The potential for primed acclimation in papaya (Carica papaya L.): Determination of critical water deficit thresholds and physiological response variables

Priming is a strategy that potentiates a plant’s response to stress. Primed acclimation (PA) in crop irrigation scheduling is the use of a temporary period of regulated deficit irrigation (RDI), imposing mild or moderate deficits, to evoke a primed state, after which full evapotranspiration replacement is resumed. Although little has been reported regarding physiological impacts of PA, the technique has been used to maintain or improve yields in agronomic crops in drought-prone regions. The first objective of this study was to test a plant-based approach to determine the appropriate level of soil water potential needed to evoke a primed response in papaya plants. This was done through a substrate (potting medium) dry-down experiment with one group of plants irrigated to field capacity and a second group of plants for which the soil was allowed to dry gradually. Physiological responses were assessed to determine at what point a mild to moderate level of stress was reached. The second objective was to test the effects of a priming stress (the level set in the first objective) on leaf gas exchange (net photosynthesis [A], stomatal conductance [gs], transpiration [E]) and chlorophyll fluorescence in papaya as compared to a control treatment in which soil water content was maintained at field capacity. The most sensitive variable in the dry-down experiment was gs, which was used to select a soil water potential target of −30kPa for subsequent priming treatments. Priming treatments using this predetermined target resulted in increased A and leaf chlorophyll index (SPAD) during the period when irrigation to field capacity was restored. According to OJIP analysis, primed plants had increased fluorescence performance indices, reduced antennae size per Photosystem II reaction center, and increased fluxes and efficiencies from plastoquinone A through Photosystem I. Thus, increased A as a result of priming had both stomatal and photochemical components. However, increased A due to priming of papaya appeared to be temporary. These results show that papaya has the capacity to reach a primed state and provide the soil water potential levels that could be utilized to achieve the appropriate level of water deficit for priming. This vital information can be used in the development of management techniques to capitalize on papaya’s priming ability under commercial irrigation management schemes.