Abstract
Plant growth regulators (PGRs) applied to cotton plants (Gossypium hirsutum L.) can be washed off by rainfall. It is expected that the closer the rainfall to spraying time, the higher the product loss and the higher the amount of product to be reapplied to reach the desired growth rate. The objective of this study was to evaluate the effects of time between rainfall and application of either mepiquat chloride or chlormequat chloride to cotton on plant growth, as well as, estimate the need for PGR reapplication. Cotton was grown in 12-L pots with soil in a greenhouse. PGRs were applied forty days after seedling emergence, when 50% of plants had one pinhead square. Rainfall was simulated 1, 2, 4, 6, or 24 h after spraying. Plant height was measured just before PGR application and then at 3-d intervals for 30 d. At harvest, the number of reproductive branches and structures were counted before dry matter phytomass determination. Both growth regulators reduced cotton dry matter yields regardless of rainfall interval. PGRs controlled excessive plant growth; however, their efficiency was reduced as the time elapsed until rainfall was shorter. Product losses were detected after all rainfall intervals, which, in field conditions would require PGR reapplication. Mepiquat chloride rates to be reapplied after rain were on average 17% higher than chlormequat chloride rates.
Highlights
Rank growth is often observed in cotton (Gossypium hirsutum L.) plantations on limed and well fertilized soils and favorable weather conditions
In Brazil, mepiquat chloride and chlormequat chloride are recommended for cotton (Carvalho et al, 1994)
There was a significant interaction between the effects of growth regulators and rainfall intervals on the number of reproductive structures but it was not possible to establish a cause-effect relation (Table 1)
Summary
Rank growth is often observed in cotton (Gossypium hirsutum L.) plantations on limed and well fertilized soils and favorable weather conditions. Growth regulators are either natural or synthetic chemical substances that can be applied directly to plants to modify either vital or structural processes by changing hormonal balance, increasing cotton yield and quality and facilitating harvest (Laca-Buendia, 1989). In Brazil, mepiquat chloride and chlormequat chloride are recommended for cotton (Carvalho et al, 1994). They have similar modes of action and effects, which is to inhibit gibberellic acid biosynthesis and reduce cellular elongation, resulting in lower growth rate (Lamas, 2001). The technique increases 100-seed weight and cotton boll size, enhancing early fruit opening, harvest efficiency and production of high quality fibers (Cruz et al, 1982; Reddy et al, 1992)
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