Lettuce: understand what tipburn is, why it occurs and how to minimize it

Tipburn, also known as tip burn or edge burn, is a physiological disorder that affects lettuce crops. It occasionally causes serious losses in production areas. This problem usually appears when there is a nutritional imbalance in the environment; or when solar radiation levels (and, consequently, air temperature) are high, which normally occurs in crops that take place during the summer and spring months. Or when environmental conditions limit the plant's transpiration.

Such disorder affects the visual quality and useful life of the product, making it unviable to sell due to consumer rejection, resulting in economic losses for the producer.

What is tipburn

O Tipburn It is a physiological disorder caused by calcium deficiency. It can occur due to the absence of the mineral in the environment. Or in situations where this element is present and available, but is not adequately distributed in the plant tissues.

Calcium is an essential macronutrient for maintaining the cell wall and membrane structure. It acts as a cementing element that aims to ensure the integrity of these structures. Plants can make cell walls more rigid or plastic; and membranes more or less permeable depending on the concentration of calcium. Therefore, when its supply is insufficient (due to deficiency) or inadequate (due to poor internal distribution), there is weakening and rupture of the cell wall and membranes, resulting in the leakage of cellular contents and, consequently, tissue necrosis. This necrosis is called Tipburn.

In lettuce (lactuca sativa), such necrosis of the edges is especially striking due to the presence of lactiferous vessels filled with latex, which is a plant product with a milky appearance, secreted in the leaves and stems of plants of the genus lactuca. Latex is responsible for the bitterness of the leaves. The higher its concentration (lower water content in the tissues), the more unpleasant the taste of the leaf will be.

Thus, when the cell ruptures and its contents leak due to the absence of calcium, latex is released into the surrounding tissue, causing the parenchyma to collapse and the xylem elements to become occluded. The affected area quickly loses turgor and scattered mesophyll cells become necrotic.

Calcium is transported from the root to the leaves via water transport in the xylem, through the transpiration flow. When stored at its destination to fulfill its structural function, it is rarely remobilized, and is therefore considered a non-mobile element or one with limited mobility in the plant. In other words, we can say that calcium can only be supplied by transpiration and not through storage or allocation, which means that its deficiency occurs primarily at the points of growth (meristematic zones).

Since the larger, older lettuce leaves (located on the outside of the rosette) transpire more, they tend to accumulate more calcium than the smaller, younger leaves (located on the inside of the rosette). Therefore, the symptoms of Tipburn They usually appear on the leaves and occupy the most internal positions of the plant.

why does it occur

As mentioned, there are three most widespread situations to which the occurrence of Tipburn: calcium deficiency, high levels of solar radiation and air temperature, and environmental conditions that limit plant transpiration.

The first situation, which is easy to understand, is due to the pure absence of the element calcium in the production system. If it is not present, this nutritional deficiency will result in problems in the formation of the cell wall, which will result in its rupture and, consequently, in the formation of necrosis in the tissues. This situation is the least frequent and also the easiest to solve.

The second and third situations presuppose the presence and availability of calcium in the production environment. However, as they deal with specific and momentary microclimatic characteristics of the crop, they are unpredictable and somewhat more complex to understand and diagnose.

In the second situation, where the levels of solar radiation (and, consequently, the air temperature) in the environment are high, the rapid growth of the plant induced by these conditions may not be accompanied by the absorption capacity of the root system and the flow of water and calcium through the conducting vessels. Thus, there will be a delay between the demand at the growth points (edges of young leaves) and the ability of the root to capture and distribute it through the vascular system. This mismatch generated by abiotic stress, which is likely to occur in spring and summer crops, will result in insufficient deposition of calcium on the edges of young leaves and, consequently, Tipburn. Several studies carried out on this topic have shown that an increase in light intensity results in a greater occurrence of this disorder in lettuce (Gaudreau et al., (1994); Wissemeier and Zühlke (2002); Sago (2016); Knoop (2019)). For Bárcena et al. (2019), high light intensity increases the growth rate of plants, which may affect the occurrence of Tipburn.

The third situation is the opposite of the second one presented. In this case, instead of an accelerated demand (and consequently a flow between soil-plant-atmosphere that is not met), there are circumstances where the flow is completely interrupted or reduced to negligible levels. As already seen, the transport and ascent of calcium in the plant occurs as a function of transpiration. Therefore, when lettuce plants are subjected to conditions that interrupt (or reduce to very small rates) transpiration through the closure of the stomata, the following will occur: Tipburn.

According to Saure (1998), there is a positive correlation between high relative humidity (RH) and the occurrence of Tipburn. In 1976, Tibbitts and Bottenberg found that growth rate increased dramatically for lettuce grown under 85% RH, compared to lettuce grown under 50% RH. This may suggest that increasing RH affects the Tipburn through its effects on plant growth rate. Knoob (2019) observed in a study that in treatments where lettuce was subjected to a combination of high light intensity and high RH, and the occurrence of Tipburn was more severe. When RH is high, the vapor saturation pressure deficit is low, therefore reducing the transpiration rate.

This situation can occur when, for example, on days with high relative humidity, there is no vapor saturation deficit between the stoma and the environment, which results in stomatal closure. With the flow interrupted, calcium does not reach the meristematic points where it is required and thus, we can also observe the occurrence of Tipburn.

How to minimize the problem

Although lettuce can be grown year-round, it is considered a temperate vegetable and tends to adapt better to mild temperatures, with an average of between 15 and 24ºC. Therefore, the biggest challenges related to the occurrence of Tipburn will occur in crops grown during the hottest periods of the year and with high light intensity (spring and summer months).

For these periods, it is recommended to use cultivars that are tolerant to edge blight both in field cultivation and in protected cultivation.

There are no recommended management procedures that guarantee the prevention of Tipburn. However, for field crops, where environmental control is limited, the problem may be reduced by harvesting before the plant is fully mature, so that symptoms do not develop sufficiently to affect the marketability of the lettuce.

For crops grown in protected environments, where microclimate management is possible, procedures such as: maintaining adequate humidity near the root system; maintaining a vapor pressure deficit in the greenhouse atmosphere provided by the movement of air over the plant canopy; and controlling radiation levels with the use of shade screens so as not to allow plants to grow at excessively accelerated rates, seem to minimize the occurrence of TipBurn.

It is important to note that in crops grown in warmer periods (average temperatures above 20ºC), early bolting may also occur, another physiological disorder that makes lettuce unviable for sale. In this case, cultivars that are also tolerant to bolting should be chosen. The so-called early bolting, in addition to inducing the plant to flower early, stimulates the production of latex, a substance that makes the leaves bitter.

By Natalia Teixeira Schwab, Federal University of Santa Maria

Article published in issue 138 of Revista Cultivar Hortaliças e Frutas