High temperature leads to a loss of isohydry in Pinus radiata

Pinus radiata has been characterised as strongly isohydric, which means that it tends to close stomata to maintain leaf water potential relatively constant under changing environmental conditions. However, under high temperatures, where leaf cooling may be necessary to prevent overheating, the ability to maintain this isohydric behaviour remains unexplored. Here, we examined the impact of increasing temperature (and associated needle-to-air vapor pressure deficit hereafter referred to as VPD_needle) on whole plant stomatal conductance (g_c), canopy transpiration (E_c), minimal conductance (g_min) and stem water potential (Ѱ_stem) in well-watered Pinus radiata plants. A decline in g_c was observed as temperature increased from 22°C to 32°C resulting in 27% stomatal closure, followed by levelling off at further higher temperatures. However, this stomatal closure was not enough to prevent an exponential rise in E_c and pronounced declines in Ѱ_stem. g_min contributed 18% of g_c at 42°C and showed an exponential rise at temperatures higher than 42°C. Our findings suggests that high temperatures may lead to drop in isohydry in Pinus radiata. This shift may be crucial to avoid overheating and damage of plant tissues but at the same time leads to more negative Ѱ_stem, thereby increasing the chances of xylem embolism. The observed shift in stomatal response reveals that even in water conservative species like Pinus radiata, high temperatures may compromise stomatal regulation of water potential.