Effects of light spectra on growth and defence in potted Actinidia chinensis var. deliciosa 'Hayward' kiwifruit plants

Light intensity and spectral composition have major effects on plant growth, development and defence. We compared growth and defence elicitation in potted Actinidia chinensis var. deliciosa 'Hayward' kiwifruit plants raised in a greenhouse under natural lighting with counterparts grown in a temperature-controlled room under high-pressure sodium lamps (HPS; peaks at 570 and 589 nm) and light-emitting diodes (LED red 670 nm and LED red/blue 440 and 670 nm). Overall, shoot extension was not significantly affected by light source during the four-week trial, but evidence of faster shoot elongation under HPS and LED red than in the greenhouse or under LED red/blue emerged towards the end of the experiment. Leaf chlorophyll content and photosynthetic rate were higher in natural lighting than under HPS or LED, whereas the opposite was true for total phenolic content, which was greatest under LED red and LED red/blue. Constitutive resistance to one of the most devastating global pathogens of kiwifruit, Pseudomonas syringae pv. actinidiae (Psa3) was greater in LED red plants than LED red/blue or HPS. Treatment with the plant defence elicitor, acibenzolar-S-methyl (ASM), induced a reduction in Psa3 leaf necrosis in greenhouse plants, relative to untreated controls, but not in kiwifruit grown under artificial lighting, suggesting that inducible resistance was affected by light spectra. This correlated with upregulation of the salicylic acid (SA) pathway marker (PR1), which was more strongly induced by ASM in the greenhouse than under artificial light. In contrast, another SA pathway marker (PR2 - β-1,3 glucosidase) was more strongly induced in ASM-treated plants after Psa inoculation under artificial light than in the greenhouse. Results show the potential to manipulate plant growth and defence by modification of the light spectrum, although further research is required.