What does the functional-structural plant model hydroshoot tell us about the reasons for grapevine (Vitis vinifera L.) photosynthesis depression?

Abstract

Grapevine is a species that get along with water deficit. Yet, it cannot always stand thirst when accompanied by high temperatures, reducing noticeably its gas-exchange rates. Elucidating the origins of this reduction is a challenge, regarded the complex hydraulic, biochemical and energy processes lying behind gas-exchanges. In this work we analyze data collected from an experiment conducted at the whole plant scale on Syrah vines with the aid of the functional-structural plant model HydroShoot. During our experiment, we submitted grapevines to a severe water stress and observed a steep drop in whole plant photosynthetic rates at midday, that was not due to stomatal closure, suggesting that both processes were decoupled at this moment. Using HydroShoot, we explore whether this decoupling results from a direct water limitation on biochemical processes. HydroShoot links xylem hydraulic transport to gas and energy exchanges processes at the organ level. It simulates the effect of water deficit on xylem and stomatal conductances. The biochemical reactions of photosynthesis are affected by water deficit both indirectly through diffusional limitation and directly, through a reduced electron transport rate. Temperature affects photosynthetic rates through Arrhenius functions. Using HydroShoot, we show that photosynthetic, midday depression could not be explained by simple hydraulic limitations. Bulk leaf water potential dropped to -1.6 MPa but this drop only affected J_max when temperatures exceeded 34 °C. Neither the Arrhenius response, nor the water limitation considered independently were sufficient to predict the observed drops. Only when responses to water and temperature were combined were we able to reproduce these observations, suggesting that photoinhibition may have occurred under these conditions. Apart from an evidence of photoinhibition, our simulations indicate that xylem cavitation could not explain the observed drop in bulk leaf water potential. By contrast, a decrease in soil water potential has dramatic effects, much stronger than changes in xylem conductivity. The hydraulic architecture did not seem to play a major role in triggering stomatal closure. We conclude that an adequate prediction of grapevines water use efficiency under water deficit conditions relies strongly on soil hydraulic properties and photoinhibition predictions.