Drought Stress Influences Gas-exchange Responses of Papaya Leaves to Rapid Changes in Irradiance

Abstract

Field-grown `Red Lady' papaya (Carica papaya L.) plants were used to measure foliar gas-exchange responses to rapid changes in irradiance levels to determine if papaya stomata are able to track simulated sun-to-cloud cover transitions. Natural sunlight and neutral shade cloth placed over the leaf were used to provide high photosynthetic photon flux (PPF) of about 2000 μmol·m-2·s-1 until leaves reached steady state within the cuvette, followed by three minutes with low PPF of about 325 μmol·m-2·s-1, and a return to PPF of about 2000 μmol·m-2·s-1. Net CO2 assimilation (A) declined from an initial 20 μmol·m-2·s-1 to about 9 μmol·m-2·s-1 within 20 seconds of initiating low PPF, and remained fairly stable for the duration of the three minutes of low PPF. Stomatal conductance (gs) declined within 60 seconds of initiating low PPF, from 385 to about 340 μmol·m-2·s-1 during the three minutes duration of low PPF. Following the return to high PPF, A rapidly increased to about 18 μmol·m-2·s-1, then gradually increased to the original value. After a lag of about 1 minute following the return to high PPF, gs began to increase and returned to the original value after three minutes. Container-grown `Tainung #1' papaya plants were used in a second study to determine the influence of mild drought stress on gas-exchange responses to rapid irradiance transitions. For drought-stressed plants, gs declined to a greater magnitude following the high-to-low PPF transition, and gs and A recovered more slowly following the transition from low-to-high PPF than for well-watered plants. Water use efficiency declined to a minimum immediately following the high-to-low PPF transition for both sets of plants, but recovered more rapidly for drought-stressed plants. These results indicate that papaya stomata are able to track rapid changes in irradiance, and mild drought stress enhances the tracking response.