Impact of heat stress of varying severity on papaya (Carica papaya) leaves: Major changes in stress volatile signatures, but surprisingly small enhancements of total emissions

Tropical plants are considered to be highly heat stress resistant, but heat waves of different frequency and duration induced by global climate change can exceed the plant heat stress tolerance in the tropics. To gain insight into the overall heat resistance, leaves of papaya were exposed to mild (37 and 41 °C), moderate (46 °C) and severe (49 °C) heat stress (applied for 5 min), and plant recovery was monitored for 48 h upon return to pre-stress conditions. Leaf photosynthetic characteristics were reduced immediately after stress in all cases, but photosynthesis rate almost fully recovered after mild heat treatment, and partly recovered in 48 h after moderate stress, while no recovery was observed under severe stress. Constitutive BVOC emissions dominated by long-chained aldehydes, and glucosinolate breakdown products, attained the emission maxima at 46 °C. Acute heat stress led only to a minor induction of BVOC emissions, and the greatest share of induced emissions was observed in leaves exposed to 46 °C. Induced emissions were dominated by oxygenated monoterpenes, indicating an increase in leaf oxidative status. Our results demonstrate that papaya could withstand moderately severe heat stress of up to 46 °C, but this level of stress will have negative impacts on plant carbon gain due to delayed photosynthetic recovery after heat stress. Although volatile profiles sensitively responded to heat stress, the magnitude and degree of elicitation were surprisingly small throughout the recovery, suggesting that papaya is relatively heat resistant and possess a suite of heat-protective traits.