Understory environment promotes photosynthetic efficiency and mitigates severity and function of an introduced, vectored pathosystem: a study of a feral citrus population in central Florida.
Feral citrus trees growing under forest overstory appeared healthy and showed no symptoms of huanglongbing (HLB; citrus greening disease), despite high infection rates and obvious symptoms in open plantings. We undertook a survey in a natural ecosystem area with abundant understory feral citrus (Citrus × paradisi Macfad.) trees to understand how natural understory conditions affect distribution of the HLB causal bacterium, Candidatus Liberibacter asiaticus, and its vector, the Asian citrus psyllid (Diaphorina citri Kuwayama). Citrus also undergoes high irradiance-induced photoinhibition which is exacerbated by HLB. We hypothesized that the understory environment would minimize photodamage and improve photochemical functions. We examined sixty citrus trees, across a high-low overstory density (gap fraction, GF) gradient, for photochemical performances and other key leaf traits associated with disease symptoms or shade acclimation. We did not detect any psyllids in the understory during early summer when peak outbreak of the vector population was expected, and only a few trees were infected with Ca. L. asiaticus. Leaf pigment stoichiometry and photosystem-II functions showed strong acclimation to overstory GF. Photochemical performance improved with decreasing overstory GF due to increased electron transport efficiencies from donor to acceptor sites. No difference in measured leaf biochemical traits and photosystem-II performances was observed between infected and uninfected understory trees, though the number of infected trees was small. We conclude that the naturally shaded understory environment greatly inhibits the HLB pathosystem by deterring the arrival of the vector and significantly improves the photosystem-II performance of citrus, making the shaded environment advantageous for the feral citrus population.