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Introduction
At the flowering stage tomato(Solanum lycopersicum L.) is very prone to both high and low temperatures. Studies conducted to find the relationship between daily temperature and reproductive stage of tomato have found that day temperatures of 21-28°C and night temperatures of 15-20 °C are optimal for proper fruit setting and size development in tomato(Rahman et al., 2015).
Any increase or decrease from optimum temperature results in premature drop of tomato fruit(Pramanik et al., 2017). Night temperature also plays an important role in the setting and development of the fruit, as night temperature below 10 °C significantly affects the fruit set in tomato(Karapanos et al., 2008). Adam et al.(2001) while evaluating the impact of temperature on growth and development of tomato fruit, reported that temperature below 14°C greatly reduces the tomato growth and causes the development of small and parthenocarpic tomatoes. Low temperature mainly reduces the pollen viability, pollen release and also delays germination of pollen on stigma, pollen tube development in style that leads to poor pollination and delays the fertilization process which are the prerequisite for proper fruit setting and development in tomato(Fernandez-munoz et al., 1995; Pressman et al., 2007).
Plant growth regulators are extensively being practiced in tomato for improving crop yield in temperature stress conditions. The impact of synthetic auxins and gibberellins to boost the production of tomatoes by setting parthenocarpic fruits has been acknowledged by many researchers(Tiwari et al., 2014; Pramanik et al., 2017). 4-Chlorophenoxy acetic acid is a synthetic auxin whose application has been reported to increase fruit set and size of tomato in temperature stress conditions(Picken and Grimmett, 1986; Ramin, 2003).
Many studies tested the growth hormones applications to improve tomato crop yield during high-temperature stress. There are few studies that check its effectiveness in low-temperature stress. However, investigations regarding 4-CPA test for low night temperature conditions are limited. In the present study effects of different 4-CPA levels were assessed in response to two minimum temperature conditions. In the first condition, different 4-CPA levels were tested during October to November when minimum night temperature was ranging between(12.5°C-23°C) and secondly the same levels of 4-CPA were tested in December-January when the minimum temperature at night was ranging between(2°C-7.5°C). The ultimate goal of the study was to find the best...