Physiological measurements of daily daylight fertigated citrus trees

[摘要] ENGLISH ABSTRACT:Daily daylight fertigation regimes (involvinq drip fertigation, on a daily basis,during the daylight hours) are becoming widely used in the South Africancitrus industry in the endeavor to enhance tree productivity. Such regimescould provide sensitive nutrient and moisture management, reducing plantstress in general as well as the response time to root-directed culturalactivities.There is a need to evaluate the efficacy of daily daylight fertigation systemsrelative to conventional irrigation systems. Standard horticultural evaluation oforchard management practices is very time consuming. We opted forphysiological studies comparing plant stress levels, in an attempt to quantifyplant performance under each system. Citrus trees under daily daylightfertigation and conventional micro-jet and drip-irrigated regimes weremonitored to establish plant stress levels as indicated by sap flow, xylemwater potential, stomatal conductance and chlorophyll a fluorescence. Plantsunder a daily daylight fertigation regime are believed to have good soil waterconditions in their rooting volume, and therefore experience negligiblebaseline levels of stress. The trees do, however, experience middaydepression in stomatal conductance, to a lesser degree, but not unlike treesunder conventional regimes. It appears as if a larger rooting volume of microjetirrigation regimes enhances recovery from the midday depression. It isrecommended that producers optimise productivity during the moming hours,by early irrigation, so that plants can function optimally, whilst environmentalconditions are most favourable for high physiological activity.We also assessed the effect of withholding water from trees adapted to a dailydaylight fertigation regime to evaluate the risk involved with short-term waterdeficits in trees adapted to this regime, as well as the usefulness ofphysiological techniques for identifying water stress. Stomatal conductanceand xylem water potential indicated water stress sooner than the otherphysiological parameters. Citrus trees seem to be relatively insensitive towater deficit stress as measured by sap flow and chlorophyll a fluorescence.Sap flow is buffered by tree capacitance, and although mediated via stomatalconductance, atmospheric conditions and not the soil water content primarilydetermine it. As daily fertigation is applied to trees under DOF regimes, theyexhibit more optimal levels of xylem water potential and stomatalconductance, compared to trees from which water is withheld. Althoughalleviating it to a degree, daily irrigation did not mitigate the middaydepression in these values. Seen over a season, even small enhancementsof stomatal conductance (and with it photosynthesis and possibly, growth) andxylem water potential, could incrementally produce higher yields.

[发布机构] Stellenbosch University