measurements of the lychee trees and assuming a low evaporation rate (k e ¼ 0.05)
– as achieved by micro-irrigation.
6.3.2 Deficit Irrigation in Longan
As limited water resources create an obstacle for increased longan production,
deficit irrigation strategies have lately been investigated, and one experiment with
commercial orchards in Thailand found that PRD with 66 % irrigation of calculated
ET c. did not cause a significant reduction in yield or fruit quality as compared to a
100 %-watered control (Ongprasert et al. 2007). As a result, under conditions of
extreme drought, DI can help ensure stable yields, added to which, lower irrigation
water use can reduce electricity costs for water pumping, even when water is free of
costs (Satienperakul et al. 2006). In a previous study, under controlled conditions
PRD (with 60 % of ET c. ) was applied to 3- year old longan trees grown under a
plastic shelter. The trees subjected to PRD showed stunted vegetative growth
without noticeable foliar wilt, and during the 28 weeks of the experiment, the
control trees gave two flushes, while those subjected to PRD gave only one flush
but with a higher number of leaves and shorter shoots than those developed during
the first flush for the control (Ongprasert and Wiriya-alongkorn 2009). Furthermore,
another study found reduced concentrations of phosphorus and potassium in the leaf
tissues of PRD irrigated longan trees (Srikasetsarakul et al. 2011), as compared to
previous reports on the nutrient content of healthy trees (Khaosumain et al. 2005).
These findings on reduced biomass formation may indicate similar long-term
effects in longan as has been reported for other tree species, such as the lower
crown volume found in almonds (Egea et al. 2010) and the reduced root-biomass
growth shown in peach trees (Abrisqueta et al. 2008). However other studies carried
out under the same climatic conditions as those found in Thailand, which has an
intensive rainy season, did not reveal any negative impacts on yields in the long-run
(Spreer et al. 2009). In light of this, there is a need for more research to be done on
the longan tree’s response to drought stress.
6.3.3 Water Stress Monitoring
Monitoring water stress during fruit tree production is a rather complex task, as –
other than in most annual crops – it is not the absolute yield or total biomass but
rather quality that determines the success of the producer. Depending on the fruit
species involved, the marketable yield is an effect of inner and outer quality
parameters. Thus, deficit irrigation is carried out, not only to save water, but also
to enhance desired properties such as acidity in wine production or obtaining a
better fruit to flesh ratio, as is the case with mango (Spreer et al. 2007). In longan,
however, the absence of water stress during the fruit growing period is fundamental
for ensuring a marketable yield as otherwise fruit crack occurs as a result of osmotic
6 Mango and Longan Production in Northern Thailand: The Role of Water Saving. . .
223
– as achieved by micro-irrigation.
6.3.2 Deficit Irrigation in Longan
As limited water resources create an obstacle for increased longan production,
deficit irrigation strategies have lately been investigated, and one experiment with
commercial orchards in Thailand found that PRD with 66 % irrigation of calculated
ET c. did not cause a significant reduction in yield or fruit quality as compared to a
100 %-watered control (Ongprasert et al. 2007). As a result, under conditions of
extreme drought, DI can help ensure stable yields, added to which, lower irrigation
water use can reduce electricity costs for water pumping, even when water is free of
costs (Satienperakul et al. 2006). In a previous study, under controlled conditions
PRD (with 60 % of ET c. ) was applied to 3- year old longan trees grown under a
plastic shelter. The trees subjected to PRD showed stunted vegetative growth
without noticeable foliar wilt, and during the 28 weeks of the experiment, the
control trees gave two flushes, while those subjected to PRD gave only one flush
but with a higher number of leaves and shorter shoots than those developed during
the first flush for the control (Ongprasert and Wiriya-alongkorn 2009). Furthermore,
another study found reduced concentrations of phosphorus and potassium in the leaf
tissues of PRD irrigated longan trees (Srikasetsarakul et al. 2011), as compared to
previous reports on the nutrient content of healthy trees (Khaosumain et al. 2005).
These findings on reduced biomass formation may indicate similar long-term
effects in longan as has been reported for other tree species, such as the lower
crown volume found in almonds (Egea et al. 2010) and the reduced root-biomass
growth shown in peach trees (Abrisqueta et al. 2008). However other studies carried
out under the same climatic conditions as those found in Thailand, which has an
intensive rainy season, did not reveal any negative impacts on yields in the long-run
(Spreer et al. 2009). In light of this, there is a need for more research to be done on
the longan tree’s response to drought stress.
6.3.3 Water Stress Monitoring
Monitoring water stress during fruit tree production is a rather complex task, as –
other than in most annual crops – it is not the absolute yield or total biomass but
rather quality that determines the success of the producer. Depending on the fruit
species involved, the marketable yield is an effect of inner and outer quality
parameters. Thus, deficit irrigation is carried out, not only to save water, but also
to enhance desired properties such as acidity in wine production or obtaining a
better fruit to flesh ratio, as is the case with mango (Spreer et al. 2007). In longan,
however, the absence of water stress during the fruit growing period is fundamental
for ensuring a marketable yield as otherwise fruit crack occurs as a result of osmotic
6 Mango and Longan Production in Northern Thailand: The Role of Water Saving. . .
223