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

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 CO 2 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 (g s ) 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, g s 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, g s declined to a greater magnitude following the high-to-low PPF transition, and g s 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.