Morphological responses and growth of tomato seedlings grown under different doses of red, and red and blue light in an artificial solar background light.

MSc-thesis abstract (submitted 20 August 2015): The spectral distribution of light has a fundamental regulating effect on the physiology of plants. The introduction of light emitting diodes (LED) in greenhouse horticulture enables accurate manipulation over the spectrum of light and therefore enables the exploitation of plant responses to light quality. As a result, information regarding plant responses to a LED light enriched daylight spectrum becomes essential.

A climate chamber experiment was conducted in order to evaluate plant responses to increasing fractions of red LED light (R), and red and blue LED light (RB) in an artificial solar background light (AS). Specifically, morphological responses and growth of early developing tomato seedlings were investigated (30 days from sowing). For every increase in the fraction of LED light, an equal decrease in the fraction of AS was applied such that a PPFD of 100 μmols m-2 s-1 was acquired for all treatments, including the control (100% AS). Red LED light treatments (R-treatments) were consisted of 30% and 50% fraction of red LED light while RB treatments were consisted of 10%, 30% and 50% fraction of red LED light and an amount of blue LED light equal to 45% of the red LED light dose found in RB treatments. The amount of blue LED light was such that all RB treatments had equal amount of blue light (400-500 nm) to the control treatment (31 μmols m-2 s-1). 

An increasingly compact plant morphology was obtained with increasing fractions of red, and red and blue LED light. Specifically, plants became shorter, as a result of shorter internodes and hypocotyl while leaf length to width ratio and petiole elevation reduced. The efficiency of light in inhibiting stem elongation was highest for blue LED light as the plant reduction per unit of μmols m-2 s-1 of light was more than twice as much for blue LED compared to red LED. Furthermore, as the amount of blue light (400-500 nm) was equal between RB treatments it was also concluded that the efficiency of light in inhibiting stem elongation is higher for blue LED compared to broadband blue light provided by the AS (400-500 nm). Plant height prediction by the PSS value of light was inconsistent for R and RB treatments, confirming the strong involvement of blue light specific photoreceptors in this response. Furthermore, plant height was found to be a highly determinant factor for the allocation of biomass to the different organs. Specifically, taller plants were found to have a larger fraction of their total dry weight in the stem rather than to the leaves; both linearly proportional to plant height. In addition, the proportionality to stem length was highest for stem dry mass indicating that despite the elongation, this response is highly depended on the supply of carbohydrates to the stem. Finally, this dependence was not affected by the spectral distribution of light as it was consistent for all light treatments.

Total leaf area was found to increase linearly to the fraction of red LED only in R treatments, suggesting an opposing positive and negative effect of red and blue LED light respectively however, differences between light treatments were found to be insignificant. A qualitative effect of LED light was observed on chlorophyll content per unit leaf area which exhibited a strong increase in the treatment with the minimum fraction of LED light (10RB = 14.5%). Chlorophyll content was found to be linearly proportional to the fraction of LED light in RB treatments indicating the lack of saturation of the response, however overall differences between LED treatments were insignificant indicating a rather small quantitative effect. In a similar fashion, the length to width ratio of leaves also exhibited a strong qualitative effect in response to LED light (10RB) but instead, no response was observed to further increase in the fraction of LED light.

Finally the vast majority of growth responses investigated were highly insignificant, stressing out the fact that more repetitions (n=2), a larger plant sample, or longer growing period were required to yield significant observations regarding growth responses. The most important of them were: total dry weight, leaf area, leaf mass area, leaf absorption and the effect of morphology on light interception expressed with the term ‘leaf productivity’ (total dry weight per unit leaf area; g m-2). Yet, the important information drawn from significant results suggest that the investigation of LED light dose responses in an artificial-solar background light could further yield important information regarding plant responses to the quality of light.