Effects of water stress on flowering, fruit drop and fruit size of 10-year-old mangosteen (Garcinia mangostana L.) trees

The effects of imposing different soil moisture levels on flowering, fruit yield and fruit growth of mature mangosteen (Garcinia mangostana L.) trees were studied in drought months at MARDI Station, Bukit Tangga, Kedah. The treatments: T1 (1 emitter/tree), T2 (2 emitters/tree), T3 (3 emitters/tree) and T4 (4 emitters/tree), were imposed at 8 weeks prior to expected flowering. Each emitter was estimated to release water at a rate of about 50 litres an hour. After flowering, similar treatments were continued on a new set of trees for another 6 weeks after which the trial had to be terminated due to heavy rainfall. Leaf water potential ( L) was used to indicate plant water status. T1 was the most severely stressed with L dropping to values below –0.80 MPa from the third week. On the eighth week, the value dropped to –1.17 MPa, remained constant for another week before occurrence of flowering at the 10th week. Within the same period, T4 had excess water supply as indicated by L values fluctuating around –0.65 MPa. However, the L in T2 and T3 were moderately stressed with values ranging from –0.65 to –0.80 MPa on most days. Flowering intensity was in descending order: T2>T3>T4>T1. This indicated that lack of water or too much of it failed to trigger profuse flowering. It was the moderately stressed water treatments (T2 and T3) that produced the highest number of flowers. From the 10th week onwards, similar treatments were imposed on a new set of trees bearing approximately equal number of flowers. The L of T1 fluctuated between –1.10 and –1.47 MPa for 8 weeks from the commencement of flowering until sudden termination of the trial due to heavy rainfall. In contrast, the L values within the same period for the other treatments were – 0.78 to –0.99 MPa (T2), –0.74 to –0.84 MPa (T3) and –0.63 to –0.78 MPa (T4). These values indicated a similar trend of L before and after flowering. Plants under severe stressed (T1) had a very high percentage of fruit drop of up to 77% at 6 weeks after flowering while plant receiving excess water (T4) had only 50% fruit drop. The moderately stressed trees (T2 and T3) had 62–71% fruit drop. Water stress also significantly reduced initial fruit size and at harvest. Good management of irrigation water that resulted in trees exposed to moderately stressed conditions prior to and during flowering is important in ensuring higher fruit production of mangosteen. Introduction Under the new Third Agriculture Policy of Malaysia, special attention is given to mangosteen (Garcinia mangostana L.) identifying it as a potential fruit type to be developed progressively and aggressively for export purpose. However, commercial production of this crop has been hampered by many limiting factors. One of them is the lack of water resulting from serious drought. Although the total annual rainfall in Malaysia is more than 2 500 mm, its uneven distribution has caused serious drought that affects growth and production of many agricultural crops (Nieuwolt 1982). Insufficient water has been a major factor limiting growth and production of fruit trees (Hanan 1972). Water stress affects a wide range of physiological processes such as stomatal behaviour, photosynthesis,