Biomass Partitioning and Gas Exchange in Dalbergia sissoo seedlings under water stress

Biomass, leaf water potential (Ψₗ), net photosynthetic rate (P N), transpiration rate (E), stomatal conductance (g ₛ), leaf to air temperature difference (T dᵢff), and instantaneous water use efficiency (WUE) were measured in the seedlings of Dalbergia sissoo Roxb. grown under irrigation of 20 (W₁), 14 (W₂), 10 (W₃), and 8 (W₄) mm. Treatments were maintained by re-irrigation when water content of the soil reached 7.4% in W₁, 5.6% in W₂, 4.3% in W₃, and 3.2% in W₄. Seedlings in a control (W₅) were left without irrigation after maintaining the soil field capacity (10.7%). Seedlings of W₁ had highest biomass that was one tenth in W₅. Biomass allocation was highest in leaf in W₂ and in root in W₄ and W₅ treatments. Difference between predawn leaf water potential (ΨPd) and midday (Ψₘᵢd) increased with soil water stress and with vapour pressure deficit (VPD) in April and May slowing down the recovery in plant leaf water status after transpiration loss. P N, E, and g ₛ declined and T dᵢff increased from W₁ to W₅. Their values were highly significant in April and May for the severely stressed seedlings of W₄ and W₅. P N increased from 08:00 to 10:00 and E increased until 13:00 within the day for most of the seedlings whereas g ₛ decreased throughout the day from 08:00 to 17:00. P N and E were highest in March but their values were low in January, February, April, and May. Large variations in physiological variables to air temperature, photosynthetically active radiation, and vapour pressure deficit (VPD) indicated greater sensitivity of the species to environmental factors. WUE increased from W₁ to W₂ but decreased drastically at high water stress particularly during hot summer showing a kind of adaptation in D. sissoo to water stress. However, low biomass and reduced physiological functions at <50% of soil field capacity suggest that this species does not produce significant biomass at severe soil water stress or drought of a prolonged period.