The distribution of four Caragana species is related to their differential responses to drought stress

Caragana species are widespread in northwest China. However, species in semiarid areas of Inner Mongolia experience summer and autumn rainfall, whereas morphologically similar close relatives in arid areas in Xinjiang Province experience wet springs, but hot dry summers. We hypothesize that the differences among species in response to soil drought help to explain their distributions. A comparison of the closely related species C. intermedia and C. microphylla from Inner Mongolia (semiarid species) and C. pruinosa and C. spinosa from Xinjiang (arid species) was conducted to examine whether responses to a water deficit in a pot experiment were associated with the environments of their habitat. In the two semiarid species, the stomatal conductance was relatively insensitive to the decrease in predawn leaf water potential (Ψleaf), and leaves did not abscise or die even when the Ψleaf decreased below -6.0 MPa, while in seedlings of the two arid species the stomata closed at a relatively high Ψleaf, and the leaves abscised gradually when Ψleaf fell below about -3.0 MPa. Furthermore, major vein density, minor vein density and the cubed ratio of the conduit wall thickness to the conduit lumen breadth [(t/b)3] of minor veins increased by an average of 60, 150 and 220 %, respectively, in the semiarid species compared with the arid species. The results indicated that semiarid species have typical anisohydric properties, while arid species have typical isohydric properties. We suggest that the divergence in water-use strategies in the genus may be associated with variation in vein architecture, and may possibly play an important role in determining the distribution of these species in the different environments of northwest China. © 2013 Springer Science+Business Media Dordrecht.