There is an increasing need for improving irrigation management around the world, mainly due to the effects of climate change such as increased temperatures and more uneven precipitation distribution. Despite increased evapotranspiration and reduced water availability due to climate change, most EU growers still schedule irrigation on an empirical basis, with very little or no awareness of the consequences on yields and on water use efficiency (WUE). Some growers monitor soil water status through various sensors, such as tensiometers or capacitance probes. Others use online services, based on the estimation of the crop potential evapotranspiration (ETc), which are available locally and can provide custom-tailored irrigation scheduling. In addition, efficient irrigation management in fruit trees is far more complex than in field crops. Considering this context, many plant-based sensors and methodologies have been developed aiming at better understanding the crop water requirements. Much attention has been paid to the plant-based methods, since they directly monitor the cultivation target, while integrating the atmospheric and soil water status.
Among the many plant-based approaches, the sap flux density methodologies present great advantages due to their automatic operation and for being non-destructive. Other great advantages of the sap flow sensors are their robustness and reliability for use in open field for extended periods of time, and its easiness to implement with data transmission systems. Several sap flow methods were developed, generally involving the measurement of heat transport by the sap flux. These approaches have already been used for research purposes in fruit trees such as in apple, pear, and peach trees.
Some authors consider that the upscaling of sap flow measurements from a single point in a tree to the whole tree is a great source of uncertainty. However, there are several reports on the importance of the radial sap flux density (Js) variability for monitoring plant water status. Some factors have been reported as responsible for the dynamic radial variation of Js, such as soil water status, water uptake from deep roots8, stomatal closure of sun-exposed leaves in response to atmospheric vapor pressure deficit7, and changes in the distribution of incoming radiation across the canopy.
Recent studies present the results of field experiments on the estimation of net assimilation (AN) and stomatal conductance (gs), based on sap flux density measurement. These results led to a common conclusion that the use of sap flux density sensors with atmospheric measurements can be an alternative for continuous measurement of photosynthesis and stomatal conductance in olive trees.
While olives are well adapted to Mediterranean conditions and are considered a drought-tolerant crop, other fruit crops are much more demanding in terms of water requirements and would greatly benefit from a rational tool to improve irrigation scheduling. For example, due to its environment of origin and its anatomical features, kiwifruit is the fruit crop requiring the highest irrigation volumes during the season. These very high water requirements represent a serious issue in times of water scarcity and rational approaches to irrigation management are urgently needed by growers, also because of the risk of supplying excessive amounts of water, causing problems to roots such as anoxia, water logging, and probably determining the conditions for “kiwifruit vine decline”, a very serious pathology that is destroying hectares of orchards in Europe.
Olive and kiwifruits present great differences in terms of hydraulics characteristics and behavior. Olive trees present smaller vessels and higher density of vessels in comparison to kiwifruit vines. Therefore, it is of great interest to obtain information on the maximum level of water stress imposed by deficit irrigation in kiwifruit vines to optimize fruit growth, quality, and dry matter content, without causing vessels cavitation. In fact, it is known how moderate drought stress can increase fruit quality levels in terms of dry matter concentration, sweetness, and taste. Furthermore, studies have demonstrated that secondary metabolites can be synthesized in presence of drought stress, potentially increasing nutraceutical properties of fruits.
To our best knowledge, there are no studies in literature on the gs estimation in kiwi trees from sap flux density and atmospheric vapor pressure deficit measurements and no attempts to optimize fruit quality based on irrigation protocols scheduled based on the actual plant water status and physiological performance.
Objectives: Considering this context, the main objective of this project was to obtain a model for the estimation of stomatal conductance through sap flux measurement in yellow-fleshed kiwifruit vines, allowing assessment of plant’s water status and correct management of irrigation, while optimizing fruit growth and quality.
