Flooding and Phytophthora cinnamomi: Effects on photosynthesis and chlorophyll fluorescence in shoots of non-grafted Persea americana (Mill.) rootstocks differing in tolerance to Phytophthora root rot

• Flooding and infection reduce stomatal conductance and photosynthesis.
• Recovery of stomatal conductance is a physiological marker for tolerance.
• There are non-stomatal limitations to photosynthesis in flooded plants.
• PSII efficiency is reduced in flooded plants.
• Flooding and infection causes damage to the photosynthetic apparatus.

Losses in the production of avocado (Persea americana (Mill.)) are incurred due to Phytophthora root rot (PRR), a disease of the feeder roots that results in tree-dieback and eventual tree death. Avocado is also a flood-sensitive species and flooding exacerbates the effects of PRR. The avocado industry relies on the use of rootstocks tolerant to PRR to minimise losses. The present study compared the gas exchange and chlorophyll fluorescence responses of avocado rootstock plants of ‘Dusa™’, the current South African industry standard, with ‘Duke 7’, and the selections R0.12 and R0.06 which show reduced and superior tolerance to PRR, respectively. A decline in stomatal conductance (gs) and net CO2 assimilation (PN) over the 30 day evaluation period were early responses to flooding. ‘Dusa™’, the more tolerant rootstock plants, demonstrated a better recovery in PN and gs in response to inoculation; however, both rootstocks performed poorly under flooded conditions. A decline in PN in infected ‘Duke 7’ plants appeared to be associated with stomatal limitations due to reduced stomatal conductance. The decline in PN and gs was not apparent in infected ‘Dusa™’ plants. Non-stomatal limitations to PN in rootstock plants exposed to flooding were also evident as indicated by increases in the ratio of internal to atmospheric CO2 concentrations (Ci/Ca). Impaired photosynthetic capacity in flooded rootstock plants was reflected by reduced photosystem II efficiency and photochemical quenching. In comparison to ‘Dusa™’, R0.12 rootstock plants showed reduced PN and gs following inoculation with Phytophthora cinnamomi whereas the more tolerant R0.06 rootstock plants revealed sustained photosynthetic activity. Interestingly R0.06 was the only rootstock able to maintain PN and gs in non-inoculated, flooded plants.