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