seedlings of four closely related geophytic species of Pelargonium.

Methods: We imposed two drought treatments when seedlings were two months old: chronic low water and acute water restriction for ten days. Plants in the acute dry-down treatment were then rewatered at control levels. We compared morphological and ecophysiological traits at two, three and six months of age and used mixed-effects models to identify traits determining tuber biomass at dormancy.     

Key results: Despite ten-fold variation in size, species had similar physiological trait values under well-watered conditions. Chronic and acute droughts negatively affected tuber size at the end of the season, but only in the two species with large tubers. Chronic drought did not affect physiological traits of any species, but in response to acute drought, larger species showed reduced photosynthetic performance. Canopy area was the best predictor of final tuber biomass. 

Conclusions: Contradictory to the hypothesis that large tubers provide greater drought resiliency, in Pelargonium seedlings smallness actually increased drought tolerance, although at the expense of more vigorous growth compared to species with larger tubers under well-watered conditions.