Water-use efficiency is higher in green stems than in leaves of a tropical tree species

Key messageStems have similar photosynthetic rates as leaves but higher water-use efficiency.AbstractPlants with green photosynthetic stems are common in sub-tropical and tropical dry woodlands worldwide, yet the benefits of photosynthetic stems in tropical species have not been studied before. Parkinsonia praecox (Ruiz & Pav. ex Hook.) Hawkins (Fabaceae) is a small tree found in the arid and semi-arid regions of northern Venezuela and has green stems. We evaluated ecophysiological traits and the role of the photosynthetic stem in the carbon gain of P. praecox in a tropical dry forest, by measuring seasonal changes in water status, gas exchange, water-use efficiency (WUE), photochemical activity of PSII, and biochemical, morphometric and functional traits of leaves and green stems. We found stem net photosynthesis with a rate of 17 mu molm(-2)s(-1), indicating that the stem contribution to the carbon balance of the species is positive. We also found 1.6 and 2.5 times higher instantaneous and intrinsic WUE, respectively, in green stems than in leaves during the rainy season, which has important implications for water balance. Drought had a negative effect on water potential, leaf P-N and photochemical activity of the stem. A similar contribution to the daily whole-plant carbon gain by each photosynthetic organ was found during both seasons; however, when leaf loss is complete during the dry season, the stem contribution would increase up to 100%.