2.1 Plant material and growth conditions
Three-week-old seedlings of tomato (Solanum lycopersicumL.) cultivar ‘Micro-Tom’ were planted in pots containing 150 g of soil composing of peat, vermiculite and perlite (4:2:1) and grown in a growth chamber with 24/20C (day and night) temperatures, constant 60% relative humidity, 16 hours photoperiod and light intensity of 150 μmol m-2 s-1. Plants were watered regularly until reproductive development. About binucleate stage of pollen development of the first flower buds, plants were exposed to two watering regimes: Well-watered (WW), soil moisture 65 ± 5%, and drought stress (DS), soil moisture 6 ± 3% by withholding watering in the DS plants. Soil moisture was monitored using HH2 Soil Moisture Meter (Delta-T Devices, Cambridge, UK) until it decreased to 5%. The soil moisture was then maintained at 6 ± 3% for 4 days, thereafter normal watering resumed. Meanwhile, the WW plants were watered normally. Pollen development stages were estimated by measurement of bud length and staining with 4, 6-diamidino-2-phenylindole (Regan and Moffatt, 1990).