Water retention potentials of Italian soils and physiological responses of potted yellow kiwifruit

Correct water management of yellow kiwifruit vines is essential for reaching high yield and fruit quality, to keep plants healthy and avoid useless water loss. The aim of the present experiment was to evaluate the physiological responses of potted Zezy002 (Actinidia chinensis var chinensis) plants to decrease of soil moisture to the wilting point, and to assess the retention curves of 5 typical soil substrates for the kiwifruit production in Italy. The 5 soils were collected from 4 Italian regions named: Basilicata, Calabria, Emilia-Romagna and Lazio (2 soils: Folie and Rosini). Plants from each soil were divided in three groups: 3 plants were irrigated maintaining soil moisture at field capacity (CONTROL); 4 plants were subjected to water stress (STRESS plants), after 48 h of water suspension, two of the four plants were irrigated as for the control plants (RECOVERY). Each pot was provided with a chalk potentiometric probe to monitor soil matric potential (Y_m). In addition, soil moisture was evaluated by weight of a soil sample oven-dried, finally daily pot evapotranspiration rate was evaluated gravimetrically by pot weight at 24-h-interval. Leaf gas exchange and stem water potential (Y_w) were measured daily. After irrigation suspension, plants rapidly (48 h) reached the wilting point evidenced by the stop of CO₂ fixation. This corresponded to stem Y_w lower than -1.75 MPa in all soils but the one from Emilia-Romagna which had the higher percentage of loam (42%) that also maintained a positive CO₂ assimilation rate longer than the other soils. In this lapse of time, the rate of leaf CO₂ assimilation, stomatal conductance and transpiration sharply decreased while intercellular CO₂ concentration increased. Similarly stem Y_w responded quickly to the suspension and re-start of irrigation, reaching values as low as – 1,9 MPa 2 days after the quit of irrigation. At wilting points soil Y_m was: -0.96 MPa for Emilia-Romagna, -0.5 MPa for Basilicata, -1.6 MPa for Calabria, -1.8 MPa for Lazio Folie and -2.3 MPa for Lazio Rosini. CO₂ assimilation was better correlated to stem Y_w than soil Y_m.

Key words: soil water moisture, chalk potentiometric probe, leaf gas exchange, stem water potential, soil matric potential