Flowering of kiwifruit (Actinidia deliciosa) is reduced by long photoperiods

Mature kiwifruit (Actinidia deliciosa ‘Hayward’) vines grown under standard orchard management were exposed to 16-h photoperiods from the longest day in summer until after leaf fall in autumn. Photoperiod extension was achieved with tungsten halogen lamps that produced 2–8 µmols m–2 s–1 photosynthetically active radiation. Long day treatments did not affect fruit dry matter or fruit weight at harvest during the growing season that the treatments were applied or during the following growing season. However, flowering was reduced by 22% during the spring following treatment application. As this reduction in flowering was not accompanied by a decrease in budbreak, the long day effect is not consistent with a delay in the onset of winter chilling. It is suggested therefore, that the observed reduction in flowering may be because of a diminution of floral evocation

Phenolic compounds in young developing kiwifruit in relation to light exposure: Implications for fruit calcium accumulation

The interaction between light availability and the biosynthesis of phenolic compounds in fruit of kiwifruit (Actinidia deliciosa var. deliciosa, C.F. Liang et A. R. Ferguson) was investigated. Fruits were exposed either to natural light or were artificially shaded while growing on mature vines and were analysed weekly during the first 11 weeks of development. Phenols were identified and quantified by using High Performance Liquid Chromatography (HPLC). Results showed that the predominant phenolic compounds were hydroxycinnamic acids (HCAs), flavonols and the flavan 3-ol epicatechin. Calcium (Ca2+), the main mineral nutrient involved in fruit quality was also determined. Light significantly increased the accumulation of both phenols and Ca2+ into the fruit. This work expands the list of known phenolics in kiwifruit and provides a possible explanation for the seasonal pattern of Ca2+ import into the fruit. Results on light–phenol interaction being apparently beneficial for fruit Ca2+ accumulation, suggest that accurate canopy management could enhance fruit quality

Early detection of Psa infection in kiwifruit by means of infrared thermography at leaf and orchard scale

© CSIRO 2014. Pseudomonas syringae pv. actinidiae (Psa), the causal agent of bacterial canker of kiwifruit, has become a worldwide threat for the kiwifruit industry. In this work, the potential of infrared thermography for early detection of physiological symptoms related to Psa-infection at leaf and at orchard block scale was assessed. At the leaf level, thermal cold spots appeared shortly after Psa-infection, well before any visual symptoms. A few weeks after infection, thermal hot spots were observed, associated with, but not limited to, spots of visible leaf necrosis. At orchard block level, Psa-infected canes were significantly warmer in both blocks and on all measurement days. A novel wet reference surface, existing of a cluster of cotton imitation leaves with similar dimensions and orientation as real leaves and remaining wet through sucking up water from a small container, was used to estimate the crop water stress index (CWSI). CWSI showed stable values of infected and uninfected areas during the day and between following days. Crop temperature and CWSI were closely correlated with leaf stomatal conductance, which was lower in infected canes. A Psa-infection map based on canopy temperature revealed that Psa infects the outer canes rather than the central part of the canopy

A new wet reference target method for continuous infrared thermography of vegetations

© 2016 Elsevier B.V.. Although infrared thermography for stress detection in plants is popular in scientific research, it is rarely used in continuous and automated applications. One of the main reasons for this is that the most precise method for generating wet reference targets, used for normalizing the leaf or canopy surface temperature for microclimatic conditions, requires manual wetting before each image capture. In this article, we present and evaluate a new type of wet reference target that remains wet while having an energy balance as similar as possible to that of the canopy. This reference target consists of a cloth knitted around a solid frame whose shape and dimensions mimic those of the leaves. The cloth remains wet by constantly absorbing water from a reservoir. The new reference target was evaluated on grapevine and kiwifruit plants in greenhouse and orchard conditions. In greenhouse conditions, measured stomatal conductance was consistently more highly correlated with the stomatal conductance index Ig when Ig was calculated with the new wet reference rather than the manually wetted reference target. Furthermore, the temperature difference between leaves and the new reference target remained stable for as long as measured, in contrast with the manually wetted leaves. Ig obtained with the new reference target method was also highly correlated with stomatal conductance (gs) of both crops in orchard conditions. A new empirical regression model to estimate gs from Ig in greenhouse conditions was introduced and evaluated. This regression model incorporates the background temperature, a parameter that needs to be included in thermographic measurements for obtaining correct surface temperatures, thus avoiding the need for any additional measurements. The same regression model can be applied on different days with differing conditions. The model performed better than other tested empirical models and provided unbiased estimates of gs on days with different conditions, resulting in a root mean square error of 22-25% of gs. Thus, it provides a promising method for continuous remote sensing of stomatal conductance or drought stress detection of plants and vegetations