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Irrigation with 5°C water and paclobutrazol promotes strong seedling growth in tomato (Solanum lycopersicon)

Summary Tomato seedlings grown in high-density plug trays are often weak, with excessively elongated stems that are easily damaged, and do not survive well when transplanted into the field. To alleviate this problem, we found that overhead irrigation with 5°C water at 1 – 2 d intervals produced strong seedlings with shorter and stronger stems, increased root and shoot dry weights (DW), and elevated levels of chlorophyll. Irrigation with water at 5°C, 10°C, or 15°C showed that 5°C yielded stems with the greatest strength (575 g mm–1) measured with a texture analyser. No significant difference in seedling height was observed with irrigation scheduled at 08.00 h, 13.00 h, or 16.00 h. Stem and growth indices based on the ratios of stem diameter:plant height, and shoot DW:plant height, respectively, indicated that irrigation with 5°C water increased the stem index to 162% and the growth index to 152%, respectively, compared to control (27°C-watered) seedlings. Seedlings were elongated by gibberellin (GA3) treatment and shortened by treatment with the inhibitor of GA3, paclobutrazol. Paclobutrazol acted synergistically with 5°C water irrigation to reduce stem elongation. Silver thiosulphate (STS), an inhibitor of ethylene action, did not inhibit the stem shortening caused by irrigation with 5°C water when the STS was applied immediately following irrigation. These results suggest the possible involvement of GA3 in stem shortening caused by irrigation with 5°C water. We conclude that irrigation with 5°C water acts as a positive stimulus to promote the growth of seedlings with stronger stems, elevated levels of chlorophyll in their leaves, and higher DWs of all tissues.