Effects of light manipulation through different photo selected net colours and LEDs (Lactuca Sativa L.) and cabbage as estimated by chlorophyl fluoressence parameters, macro and micro element content and physical measurements.

Thesis (MA)--Stellenbosch University, 2018.

ENGLISH ABSTRACT: Light quantity and quality plays a fundamental role in seed germination, plant growth and development. Plants grown in high radiation regions can experience light stress, resulting in photoinhibition when the photosynthetic capacity of the light harvesting complexes in the photosystems are over-exposed. Shade nets are a cost-effective way to reduce high light quantities and to create a more desirable growing environment. Black shade nets are used widely for vegetable seedling and mature crop production in South Africa, due to its cost effectiveness and availability. Although, recent studies have indicated that coloured shade nets can increase plant responses and physiological processes like seed germination, plant architecture, circadian rhythms as well as plant growth and development. Chlorophyll fluorescence is a non-destructive method used to determine the amount and type of plant stress when a plant is exposed to sub-optimal growing conditions. Unfortunately, no literature could be found to indicate specific chlorophyll fluorescence parameter values, for lettuce and cabbage seedlings and mature crops, grown under different coloured shade nets in high solar radiation environments, and low radiation LEDs. The objective of this study was two-fold. Firstly, to determine the chlorophyll fluorescence parameters for lettuce and cabbage seedlings and mature plants produced under different coloured shade nets with high solar radiation, and the leaf macro- and micro-element composition of lettuce and cabbage seedlings and mature plants, and physical analyses of mature cabbage plants. Secondly, to determine chlorophyll fluorescence parameters, and leaf macro- and micro-element composition of lettuce seedlings, under different colour combination low radiation LEDs. The research was conducted by means of three replicates of five different coloured shade nets – with the black net as the control. ‘Grand Slam’ and ‘Islandia’ lettuce, and ‘Conquistador’ and ‘Sapphire’ cabbage cultivars were grown under each high solar radiation coloured net. The second trial entailed where ‘Robinson’, ‘Grand Slam’, ‘Lolla Rossa’ and ‘Multi Red’ lettuce seedlings were grown under high radiation under the same coloured shade nets and were compared to different LED combinations of low radiation. An analysis of variance (ANOVA) was used for data analysis, and Fisher’s least significant differences were used to determine the mean data comparisons. Dark pigmented lettuce had higher RC/ABS values than green lettuce, and these values differed statistically per cultivar. The RC/ABS values did not differ between the B+DR and B+FR LEDs, although they were significantly lower than under the coloured shade nets. The PItotal values decreased after head formation and this indicates decreasing PN values. All chlorophyll fluorescence parameters were greatly influenced by plant age. The OJIP transient curve is indicative of P levels in lettuce. The largest P uptake differences between lettuce cultivars were under Photon Red and white nets - while the blue nets produced the least variance for P uptake in different lettuce cultivars. All macro- and micro-element uptake for lettuce seedlings was significantly higher under the low radiation B+DR LEDs than different coloured shade nets under high solar radiation. The B+DR LEDs vastly increased the uptake of Cu, followed by Na, Zn, Ca Cabbages grown under white nets averaged 8.16 kg, and were 86% heavier than cabbages produced under black nets. Cabbage leaf length and width values were significantly higher under white nets. Also, it produced the lowest N, P and K leaf levels under the Photon Red and white nets, while cabbages under the black nets had the shortest and narrowest leaves - but the highest N, P and K values. The blue nets once again produced the smallest variance regarding the uptake ratio of N, P and K for the different cabbage cultivars.

AFRIKAANSE OPSOMMING: Lig kwaliteit en kwantiteit speel ‘n deurslaggewende rol in saad ontkieming, plantgroei en ontwikkeling. Dit kan ook fotoinhibisie meebring wanneer die fotosintetiese kapasiteit van die lig absorberende komplekse in die fotosisteme oorbelig word, indien lig kwantiteit nie verminder word nie. Plantgroei reaksies en fisiologiese prosesse van ‘n plant word beïnvloed deur lig-kwantiteit en kwaliteit te manipuleer, om sodoende ‘n gunstiger groei omgewing te skep. Chlorofil fluoresensie is ‘n nie-vernietigende metode wat gebruik word om die hoeveelheid en tipe stres te bepaal waaraan ‘n plant blootgestel word in sub-optimale groei omstandighede. Skadunette word in die algemeen gebruik om plant stres te verminder, deur die beskikbare sonlig te manipuleer na aanvaarbare vlakke. In Suid-Afrika word swart skadunette hoofsaaklik gebruik vir groentesaailing sowel as volwasse groente produksie, as gevolg van die koste-effektiwiteit en bekostigbaarheid daarvan. Verskeie gewasse wat onder verskillende gekleurde nette gegroei was, het veel beter presteer as onder swart nette. Geen literatuur kon bekom word om die Chlorofil fluoresensie parameter waardes aan te dui vir slaai en kool saailinge en volwasse gewasse wat onder gekleurde nette in hoë son radiasie gebiede gegroei word nie. Die doel van die studie is twee-voudig. Eerstens, om die Chlorofil fluoresensie parameters vir slaai en kool saailinge en volwasse plante wat onder verskillende gekleurde nette onder hoë son radiasie geproduseer is, te bepaal. Dit sluit ook in die vastelling van blaar makro- en mikro- element komposisie van slaai- en koolsaailinge wat geproduseer is onder verskillende kleur kombinasies van lae radiasie LEDs. Die navorsing was gedoen deur drie replikasies van vyf verskillende gekleurde nette – met die swart net as die kontrole. “Grand Slam” en ‘Islandia’ slaai en ‘Conquistador’ en ‘Sapphire’ kool kultivars was gegroei onder hoë son radiasie onder gekleurde nette, terwyl ‘Robinson’, ‘Grand Slam’, ‘Lolla Rossa’ en ‘’Multi Red’ slaai saailinge onder verskillende kleur kombinasies LEDs gegroei is. ‘n Analise van die variasie (ANOVA) was gebruik vir data analise, en Fisher’s minste beduidende verskille was gebruik om die gemiddelde data te vergelyk. Donker gepigmenteerde slaai het hoër RC/ABS waardes as groen slaai getoon, en hierdie waardes het statisties verskil per kultivar. Die RC/ABS waardes het nie van mekaar verskil onder B+DR en B+FR LEDs nie, en was noemenswaardig laer as onder al die gekleurde nette. Die PItotal waardes het afgeneem na kop formasie en dit dui op die afname van PN waardes. Alle Chlorofil fluoresensie parameters was grootliks beinvloed deur plant ouderdom. Die OJIP oorgangskurwe is aanduidend vir die P vlakke in slaai. Die grootste P opname verskille tussen die verskillende slaai kultivars, was onder foton rooi en wit nette – terwyl blou nette die minste variasie in P opname tussen die verskillende kultivars getoon het. Alle makro- en mikro-element opname vir slaai saailinge was noemenswaardig hoër onder die lae radiasie B+DR LEDs in vergelyking met die verskillende gekleurde nette onder hoë son radiasie. Die B+DR LEDs het die opname van Cu noemenswaardig laat toeneem, asook in ‘n mindere mate die opname van Na, Zn en Ca. Kool het ‘n gemiddelde gewig van 8.16 kg onder die wit nette geweeg, en was 86% swaarder as die kool wat onder die swart nette geproduseer is. Die blaar lengte en breedte waardes van volwasse kool was noemenswaardig hoër onder die wit nette, maar het die laagste N, P en K vlakke gehad, terwyl die kool onder swart nette die kortste en smalste blare met die hoogste N, P en K waardes gehad het. Die blou nette het die kleinste variasie in verband met die N, P en K opname verhoudings getoon vir die verskillende kool kultivars.