Posts Tagged ‘heat stress’

Sweet Corn Pollination Problems

Friday, July 27th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Growers are experiencing quality problems in sweet corn this year related to poor pollination as a result of high heat. This problem is more severe in less stress tolerant varieties and where irrigation is inadequate.

In corn silk elongation begins 7 to 10 days prior to silk emergence from the husk. Every potential kernel (ovule) on an ear develops its own silk that must be pollinated in order for the ovary to be fertilized and develop into a kernel. The silks from near the base of the ear emerge first and those from the tip appear last. Under good conditions, all silks for an ear will emerge and be ready for pollination within a span of 3 to 5 days and this usually provides adequate time for all silks to be pollinated before pollen shed ceases.

Pollen grains are borne in anthers, each of which contains a large number of pollen grains. The anthers open and the pollen grains pour out after dew has dried off the tassels. Pollen is light and can be carried considerable distances (up to 600 feet) by the wind. However, most of it settles within 20 to 50 feet. Pollen shed is not a continuous process. It stops when the tassel is too wet or too dry and begins again when temperature conditions are favorable.

Under favorable conditions, a pollen grain upon landing on a receptive silk will develop a pollen tube containing the male genetic material, develop and grow inside the silk, and fertilize the female ovary within 24 hours. The amount of pollen is rarely a cause of poor kernel set. Each tassel contains from 2 to 5 million pollen grains, which translates to 2,000 to 5,000 pollen grains produced for each silk of the ear shoot.

Poor seed set is often associated with poor timing of pollen shed with silk emergence (silks emerging after pollen shed). Shortages of pollen are usually only a problem under conditions of extreme heat and drought. Extreme heat and desiccating winds can affect pollen germination on silks or pollen tube development leading to poor seed set. Insects that clip silks during pollination can cause similar problems.

Heat Effects on Vegetable and Fruit Crops

Friday, July 27th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

With excessive heat being an issue again in 2012, I wanted to reprint an article that I wrote in 2011 about heat effects in vegetable and fruit crops.

The plant temperature at which tissue dies is around 115°F. Normally, plant temperature is just above air temperature. However, plant temperature can rise to a critical level under certain conditions. Plants have 3 major ways in which they dissipate excess heat: 1) long-wave radiation, 2) heat convection into the air and 3) transpiration.

A critical factor is transpiration. If transpiration is interrupted by stomatal closure due to water stress, inadequate water uptake, injury, vascular system plugging or other factors, a major cooling mechanism is lost. Without transpiration, the only way that plants can lose heat is by heat radiation back into the air or wind cooling. Under high temperatures, radiated heat builds up in the atmosphere around leaves, limiting further heat dissipation.

Dry soil conditions start a process that can also lead to excess heating in plants. In dry soils, roots produce Abscisic Acid (ABA). This is transported to leaves and signals to stomate guard cells to close. As stomates close, transpiration is reduced. Without water available for transpiration, plants cannot dissipate much of the heat in their tissues. This will cause internal leaf temperatures to rise.

Vegetables can dissipate a large amount of heat if they are functioning normally. However, in extreme temperatures (high 90s or 100s) there is a large increase the water vapor pressure deficient (dryness of the air). Rapid water loss from the plant in these conditions causes leaf stomates to close, again limiting cooling, and spiking leaf temperatures, potentially to critical levels causing damage or tissue death.

Very hot, dry winds are a major factor in heat buildup in plants. Such conditions cause rapid water loss because leaves will be losing water more quickly than roots can take up water, leading to heat injury. Therefore, heat damage is most prevalent in hot, sunny, windy days from 11 a.m. to 4 p.m. when transpiration has been reduced. As the plants close stomates to reduce water loss, leaf temperatures will rise even more. In addition, wind can decrease leaf boundary layer resistance to water movement and cause quick dehydration. Wind can also carry large amounts of advected heat.

Photosynthesis rapidly decreases above 94°F, so high temperatures will limit yields in many vegetables and fruits. While daytime temperatures can cause major heat related problems in plants, high night temperatures have great effects on vegetables, especially fruiting vegetables. The warmer the night temperature, the faster respiration processes. This limits the amount of sugars and other storage products that can go into fruits and developing seeds.

Heat injury in plants includes scalding and scorching of leaves and stems, sunburn on fruits and stems, leaf drop, rapid leaf death, and reduction in growth. Wilting is the major sign of water loss which can lead to heat damage. Plants often will drop leaves or in severe cases will “dry in place” where death is so rapid, abscission layers have not had time to form.

On black plastic mulch, surface temperatures can exceed 150°F. This heat can be radiated and reflected onto vegetables causing tremendous heat loading. This is particularly a problem in young plants that have limited shading of the plastic. This can cause heat lesions just above the plastic. Heat lesions are usually first seen on the south or south-west side of stems.

High heat and associated water uptake issues will cause heat stress problems. As heat stress becomes more severe a series of event occurs in plants starting with a decrease in photosynthesis and increase in respiration. As stress increases, photosynthesis shuts down because closure of stomates stops CO2 capture and increases photo-respiration. This will cause growth inhibition. There will be a major slow-down in transpiration (cooling process loss and internal temperature increase). As stress becomes more severe there will be membrane integrity loss, cell membrane leakage and protein breakdown. Toxins generated through cell membrane releases will cause damage to cellular processes. Finally, if stress is severe enough there can be plant starvation through rapid use of food reserves, inefficient food use, and inability to call on reserves when and where needed.

The major method to reduce heat stress is by meeting evapotranspirational demand with irrigation. Use of overhead watering, sprinkling, and misting can reduce of tissue temperature and lessen water vapor pressure deficit. Mulches can also help greatly. You can increase reflection and dissipation of radiative heat using reflective mulches or use low density, organic mulches such as straw to reduce surface radiation and conserve moisture. In very hot areas of the world, shade cloth is used for partial shading to reduce advected heat and total incoming radiation.

 

Heat Affects Early Lima Beans

Friday, July 27th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

We are seeing heat effects on early planted lima beans again in 2012, with heavy bud, blossom, and pod drop. A split set is also present because of changes from high heat where pod set was limited to more moderate temperatures favoring pod set back to high heat again causing pod loss. Another problem is misshapen pods and irregular, dimpled, or misshapen seed which may be caused by incomplete pollination due to the heat or direct piercing/sucking insect damage to the seed.

Tomato Pollination and Excessive Heat

Thursday, July 12th, 2012

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

The extreme heat we had will play havoc on tomato fruit that was just flowering or ripening, causing problems in fruit development due to poor pollination. Constant exposure of a tomato plant to high temperatures (day/night temperatures of 95/80°F) significantly reduces the number of pollen grains produced and released per flower and decreases the pollen’s viability. Most pollen is shed between 10:00 a.m. and 4:00 p.m. and 3-hours or more at 103oF on two consecutive days can cause fruit set failure. Temperatures at night may play a more important role in determining whether or not pollination takes place than day time temperatures. This is because ideal fruit set occurs within a very narrow range of night temperatures (60°-70° F). If tomato plants experience night temperatures above 75°F, interference with the growth of pollen tubes can occur preventing normal fertilization and causing blossom drop (Fig. 1). Prolonged high humidity (>80%) also will hinder good fruit set as the pollen either will not shed freely or the pollen grains may bind together, resulting in poor pollination. Poor pollination may result in under-size fruit that looks ‘normal’ but is just a great deal smaller. Other problems include poor development of the gel inside the fruit. This causes the fruit to appear angular and soft when squeezed (Fig. 2). When this type of fruit is cut in half, open cavities can be seen between the seed gel and the outer wall (Fig. 2). High temperatures during the ripening period additionally can cause ‘internal whitening’ in tomato fruit (Fig. 2). This white tissue only is noticeable when the fruit is cut. The hard, white areas tend to be in the vascular tissues in the outer and center walls of the fruit. Low potassium levels are also associated with ‘internal whitening’. There is not a great deal that can be done about any of the environmental problems other than to be sure to water enough and do not over fertilize during these extreme conditions. Although growth regulating chemicals can be used sometimes to help fruit set under cooler than ideal conditions there is no growth regulator that will induce normal fruit development under high temperature conditions.

 

Figure 1. Blossom drop (arrows) in tomato due to high night temperatures

Figure 2. Angular sides of fruit due to poor pollination. When cut open you can see the lack of gel resulting in pockets inside the fruit as well as ‘internal whitening’–spots in the outer wall.

Leaf Scald in Sweet Corn Again in 2012

Thursday, July 12th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Several sweet corn varieties in our fresh market bicolor variety trial are showing leaf scald symptoms in 2012. We saw similar leaf scald last year in processing varieties. Leaf scald is a physiological disorder similar to necrotic sunburn in fruits and vegetables. It occurs when leaf temperatures rise above a critical level, cells die rapidly, leaving a bleached white appearance. While newly emerged leaves in the upper canopy of susceptible varieties that are the most exposed are the most likely to scald, some of the leaf scald we are seeing this year has progressed deeper into the canopy, even showing up on some of the corn husks. Leaf scald occurs most commonly when temperatures are in the high 90s or over 100, skies are clear (high solar radiation), and humidity is low. While effect on yield is usually minimal, leaf scorch at the ear leaf level can affect kernel fill.

Leaf scald symptoms on fresh market sweet corn in a 2012 trial of bicolor varieties.

Pollination Disorders in Cucurbits

Thursday, July 12th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Watermelon harvest is underway on Delmarva; cantaloupe harvest started early this year, squash and cucumbers have been producing for over a month; and pumpkins and winter squash are setting fruit in earlier plantings. Each year, we see pollination problems with vine crop fruits, especially when weather conditions are unfavorable.

Signs of incomplete pollination in cucurbits include bottlenecked fruit or fruit with a pinched end, crooked or lopsided fruit, fruit small in size or nub-like; and fruits with prominent lobes or that are triangular in shape. Causes of incomplete pollination may be inadequate pollen transfer by pollinating insects; inadequate pollen sources (pollenizers); or hot, dry weather that reduces pollen viability or that desiccates flower parts during pollination. Research has shown that a minimum of 1,000 grains of pollen are required to be distributed over the three lobes of the stigma of the female flower of a watermelon to produce a uniformly shaped fruit.

Hollow cavities in fruit and vacant seed cavities are related to lack of seed formation, again traced back to poor pollination. Fruit tissue separation, such as hollow heart in watermelon, may also be due to inadequate pollination and may be worsened by rapid fluctuation in environmental conditions affecting fruit development.

Early Planted Lima Beans

Thursday, May 24th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Lima bean planting has begun in the region. With the expected warming trend, there is good potential for rapid germination and emergence this year due to higher soil temperatures. It is interesting to note that the variety Cypress was bred for good emergence under cooler planting conditions in Canada. We planted a trial using Cypress the first week in May this year and had excellent germination and emergence. We will harvest this trial the last week in July and then allow it to regrow and harvest a second time from the regrowth in October. Unfortunately, Cypress is very susceptible to pod drop due to heat. May planted lima beans, both at our research station and on growers farms in 2011 had very poor yields in the summer due to severe pod drop, even though some fields were well irrigated.

This illustrates the problem with May and early June planted lima beans: they most often have a lower yield potential than late June and early July plantings because they flower and set pods during summer conditions when day and night temperatures are high. Day temperatures greater than 90°F cause stomates to close early during the day to limit water loss, reducing lima bean photosynthesis. This results in fewer pods being carried by the plant. Night temperatures in the 70s or higher will also adversely affect yields because higher levels of carbohydrates are consumed in night respiration, limiting the plants ability to set and retain pods. Plants will reflower when cooler conditions recur, but this may lead to split sets.

Unfortunately, until more heat tolerant varieties are available (at the University of Delaware, one of our lima bean breeding objectives is to select for greater heat tolerance) , growers are limited in what they can do to maintain yields in early lima bean plantings. Fields closer to water bodies were temperatures are moderated by fog, heavy dew, high humidity, and cooling breezes during summer are the best candidates for early plantings. In addition, irrigate early planted fields, paying particular attention to the flowering and early pod set period and do not plant early lima beans dryland. Daytime irrigation can also help to moderate high temperature effects during hot summer periods. It is critical to keep early planted lima bean plants from being water stressed during this period.

Pod and Seed Disorders in Lima Beans

Friday, July 22nd, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

This is the time of year that we start to see pod disorders in early planted lima beans. This includes partially filled, misshapen, and yellowing pods and irregular, dimpled, or misshapen seed. These disorders are most commonly related to problems with seed development in the pod, either due to incomplete pollination or direct piercing/sucking insect damage to the seed.

Due to the high heat, we are also seeing high amounts of pod drop. In one early planted field that I looked at last week there was fair pod set. This week, virtually all pods had dropped off of the plants. These plants are now reflowering and have the potential to produce a later set. This phenomenon also occurred last year where the consistent high heat did not allow for pod set until August, delaying harvest but still allowing for a good yield in earlier planted lima beans. What is not desired is a split set. The occurs when there is enough heat or drought stress to abort some flowers or pods, but not all of them , and the plant then reflowers in less stressful weather. This causes both mature and immature pods on the plant at the same time, making harvest decisions difficult.

Leaf Scald in Sweet Corn and Other Crops

Friday, June 17th, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

We have seen a few processing sweet corn varieties in our variety trials that are showing leaf scald symptoms in the upper canopy (see photos below). Most of the varieties in the trial had little or no leaf scald.

This type of leaf scald is a physiological disorder and is not caused by a disease pathogen. Newly emerged leaves in the upper canopy of susceptible varieties that are the most exposed will be the most likely to scald.

Leaf scald occurs most commonly when temperatures are in the high 90s. At these air temperatures, crop leaf temperatures may rise to a critical level where plant cells are damaged and they desiccate quickly, leaving the scalded appearance. Upper leaves are the most exposed to radiation from the sun and therefore the most susceptible. Drying winds and low humidity will make scald more severe. Any interruption in transpiration during this period will increase leaf temperature even more and make scald more severe.

Three rows of a variety showing scald flanked by unaffected varieties.

Variety on the right affected by scald, variety on the left unaffected.

Close up of scalded leaves.

Scald is a very rapid tissue death. Leaf scorching, especially on margins, is more common and related to water stress, soil moisture deficits, or root system problems where inadequate water cannot be taken up. This is often most prevalent on larger leaves. Leaf tissues that are furthest from the veins (the margins and areas between the veins) are the first ones affected by the lack of water, leading to this scorch.

Leaf scald and scorch can also be cause by disease organisms in some vegetables. Most commonly we see this with bacterial and fungal disease organisms that affect vascular systems of plants. Verticillium wilt would be a good example. In Nebraska, Colorado, and some Midwestern states, Goss’s wilt in corn causes upper canopy leaf scorching. This is a bacterial disease that is not present in the East. However, Stewart’s wilt is common in our area in susceptible sweet corn varieties and will cause scorch-like symptoms. Use of resistant varieties is the best control for vascular diseases that cause scorch.

 

How High Heat Affects Vegetables and Other Crop Plants

Friday, June 17th, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

The plant temperature at which tissue dies is around 115°F. Normally, plant temperature is just above air temperature. However, plant temperature can rise to a critical level under certain conditions.

Plants have 3 major ways in which they dissipate excess heat: 1) long-wave radiation, 2) heat convection into the air and 3) transpiration.

A critical factor is transpiration. If transpiration is interrupted by stomatal closure due to water stress, inadequate water uptake, injury, vascular system plugging or other factors, a major cooling mechanism is lost. Without transpiration, the only way that plants can lose heat is by heat radiation back into the air or wind cooling. Under high temperatures, radiated heat builds up in the atmosphere around leaves, limiting further heat dissipation.

Dry soil conditions start a process that can also lead to excess heating in plants. In dry soils, roots produce Abscisic Acid (ABA). This is transported to leaves and signals to stomate guard cells to close. As stomates close, transpiration is reduced. Without water available for transpiration, plants cannot dissipate much of the heat in their tissues. This will cause internal leaf temperatures to rise.

Vegetables can dissipate a large amount of heat if they are functioning normally. However, in extreme temperatures (high 90s or 100s) there is a large increase the water vapor pressure deficient (dryness of the air). Rapid water loss from the plant in these conditions causes leaf stomates to close, again limiting cooling, and spiking leaf temperatures, potentially to critical levels causing damage or tissue death

Very hot, dry winds are a major factor in heat buildup in plants. This causes rapid water loss because leaves will be losing water more quickly than roots can take up water leading to heat injury. Therefore, heat damage is most prevalent in hot, sunny, windy days from 11 am to 4 pm when transpiration has been reduced. As the plants close stomates to reduce water loss, leaf temperatures will rise even more. In addition, wind can decrease leaf boundary layer resistance to water movement and cause quick dehydration. Wind can also carry large amounts of advected heat.

Photosynthesis rapidly decreases above 94°F so high temperatures will limit yields in many vegetables. While daytime temperatures can cause major heat related problems in plants, high night temperatures have great effects on vegetables, especially fruiting vegetables. The warmer the night temperature, the faster respiration processes. This limits the amount of sugars and other storage products that can go into fruits and developing seeds.

Heat injury in plants includes scalding and scorching of leaves and stems, sunburn on fruits and stems, leaf drop, rapid leaf death, and reduction in growth. Wilting is the major sign of water loss which can lead to heat damage. Plants often will drop leaves or in severe cases will “dry in place” where death is so rapid, abscission layers have not had time to form.

On black plastic mulch, surface temperatures can exceed 150°F. This heat can be radiated and reflected onto vegetables causing tremendous heat loading. This is particularly a problem in young plants that have limited shading of the plastic. This can cause heat lesions just above the plastic. Heat lesions are usually first seen on the south or south-west side of stems.

High heat and associated water uptake issues will cause heat stress problems. As heat stress becomes more severe this series of event occurs in plants:

1. decrease in photosynthesis

2. increased respiration

3. closing down of photosynthesis -closed stomates stops CO2 capture and increases photo-respiration

4. major slow-down in transpiration (cooling process loss and internal temperature increase)

5. cell membrane leakage (signals changes in protein synthesis)

6. continued physical water loss

7. growth inhibition

8. plant starvation through rapid use of food reserves, inefficient food use, and inability to call on reserves when and where needed

9. toxins generated through cell membrane releases and respiration problems

10. membrane integrity loss and protein breakdown

The major method to reduce heat stress is by overhead watering, sprinkling, and misting for improved water supply, reduction of tissue temperature, and lessening of the water vapor pressure deficit. Mulches can also help greatly. You can increase reflection and dissipation of radiative heat using reflective mulches or use low density, organic mulches such as straw to reduce surface radiation and conserve moisture.

In very hot areas of the world, shade cloth is used for partial shading to reduce advected heat and total incoming radiation. We will be demonstrating the use of shade cloth for summer production of day-neutral strawberries.

Much of this information was adapted from an article Heat Stress Syndrome by Kim D. Coder, Professor, Silvics/Ecology, Warnell School of Forest Resources, the University of Georgia