Water-deficit Priming Increases Leaf Gas Exchange of Papaya (Carica papaya L.)

Priming, the imposition of a defined duration of mild stress during early plant development, is a relatively recent focus of research with potential as a management technique for improving crop tolerance to biotic and abiotic stress.  However, priming capacity has not been tested in many horticultural crops and the physiological mechanisms involved in priming have not been well described.  In addition, there is little information about the priming “memory,” or the duration of the effects after the priming has been completed.  Hypothesized mechanisms have included increased photosynthetic capacity, increased anti-oxidant capacity, and increased partitioning of carbohydrates to roots, each of which may play a different role in determining priming memory. The objectives of this study were to (1) test whether papaya (Carica papaya L.) has the potential to be primed with mild water deficit; (2) test whether priming could elicit up-regulation of leaf gas exchange of vegetative plants; and (3) determine the relative duration of priming memory in papaya.  ‘Red Lady’ papaya plants in pots were subjected to one of 3 treatments: (1) early priming (EP) initiated 6 months after germination, (2) late priming (LP) initiated 7 months after germination, and (3) an unprimed control.  Soil water tension in the control was maintained at an average of 4 kPa via daily irrigation. Priming treatments consisted of a 3-week period of reduced irrigation in which soil water tension averaged approximately 20 kPa, with periods of greater than 30 kPa.  Leaf gas exchange variables were measured at 1-week intervals for 4 weeks, beginning 3 days after the completion of the LP treatment. Measurements included net CO₂ assimilation, stomatal conductance, and transpiration.  The LP treatment increased net CO₂ assimilation, stomatal conductance, and transpiration, while the EP treatment decreased these variables compared with the control.  On the first measurement date, net CO₂ assimilation was approximately 25 percent higher for the LP treatment than the control treatment.   At the end of the 4-week measurement period, all variables were identical across treatments, indicating a relatively short priming memory. Results indicate that papaya has the capacity to be primed using mild water deficits resulting in increased growth-related physiological processes, but that this increase is transitory.