Winai Wiriya-Alongkorn

Alternative Techniques for Water-Saving Irrigation and Optimised Fertigation in Fruit Production in Northern Thailand

The acreage of irrigated orchards in northern Thailand has dramatically increased since the discovery of flower induction by potassium chlorate in 1998 (see Chapter 3.1). As the availability of water is limited, alternatives for the more efficient use of water and fertilisers in fruit production are urgently needed. As longan and mango represent an important share of fruit production in the area under study, they have been the main focus of research on water-saving irrigation techniques.

The Plant-Physiological Basis of Flower Induction in the Control of Fruit Production

In the last four years, research has focused on off-season flower induction of longan, lychee and mango trees (Chapter 3.3). In order to achieve control over the flower induction process of fruit trees, it is necessary to address the key factors responsible for the transition from vegetative to generative bud development. Various, partly competing theories have been developed in the past about the physiological ‘Who’s Who’ in flower induction (Bernier et al., 1993). One theory favours the role of carbohydrates, which need to be present in sufficient amounts as a prerequisite for flower induction (Sachs, 1977). Other theories of flower induction focus either on the genetic control of a developmental switch from vegetative to generative development (Levy and Dean, 1998), control by particular hormones (Bernier et al., 2002), the existence of specific promoting or inhibiting factors or a mixture of both. However these theories do not apply to adult perennial fruit trees (Goldschmidt and Samach, 2004). Knowledge and understanding of the hormonal changes associated with the treatments previously described (Chapter 3.3) can be beneficial for future trials to induce flowering in mango, lychee and other fruit trees.

Strategies for Flower Induction to Improve Orchard Productivity: From Compensation of Alternate Bearing to Off-Season Fruit Production

Due to alternate and irregular bearing of fruit trees, which occurs at various extent amongst different species and cultivars, the yield of many species of fruit tree is erratic. Uncertainties regarding the time of harvest and the quality and quantity of fruit can seriously affect the marketability of the product (Monselise and Goldschmidt, 1982; Westwood, 1995; Subhadrabandhu, 1999; Souza et al., 2004). Unfavourable climatic conditions during flower induction (FI) or the flowering period are amongst the most important causes of this phenomenon. Often large areas or even whole countries face the same problem simultaneously leading to overproduction and low prices in one year and a low return from fruit production the next. Equalising these fluctuations therefore would help to make fruit production more profitable and sustainable. Another option for raising the return from fruit production would be to extend or totally shift the harvest season by artificially influencing conventional and off-season flowering.

Influence of Water Supply on CO2 Concentration in the Rootzone of Split-Root Potted Longan Trees

Longan trees are irrigated in Thailand as fruit growth takes place during the dry season. Due to the scarcity of water resources, ways for water saving irrigation are investigated. As deficit irrigation was found to have a high water saving potential, the focus was on the investigation of plant stress responses to drought, which can be used for optimizing the deficit irrigation regime. Five split-root potted longan trees in sand culture were subjected to partial rootzone drying PRD, and during six months the CO2 concentration in the rootzone was measured by rootzone probes and compared to a well-watered control. The CO2 efflux from the rootzone was found to be well correlated to the moisture regime in the substrate. However, it was necessary to correct the measured values by the values from the control to obtain a significant correlation between CO2 concentration and soil moisture. The main observed external factor influencing CO2 was the ambient temperature. It was shown that the CO2 efflux from the soil can be used as a non-destructive method for drought stress monitoring, but continuous measurement will be necessary to externalize disturbing environmental effects.

Mango and Longan Production in Northern Thailand: The Role of Water Saving Irrigation and Water Stress Monitoring

With higher fruit exports from Thailand, the importance of irrigated fruit production has increased over recent years and, as a consequence, water resources have become more and more scarce. At the same time, farmers in northern Thailand are being increasingly confronted with weather anomalies. Scheduling based on plant stress avoidance or the exploitation of stress signals can optimize water use efficiency and the profitability of irrigation under changing climatic conditions. Traditional irrigation through the use of water hoses is a common management practice in northern Thailand; however, this method is inefficient, labor intensive and relatively inflexible. The introduction of micro-irrigation systems is one important component in the development of water saving irrigation practices, with cost-benefit analyses showing that investment in a micro-sprinkler system can be beneficial. As with improved irrigation, the marketable fruit yield can be increased substantially during drought years; thus, it is worthwhile changing from traditional to modern, water efficient and flexible irrigation systems. The emphasis of irrigation research is placed on increasing water use efficiency (WUE) and deficit irrigation strategies, in particular, partial root-zone drying (PRD) were found to increase WUE substantially. Under PRD only one side of a tree row is watered, while the other is left to dry-out to a predetermined level before next being irrigated. Stress responses in general decrease water consumption and vegetative growth while yield decline is usually minor. In on-station experiments, PRD treated mango and longan trees maintained yields when compared to well-watered trees. Sophisticated deficit irrigation strategies can only be applied when robust stress monitoring methods are available, and thermal imaging is one such new, non-invasive method. In the research study outlined here, the use of this method was investigated under field conditions, with our findings showing that it has the potential to be used within future irrigation scheduling systems.