Use of polytunnel material with different UV transmission rates to manipulate lettuce growth and quality

Figure 1. Lettuce plots covered with cloches lined with material with different UV transmission rates.

Figure 1. Lettuce plots covered with cloches lined with material with different UV transmission rates.

Use of polytunnel material with different UV transmission rates to manipulate lettuce growth and quality

by Michael Gordon

Ultraviolet (UV) light in sunlight occupies a narrow range of the electromagnetic spectrum, corresponding to wavelengths slightly shorter (200-400 nm) than those of the visible spectrum (400-700 nm) which humans can see.

         

UV light represents a stress to both humans (sunburn) and plants, with prolonged exposure increasing the frequency of mutations. Standard glass blocks most of the UV in sunlight, while polytunnel covering material can be obtained with different degrees of UV transmission.       

In this study, four lettuce cultivars were grown under cloches covered with material with different UV transmittances, and yield and quality were measured in each.

Figure 2. Effect of 0% (Sterilite SuperThermic, top) and 100% (SunMaster SuperThermic, bottom) UV transmission on anthocyanin content of lettuce cv. Lollo Rossa

Figure 2. Effect of 0% (Sterilite SuperThermic, top) and 100% (SunMaster SuperThermic, bottom) UV transmission on anthocyanin content of lettuce cv. Lollo Rossa

 

Seven days after transfer of ‘Lollo Rossa’ seedlings from under the UV blocking material Sterilite SuperThermic to under the UV-transparent material, SunMaster SuperThermic, a clear increase in reddening of the foliage was evident (Figure 2). This was associated with an increase in anthocyanin content (Table 1).

Table 1. Effect of UV exposure on anthocyanin content (µg/ g dry weight) in three lettuce cultivars

         Cultivar

    UV-transparent

   UV-blocking

      ‘Lollo Rossa’

             593 d

         142 b

      ‘Green Oak Leaf’

               11 a

             8 a

      ‘Red Oak Leaf’

             289 c

           87 b

               Any two medians with a common letter were not significantly different (P>0.05)

 

Not only were the red-leaved plants more deeply coloured when grown under UV-transparent film, but they were also more efficient at conserving water (Table 2) and exhibited less feeding damage by the slug Deroceras reticulatum (data not shown)

Table 2. Effect of UV exposure on % water loss under restricted water supply in three lettuce cultivars

         Cultivar

    UV-transparent

   UV-blocking

      ‘Lollo Rossa’

             14.7 a 

            30.2 ab

      ‘Green Oak Leaf’

              52.6 c

            55.8 c

      ‘Red Oak Leaf’

              22.0 a

            41,2 bc

               Any two medians with a common letter were not significantly different (P>0.05)

 

When the UV treatments were extended to four cultivars in  field trials, the stimulatory effects of UV on foliage coloration was confirmed, as well as effects on yield (Figure 3).

Figure 3. Effects on lettuce head size and colour of A. UV-blocking and B. UV-transparent film 

Figure 3. Effects on lettuce head size and colour of A. UV-blocking and B. UV-transparent film

The UV-transparent film resulted in more intense foliage colour but smaller heads (fewer and smaller leaves) in the red cultivars (Figure 3); the smaller heads were due to the stress associated with UV exposure. The effect of UV on yield was surprisingly much smaller in the green cultivar (Figure 3) than in the red ones.

  These preliminary studies indicate that manipulation of UV exposure by the use of appropriate polytunnel films can have a marked effect on commercial parameters such as yield, quality and pest resistance. The rapid response of anthocyanin content to UV (Figure 2) suggests that red-leaved cultivars could be grown under UV-blocking film until marketable size (to benefit from the larger heads) which could then be removed for 7 days to achieve UV exposure and hence leaf colouration.

Centre for Organic Horticulture Research

School of BEES, Butler Building, Distillery Field, North Mall, Cork

Top