Photosynthetic gas exchange, growth and biomass allocation of two Eucalyptus and three indigenous tree species of Ethiopia under moisture deficit

The effects of water deficit on growth, biomass allocation and gas exchange of Cordia africana Lam., Croton macrostachyus Del., Eucalyptus camaldulensis Dehnh, Eucalyptus globulus Labill. and Millettia ferruginea (Hochst.) Baker seedlings were studied under glasshouse conditions for 104 days. Plants were subjected to four watering regimes, viz. control (well-watered), mild-, moderate- or severe-water-deficit conditions corresponding to 25, 50, or 75% of the control moisture level, respectively. Well-watered plants produced about 4-6 times higher biomass compared to severely water deficient plants. All species had comparable biomass production under sufficient water or mild water deficit. However, the eucalypts produced more biomass than the deciduous species under severe water deficit. C. africana and C. macrostachyus invested more biomass to root unlike the other species. Increased water supply increased biomass allocation to leaves in M. ferruginea and the eucalypts whereas it increased biomass allocation to roots in C. macrostachyus. Water deficit reduced predawn and midday leaf water potentials in all the studied species with large decline at the midday in the eucalypts. Water deficit had marked effect on leaf relative water content (RWC) in C. macrostachyus and C. africana in which severely stressed plants had greatly reduced RWC than the controls during both predawn and midday. M. ferruginea maintained high predawn and midday RWC under all treatment conditions. Both stomatal conductances and photosynthetic rates declined in response to increasing water stress, however, the reductions followed different patterns for the different species. Water losses from C. macrostachyus and C. africana leaves were about 2-3 times those of E. camaldulensis and E. globulus across all the treatments. Water stress reduced whole plant water use efficiencies (WUEWL) from 5 ± 0.9 to 2 ± 0.6 g dry mass kg−1 water depending on the species. WUEWL of C. africana and C. macrostachyus were lower compared to the other species in all treatments. M. ferruginea showed superior water stress avoidance as evidenced by its high tissue water potential, RWC and photosynthetic rate under severe water deficit.