This in turn may cause crops to grow rank following the stress. Extremely hot temperatures at flowering may also cause ‘parrot beaking’ and during early boll growth may lead to boll death (sometimes called ‘cavitation’ or boll freeze). These symptoms can also contribute to loss of demand by fruit for assimilates. Fibre quality can also be affected by heat stress reducing fibre length and increasing micronaire.
The consequences of hot conditions for yield and quality are exaggerated if water stress also occurs during these periods.
Information compiled by Dr Mike Bange and Dr Rose Brodrick for CottonInfo (2018).
Managing for heat stress:
Heat stress causing fruit loss (through shedding, boll freeze, or parrot beaking) can cause the crop to grow rank following heat stress. Management during and after periods of high temperatures involves closely monitoring vegetative growth rate (VGR), fruit retention and boll size. If excessive vegetative growth is detected the use of mepiquat choride (PIX®) is recommended. Using a growth regulator to limit vegetative growth rather than limiting water is preferred as this may increase fruit shedding. Where irrigation is available, decreasing the interval between irrigations maybe necessary to avoid water stress.
Physiology of heat stress:
At high temperatures many biological processes no longer have the same responses to changes in temperature as they do at moderate temperatures. When air temperature and sunlight heat up cotton plants they attempt to maintain optimum growing temperatures by opening stomates in the leaves, allowing water to pass out and evaporate and cool leaves.
If leaf temperatures rise above 32ºC during the day, this slows the function of plant enzymes for photosynthesis and growth. Night temperature is also important for optimum cotton growth. Warm nights (above 26ºC minimum) mean that leaf temperature remains high, and respiration remains high, consuming stored assimilates that would have been used for additional growth.
Excessively high temperatures (greater than 35ºC maximum) significantly decrease available assimilates because plant can no longer maintain its temperature in the optimal zone for fruit growth and in turn cause increase in square and boll shedding, and reduce seed number per boll which can all lead to decrease in yield. Loss of fruit may cause the crop to grow excessively vegetative (rank) following the period of heat stress.
In addition to reductions in assimilates available for growth, heat stress can also directly damage cotton plant tissue. At night the plant loses the ability to cool evaporatively (no transpiration) thus tissue temperatures approach air temperature. During the day very high relative humidity (which restricts evaporative cooling) in combination with clear skies can also increase tissue temperature to approach or exceed air temperature. Two known consequences of direct tissue damage from severe heat stress are:
-
Parrot Beaked Bolls – Small bolls with uneven seed numbers between the locks caused by poor pollination /seed set particularly in one lock. High temperatures reduce the viability of the pollen at flowering. This reduces boll size and can reduce yield. There are no known studies to show if the plant compensates for parrot beak bolls by having other normal bolls grow bigger.
-
Boll Freeze or Cavitation or Boll Dangle – Occurs when young bolls die before the abscission layer forms. The fruit hangs on the plant by a dangling piece of tissue. Again this loss of fruit may cause the crop to grow excessively vegetative (rank) following these symptoms.
Increases in temperature also reduce the interval between flowering and boll opening, shortening the time to maturity and reducing yield. This may increase final micronaire by limiting the number late set bolls that can have lower micronaire. Fibre length can also affected by sustained periods of high temperatures as the time required for fibre elongation is reduced, not allowing for genetic potential for fibre length to be reached.
Figure: Summary of consequences of the effect of daily temperature on rate of crop growth and development
Summary of heat stress symptoms:
Symptoms of well water watered cotton that has encountered heat stress may include one or more of the following:
-
‘Parrot beaking’ caused by high temperatures causing reducing pollen viability at flowering.
-
‘Cavitation or Boll dangle’ (development of a young boll is ‘frozen’ and the immature small (up to 15mm) boll, bracts and stalk dies and dries on the plant) resulting in fruiting loss.
-
Increased rates of respiration at night reducing available photoassimilates causing low seed count per boll and increased shedding of squares and young bolls.
-
Excessive vegetative growth following the heat stress resulting from the increased square and boll shedding.
-
Shortening of the period between flowering and boll opening reducing the time to maturity reducing yield.
-
Higher final micronaire associated with reduction in overall boll number and/or a shorter boll setting period limiting the number late set bolls that can have lower micronaire.
-
Reduced fibre length.
-
Reduced final yield and/or fibre quality.