Effects of pot-in-pot production system on water consumption, stem diameter variations and photochemical efficiency of spindle tree irrigated with saline water

Our aim was to determine whether the pot-in-pot (PIP) system can improve the salinity tolerance of spindle tree (Euonymus japonicus) compared to above-ground pot (AGP) systems in terms of water use efficiency (WUE), irrigation water evapotranspired (IWET), stem diameter variations and photochemical efficiency. The PIP system involves burying a “holder pot” in the ground up to the rim. A second, “cultivation pot” containing the growing plant is placed inside the holder pot, while AGP is the traditional above-ground pot production system. In the present experiment, the cultivation pots in both systems contained a mixture of white peat, coconut fibre and perlite (40/40/20, v/v/v). The following treatments were studied: AGP and PIP with control irrigation using water of 1.76 dS m−1 (AGPc and PIPc); and AGP and PIP using saline irrigation water of 9.04 dS m−1 (AGPs and PIPs). A soil moisture sensor-controlled irrigation system was used to irrigate all the treatments when AGPc reached a substrate volume water content (θ) of around 0.34 m3 m−3. The θ of the saline treatments increased as the experiment progressed, particularly in PIP. The leaching fraction was 33% in AGPc, 41% in PIPc, 43% in AGPs and 54% in PIPs. PIPs produced the lowest amount of IWET (9.94 L) and AGPc the highest (13.20 L). Salinity reduced the plant dry weight (DW) in AGP (5.39 g), but not in PIP (10.2 g). AGPs led to the lowest WUE (0.52 g DW L−1), while PIPs led to 1.23 g DW L−1. Daily IWET per plant was related with the amount of photosynthetic light received each day (daily light integral, DLI) and age in the control treatments, while, under salinity, IWET was only related with DLI. Stem diameter growth was greater in PIPc (1278 μm) and lower in AGPs (60 μm). The discrepancy between the photosystem II efficiency and photosynthesis rate in AGPs pointed to an increasing rate of photorespiration.