Apple Rootstock Response to Threshold Water Management during 6 Weeks of Progressing Drought

Containerized second-leaf apple, Malus domestica , ‘Honeycrisp’ scion grafted to rootstock cultivars (‘B.10’, ‘B.9’, ‘G.11’, ‘G.935’, and ‘G.969’) were subjected to 6 weeks of data-driven progressive drought and whole-tree physiology assessment. An autonomous irrigation control system, Open_Irr, was programmed to progressively decrease matric potential of a mineral soil matrix over a range of common management thresholds, −33 to −75 kPa. Observed growth and aspects of photosynthetic function were assessed by infrared gas analyzer, fluorometer/porometer, and Scholander chamber pressure to facilitate observation of rootstock cultivar conveyance of response dynamics during progressing drought conditions. The ‘G.935’ rootstock conveyed strong anisohydric tendencies in response to decreasing hydraulic gradients with large absolute (2 mmol H 2 O·m −2 ·s −1 ) and relative, 42%, declines in transpiration over 2 weeks. The remaining cultivars maintained or increased transpiration rates (+0.5 mmol H 2 O·m −2 ·s −1 , pooled) over the same period; the B.10 cultivar displayed the strongest isohydric strategy. Midday leaf water potential confirmed sensitivity of the G.935 cultivar to moderate drought as midday leaf water potential, −2.7 MPa, exceeded the traditional −1.5 MPa indicator in week 1 measures during a −37-kPa watering event threshold. Upon excavation, ‘B.10’ observed C allocational plasticity with elevated root mass fraction relative to pooled remaining rootstock cultivars, 0.67 versus 0.52. Results imply meaningful selection of finished trees in risk mitigation planning, particularly in new plantings and rainfall-dependent operations. Anisohydric tendencies suggest irrigation prioritization of ‘G.935’ relative to ‘B.10’, ‘B.9’, ‘G.11’, and ‘G.969’.