Using Physiological Parameters to Monitor Plant Water Status Under Different Irrigation Systems.

Plant physiological parameters like photosynthesis and stomatal conductance are closely related and influenced by leaf water potential. Photosynthesis can be an important indicator of plant water status. Two greenhouse experiments were conducted to evaluate and compare physiological parameters of biochar-amended Hydrangea paniculata ‘Silver Dollar’ under three irrigation-scheduling regimes. Pine bark substrate was amended with 0, 10 and 25% by volume of hardwood biochar with three different irrigation strategies and eight replications per treatment combination. The irrigation systems were conventional irrigation, delivering 1.8 cm (0.7 inches) of water in one event each day, and two sensor-based irrigation systems. The two sensor-based irrigation systems included a physiological-based on-demand irrigation system predicated on an estimate of the relationship between substrate moisture and photosynthetic rate. The second system was a physical properties based, on-demand irrigation system where the set points were derived from the relationship between substrate moisture and tension. Results were similar in both experiments in terms of how biochar type affected plant physiological parameters under different irrigation systems. There was an interaction (p<0.002) between biochar rate and irrigation system for photosynthetic rate, transpiration rate and stomatal conductance. This was caused by increasing photosynthetic rate from 13.8 to 15 mol CO₂ m⁻² s⁻¹, transpiration rate from 3.7 to 5.2 mmol H₂O m⁻² s⁻¹ and stomatal conductance from 0.24 to 0.4 mmol H₂O m⁻² s⁻¹ under plant physiology-based irrigation regime as biochar rate went from 0 to 10%. Photosynthetic rate and stomatal conductance were not affected by biochar rate for the conventional or the physical properties-based irrigation regimes.