The influence of drought stress on leaf gas exchange and chlorophyll fluorescence characteristics of field-grown papaya (Carica papaya L.) plants was determined under a range of incident light fluxes and times of day. These data may aid in improving management systems for papaya production which minimize detrimental effects from suboptimal environmental conditions. Water was withheld from field-grown `Red Lady' plants in one study and `Tainung #2', `Red Lady', and `Sunrise' plants in a second study until soil matric potential was -60 to -70 kPa. Drought-stressed plants exhibited reduced net CO2 assimilation (ACO2) above light saturation, photosynthetic photon flux (PPF) at which light saturation for ACO2 occurred, and apparent quantum yield compared to well-watered plants. The light compensation point of drought-stressed plants was greater than that of well-watered plants. Leaf chlorophyll fluorescence characteristics were not influenced by drought stress. The daily pattern of leaf gas exchange was dependent on climatic conditions. For sunny days, ACO2, stomatal conductance of water (gs), and water use efficiency of well-watered plants were maximal at mid-morning, declined during midday, and then partially recovered during late afternoon. In drought-stressed plants, leaf gas exchange was relatively constant after a brief early morning maximum. On overcast days, the responses of gas exchange variables in relation to time of day followed smooth bell-shaped patterns regardless of the level of drought stress. Combined with previously published data, these results indicate that the influence of drought stress on gas exchange is highly dependent on time of day, ambient sky conditions, plant size, and speed with which drought stress occurs.
Drought, Leaf Gas Exchange, and Chlorophyll Fluorescence of Field-grown Papaya