Posts Tagged ‘abiotic stress’

Sunburn in Fruits and Fruiting Vegetables

Friday, July 6th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

High temperatures, clear skies and high light radiation, and long daylengths are a recipe for developing sunburn in fruits and fruiting vegetables. We commonly see sunburn in watermelons, tomatoes, peppers, eggplants, cucumbers, apples, strawberries, and brambles (raspberries and blackberries).

There are three types of sunburn which may have effects on the fruit. The first, sunburn necrosis, is where skin, peel, or fruit tissue dies on the sun exposed side of the fruit. Cell membrane integrity is lost in this type of sunburn and cells start leaking their contents. The critical fruit tissue temperature for sunburn necrosis varies with type of fruit. For cucumbers research has shown that the fruit skin temperature threshold for sunburn necrosis is 100 to 104°F; for peppers, the threshold is 105 to 108°F, and for apples the critical fruit skin temperature is 125-127 °F. Fruits with sunburn necrosis are not marketable.

The second type of sunburn injury is sunburn browning. This sunburn does not cause tissue death but does cause loss of pigmentation resulting in a yellow, bronze, or brown spot on the sun exposed side of the fruit. Cells remain alive, cell membranes retain their integrity, cells do not leak, but pigments such as chlorophyll, carotenes, and xanthophylls are denatured or destroyed. This type of sunburn browning occurs at a temperature about 5°F lower than sunburn necrosis (115 to 120° F in apples). Light is required for sunburn browning. Fruits may be marketable but will be a lower grade.

The third type of sunburn is photooxidative sunburn. This is where shaded fruit are suddenly exposed to sunlight as might occur with late pruning, after storms where leaf cover is suddenly lost, or when vines are turned in drive rows. In this type of sunburn, the fruits will become photobleached by the excess light because the fruit is not acclimatized to high light levels, and fruit tissue will die. This bleaching will occur at much lower fruit temperatures than the other types of sunburn.

Genetics also play a role in sunburn and some varieties are more susceptible to sunburn. Varieties with darker colored fruit, those with more open canopies, and those with more open fruit clusters have higher risk of sunburn. Some varieties have other genetic properties that predispose them to sunburn, for example, some blackberries are more susceptible to fruit damage from UV light.

Control of sunburn in fruits starts with developing good leaf cover in the canopy to shade the fruit. Fruits most susceptible to sunburn will be those that are most exposed, especially those that are not shaded in the afternoon. Anything that reduces canopy cover will increase sunburn, such as foliar diseases, wilting due to inadequate irrigation, and excessive or late pruning. Physiological leaf roll, common in some solanaceous crops such as tomato, can also increase sunburn.

In crops with large percentages of exposed fruits at risk of sunburn, fruits can be protected by artificial shading using shade cloth (10-30% shade). However, this is not practical for large acreages. For sunburn protection at a field scale, use of film spray-on materials can reduce or eliminate sunburn. Many of these materials are Kaolin clay based and leave a white particle film on the fruit (such as Surround, Screen Duo, and many others). There are also film products that protect fruits from sunburn but do not leave a white residue, such as Raynox. Apply these materials at the manufacturer’s rates for sunburn protection. They may have to be reapplied after heavy rains or multiple overhead irrigation events.

Early Planted Lima Beans

Thursday, May 24th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

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.

Observations After the Hurricane

Friday, September 2nd, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Hurricane Irene caused less damage than expected in the region. However, there were still significant impacts on crops in Delaware. The most obvious is lodging in field corn. On the vegetable side, lodging in sweet corn varied considerably from field to field, with many escaping damage.

High winds had the potential to batter many vegetables. The largest acreage currently in the field is lima beans and looking at many fields, they weathered the storm well with minimal pod drop and foliage damage. Cucurbit fields (watermelons, cucumbers, squash, pumpkins) were much more variable with significant foliage damage in many fields. Tomatoes, peppers, and eggplants also suffered considerable damage.

Peach and apple growers had considerable fruit drop and bruising of fruits due to wind and branch contact that will reduce marketable volumes.

Rainfall totals ranged from 6 to 14 inches depending on the site but flooding was much more limited than expected. However, wet fields have led to disease issues and quality problems in snap beans, tomatoes, pickles, and other crops with increases diseases such as white mold and Phytophthora. In pickles, excess soil makes washing much more difficult and has increased fruit rots. Excess water has caused severe cracking in tomatoes and cantaloupes with much of the late summer crop ruined.

The storm has affected later watermelons to a great degree. In 2010, because of the dry year, farmers were able to keep vines healthy and continue cropping past Labor Day in many fields. Later planted fields yielded well. This year, because of the extra heat stress in July and early August followed by Irene and the current cold night temperatures, vine health has declined greatly in many fields and growth has slowed, limiting late yields. Excess water has increased water-soaking in some varieties. Later plantings that were wind damaged by Irene have open canopies causing bleaching in some fields. Volumes are down, quality has suffered, and some buyers have left the region.

While it could have been worse, Irene has cost vegetable and fruit growers significant economic losses in the region.

Flooding and Vegetables

Friday, August 26th, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist;

There is still considerable acreage of watermelons, sweet corn, pumpkins, beans, cabbage, potatoes, and other fresh market vegetable crops in the field on Delmarva. On the processing side, the majority of lima beans have yet to be harvested and there are significant acres of pickles, snap beans, and other processing crops in the field. Many of these crops will be at risk in the coming days due to hurricane Irene.

A late summer hurricane or tropical storm with both wind damage and excess rain can cause major issues in vegetable crops, most notably:

● Damage due to flooded soils in all vegetable crops

● Increased disease incidence in all vegetable crops

● Lodging damage in crops like sweet corn

Other articles will address diseases in with excess rainfall. I will focus on flooding effects on the physiology of vegetable plants.

Flooded and Waterlogged Soils
In flooded soils, the oxygen concentration drops to near zero within 24 hours because water replaces most of air in the soil pore space. Oxygen diffuses much more slowly in water filled pores than in open pores. Roots need oxygen to respire and have normal cell activity. When any remaining oxygen is used up by the roots in flooded or waterlogged soils, they will cease to function normally. Therefore, mineral nutrient uptake and water uptake are reduced or stopped in flooded conditions (plants will often wilt in flooded conditions because roots have shut down). There is also a buildup of ethylene in flooded soils, the plant hormone that in excess amounts can cause leaf drop and premature senescence.

In general, if flooding or waterlogging lasts for less than 48 hours, most vegetable crops can recover. Longer periods will lead to high amounts of root death and lower chances of recovery.

While there has not been much research on flooding effects on vegetables, the following are some physiological effects that have been documented:

● Oxygen starvation in root crops such as potatoes will lead to cell death in tubers and storage roots. This will appear as dark or discolored areas in the tubers or roots. In carrots and other crops where the tap root is harvested, the tap root will often die leading to the formation of unmarketable fibrous roots.

● Lack of root function and movement of water and calcium in the plant will lead to calcium related disorders in plants; most notably you will have a higher incidence of blossom end rot in tomatoes, peppers, watermelons, and several other susceptible crops.

● Leaching and denitrification losses of nitrogen and limited nitrogen uptake in flooded soils will lead to nitrogen deficiencies across most vegetable crops.

● In bean crops, flooding or waterlogging has shown to decrease flower production and increase flower and young fruit abscission or abortion.

● Ethylene buildup in saturated soil conditions can cause leaf drop, flower drop, fruit drop, or early plant decline in many vegetable crops.

Recovering from Flooding or Waterlogging
The most important thing that you can do to aid in vegetable crop recovery after floods or waterlogging is to open up the soil by cultivating (in crops that still small enough to be cultivated) as soon as you can get back into the field. This allows for oxygen to enter the soil more rapidly. Nutritionally, sidedress with 50 lbs of N where possible.

In fields that are still wet, consider foliar applications of nutrients. According to Steve Rieners at Cornell “Use a low salt liquid fertilizer to supply 4 to 5 lb nitrogen, 1 lb phosphate (P2O5) and 1 lb potash (K2O) per acre. Since nitrogen is the key nutrient to supply, spraying with urea ammonium nitrate (28 % N solution) alone can be helpful. These can be sprayed by aerial or ground application. Use 5 to 20 gallons of water per acre. The higher gallons per acre generally provide better coverage”. As with all foliar applications, keep total salt concentrations to less than 3% solutions to avoid foliage burn.

Sunscald Very Prevalent in Peppers This Year

Thursday, July 21st, 2011

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

I know this seems very obvious to most growers, as we have sunscald every year in our vegetable plantings. This year just seems to be especially bad as I have gotten several calls from growers about a strange problem in their peppers that looks like sunscald, but couldn’t be. The reason given that it could not be sunscald is because the plants have thick foliage and the fruit seem well covered. Sunscald occurs when peppers or other vegetables are exposed to the direct rays of the sun during hot weather; the damaged areas may become papery and bleached or tan colored, and these areas often are covered with a black fungal growth (Photo 1). It is more apparent on plants that have sparse foliage or that have lost a large proportion of leaves to a leaf-defoliating disease. But almost all the farms I visited had plants that looked very good (Photo 2). The problem is that pepper plants often lean to one side or the other because of winds blowing them in certain directions. When this occurs sunscald can be especially prevalent on previously shaded pepper fruit that are suddenly exposed to the sun, even for a short period time. Photo 3 shows one of these leaning pepper plants and several fruit that were damaged by this sudden exposure resulting in sunscald. The damaged areas are vulnerable to infection by fungi (Black mold), and bacteria, so that at times a pepper fruit will be found that is a soupy, smelly watery mess. Sunscald is most prevalent on green fruit. Staking and tying pepper plants will greatly decrease the leaning plants and greatly decrease sunscald. The pepper plants do not have to be tied often, usually once is all it takes and stakes do not need to be any taller than the pepper plants (so broken tomato stakes work well) (Photo 4). Peppers in a tied vs. non tied section of field had vastly different sunscald problems. The stake and tied section had less than 2% of fruit sunscald damaged; the non-tied section had 17% sunscald damaged fruit for the same variety planted the same day.

Photo 1. Pepper fruit with small and large areas of sunscald

Photo 2. Pepper plant that appears to have good foliage and fruit cover but still has sunscald fruit

Photo 3. Leaned-over plant exposing covered peppers, resulting in several sunscald fruit

Photo 4. Staked and tied pepper plants, 4-5 plants between stakes, one string

Stress in Vegetables

Friday, July 15th, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist;

In troubleshooting vegetables in the summer months, we see fields where the major symptom is an overall lack of vigor and this poor vigor is due to one or more stress factors. Hot weather makes this stress more pronounced. Pests such as root and crown rot fungi, bacterial and fungal wilt organisms, and insects such as squash bugs can damage plant roots, stems, and vascular systems, limiting water uptake, and causing excess stress. However, there are many stresses that don’t involve diseases or insects. The following are some other causes of excess stress in vegetables this time of year.

Soil Compaction
Plants will have limited rooting in compacted areas and therefore cannot take up adequate water or mineral nutrients. In addition, compacted soils have reduced air exchange. Plants will often be stunted and will wilt early in the day in high temperatures. Cultivation can alleviate surface compaction but will be ineffective on deeper compaction.

High Soil Temperatures
Soils that have limited water holding capacity can have excessively high soil temperatures during long hot days in late spring and early summer. Late planted crops on black plastic mulch are very likely to be exposed to high soil temperatures and surface roots will often be damaged. Overhead irrigation over the black plastic mulch is very beneficial to reduce heat loads until plants have sufficient canopies to shade over the mulch.

Drip Tape and Drip Irrigation “Diseases”
Issues with drip irrigation can often be the cause of plant stress due to inadequate water. This includes plugged emitters; leaks due to insect or animal chewing that limit water flow further down the tape; leaky connections reducing flow; tape twisting and binding, again limiting flow past the point of the bind,; improper tape selection or improper irrigation timing leading to under application of water; limited well capacity also leading to under application of water; too wide of emitter spacing for the crop or soil; too wide of bed for a single tape (with double rows) and others drip irrigation problems. Over application of water in drip irrigation also can be an issue, especially in lower field areas and where soil types change in the field. This can lead to saturated beds limiting oxygen for roots. The keys to avoiding drip irrigation associated problems is to monitor fields closely, note any areas that look stressed, and investigate whether or not the drip irrigation is functioning properly. Soil moisture monitoring devices can aid greatly in detecting problems.

Inadequate Overhead Irrigation
Under-watering can lead to additional plant stress. Plugged nozzles are a major problem that often goes uncorrected. Excessive runoff due to compacted soils can lead to reduced water intake.

Excessive Fertilization
Salt induced stress conditions can occur when excess fertilizer, manure, or high salt compost is applied or when high salt index fertilizer is applied too close to vegetable plants.

Air Pollution in Vegetables

Friday, June 24th, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist;

We are starting to see evidence of air pollution damage in sensitive vegetable plants. Those vegetables most susceptible include potatoes, watermelons, cantaloupes, snap beans, pumpkins, and squash.

Damage is most common during hot, humid, hazy weather with little wind. Air inversions, when warm air at the surface is trapped by even hotter air in the atmosphere above, lead to build up of air pollutants that cannot disperse and, consequently, plant injury. The most common form of air pollution injury to plants is ozone damage. Ozone is a strong oxidant and is formed by the action of sunlight on products of fuel combustion. It is moved from areas of high concentration (cities, heavy traffic areas) to nearby fields.

Ozone injury in susceptible vegetable varieties develops when ozone levels are over 80 ppb for four or five consecutive hours, or 70 ppb for a day or two when vegetable foliage at a susceptible stage of growth. Because it occurs in areas with high levels of automobile exhausts, crop injury is often visible on fields in close proximity to roads, especially with heavy summer weekend traffic. High pollution indexes in Baltimore and Washington are also a good indication that ozone damage may occur.

In potatoes, symptoms of ozone damage occur on the most recently emerged leaves and can be seen as a black flecking. Early red varieties are most susceptible.

Injury on watermelon leaves consists of premature chlorosis (yellowing) on older leaves. Leaves subsequently develop brown or black spots with white patches. Watermelons are generally more susceptible than other cucurbits to ozone damage. Damage is more prevalent when fruits are maturing or when plants are under stress. Injury is seen on crown leaves first and then progresses outward. Seedless watermelon varieties tend to be more resistant to air pollution injury than seeded varieties, so injury often shows up on the pollenizer plants first. “Ice box” types are the most susceptible.

Ozone injury on watermelon

In muskmelons and other melons, the upper surface of leaves goes directly from yellow to a bleached white appearance.

Ozone injury on squash and pumpkins is intermediate between watermelon and cantaloupe starting with yellowing of older interior or crown leaves. These leaves subsequently turn a bleached white color with veins often remaining green.

In snap and lima beans, ozone causes small bleached spots giving a bronze appearance on upper leaf surfaces and pods. Leaves may ultimately turn chlorotic and senesce (drop).

Ozone injury can be easily misdiagnosed as mite injury, pesticide phytotoxicity, or deficiencies.

The key to avoiding air pollution injury is to plant varieties that are of low susceptibility and to limit plant stresses. Certain fungicides such as thiophanate methyl (Topsin and others) offer some protection against ozone damage.


Leaf Scald in Sweet Corn and Other Crops

Friday, June 17th, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist;

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;

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


More Heat

Friday, June 10th, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist;

A second early heat wave has growers concerned about effects on vegetables. As stated last week, peas will have reduced yields and will mature more quickly. Moisture deficit portions of pea fields in field corners, sandy knolls, compacted, and low organic matter areas have been hard hit.

Providing adequate moisture through irrigation is critical in these high heat periods. However, water cannot completely compensate for extreme heat.

All vegetables will have reduced photosynthesis once temperatures reach a critical point. Plant stomates will close earlier in the day thus limiting gas exchange. Respiration increases with temperature, and high night temperatures can be a major factor in limiting yield. Because of this increased respiration the plant uses up photosynthates that do not go into yield components.

High air temperatures may result in high leaf temperatures, especially where water is deficient. High leaf temperature may result in heat damage to proteins. Very high leaf temperatures may result in sunburn and scorching. Sunscald of fruits will increase, especially where leaves wilt and reduce fruit cover.

In flowering and fruiting crops, high heat will affect pollen production, often reducing viable pollen numbers. Reproductive parts in plants (anthers, stigmas) may not form properly or function properly. If pollen is transferred to stigmas, pollen germination may be reduced or halted due to heat and desiccation. Reduced pollination can result in smaller fruit or misshapen fruit. Reduced pollination will also reduce seed set in pod crops and sweet corn.

If pollination is successful, early fruit abortion may occur due to lack of photosynthates or heat damage. In heat stressed plants, the hormone balance is affected and there is an increase in abscisic acid that is involved in these abortions.

High soil temperatures can damage surface roots, limiting water and nutrient uptake. This is particularly an issue in crops grown on black plastic mulch. High temperatures affect root crops such as potatoes, especially near the soil surface, by damaging tubers and roots.