Periodic Reporting for period 1 - KIWIQUALI (Optimizing KIWIfruit QUALIty through estimation of leaf stomatal conductance from sap flux density)
Reporting period: 2023-03-15 to 2025-03-14
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.
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.
In 2023 we performed an experiment in yellow-fleshed kiwifruit vines with four irrigation treatments based on the crop evapotranspiration (ETc): 100, 68, 57 and 40% of ETc. Four vines per treatment were monitored with sap flow sensors, and leaf gas exchange measurements were performed in these vines during nine days throughout the irrigation season. We assessed the link between leaf stomatal conductance (gs) and sap flux density (Js). The fruit quality was also assessed at harvest.
The amount of gs measurements allowed us to perform a thorough analysis regarding its relation with vapor pressure deficit (D). It was clear, based on the data collected, that gs presented a decrease when D is over a certain threshold, which could be defined between 1.8 and 2 kPa. Additionally, fruit dry matter was higher in the deficit irrigation treatments.
In 2024 we performed another experiment in yellow-fleshed kiwifruit vines with two sustained deficit irrigation (SDI) treatments (100 and 70% ETc); and two regulated deficit irrigation (RDI) (100% and 70% ETc during the first phase of fast fruit growth, and a restriction of about 50% on the second phase). Four vines per treatment were monitored with sap flow sensors, and leaf gas exchange measurements were performed in these vines during nine days throughout the irrigation season. We assessed the link between leaf stomatal conductance (gs) and sap flux density (Js). The fruit quality was also assessed at harvest. The same behavior regarding gs decrease at D above a threshold value was observed in these treatments. The fruit quality indicated that the treatments SDI100 and RDI70.50 (70% first phase and 50% second phase) induced maturation earlier than the other treatments. This indicates that the kiwifruit vines were under stress, either by water excess (SDI100) or by deficit (RDI70.50).
The amount of gs measurements allowed us to perform a thorough analysis regarding its relation with vapor pressure deficit (D). It was clear, based on the data collected, that gs presented a decrease when D is over a certain threshold, which could be defined between 1.8 and 2 kPa. Additionally, fruit dry matter was higher in the deficit irrigation treatments.
In 2024 we performed another experiment in yellow-fleshed kiwifruit vines with two sustained deficit irrigation (SDI) treatments (100 and 70% ETc); and two regulated deficit irrigation (RDI) (100% and 70% ETc during the first phase of fast fruit growth, and a restriction of about 50% on the second phase). Four vines per treatment were monitored with sap flow sensors, and leaf gas exchange measurements were performed in these vines during nine days throughout the irrigation season. We assessed the link between leaf stomatal conductance (gs) and sap flux density (Js). The fruit quality was also assessed at harvest. The same behavior regarding gs decrease at D above a threshold value was observed in these treatments. The fruit quality indicated that the treatments SDI100 and RDI70.50 (70% first phase and 50% second phase) induced maturation earlier than the other treatments. This indicates that the kiwifruit vines were under stress, either by water excess (SDI100) or by deficit (RDI70.50).
To the best of our knowledge, the experiment performed in 2023 was the first time in which sap flux density measurements were performed and reported in kiwifruit vines under water deficit conditions. We are currently working at a scientific publication regarding these results.
Additionally, we are also working to submit a scientific paper on the use of mechanistic and machine learning models (Artificial neural networks) to model leaf stomatal conductance from environmental variables and from sap flux density measurements.
Sap flux density was already used in literature to model stomatal conductance in olive trees. However, kiwifruit vines have a different behavior regarding stomatal closure under water deficit. Kiwifruit vines respond quickly to water deficit, closing stomata. Therefore, we found, analyzing the results from the 2023 experiment, that in kiwifruit vines the sap flux density is not greatly correlated to leaf stomatal conductance.
The paper on the use of models helped us understand the importance of other variables that could be monitored to estimate stomatal conductance, such as leaf temperature.
Additionally, we are also working to submit a scientific paper on the use of mechanistic and machine learning models (Artificial neural networks) to model leaf stomatal conductance from environmental variables and from sap flux density measurements.
Sap flux density was already used in literature to model stomatal conductance in olive trees. However, kiwifruit vines have a different behavior regarding stomatal closure under water deficit. Kiwifruit vines respond quickly to water deficit, closing stomata. Therefore, we found, analyzing the results from the 2023 experiment, that in kiwifruit vines the sap flux density is not greatly correlated to leaf stomatal conductance.
The paper on the use of models helped us understand the importance of other variables that could be monitored to estimate stomatal conductance, such as leaf temperature.