Genótipos de maconha (Cannabis sativa L.) submetidos a déficit hídrico durante a floração: respostas fotossintéticas e produtivas
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Universidade Federal de Viçosa
Abstract
A Cannabis é uma das plantas mais antigas domesticadas pelo ser humano, que ao longo de sua ampla dispersão geográfica adaptou-se as mais diversas condições edafoclimáticas. Estudos recentes indicam que as condições de cultivo alteram a expressão quimiotípica das plantas, impactando diretamente o produto final. Tendo em vista as mudanças climáticas, a seca se destaca como um dos principais estresses abióticos, limitando o crescimento e a produção de compostos do metabolismo secundário, como os canabinoides. O presente estudo surge diante da escassez de dados acerca dos mecanismos fisiológicos que explicam a variação na resposta de genótipos de Cannabis a seca. Foram avaliados os efeitos do déficit hídrico em dois genótipos quimiotipicamente contrastantes (denominados CBD e THC), submetidos a seca (30% da capacidade de campo) a partir da fase inicial da floração e comparados aos grupos controle continuamente irrigados (100% da capacidade de campo). O potencial hídrico foliar foi similarmente reduzido entre os genótipos sob seca, sem indícios de ajustamento osmótico ou elástico. O déficit hídrico levou a redução na taxa liquida de assimilação do carbono de ambos os genótipos, entretanto as plantas do genótipo THC dependeram principalmente da regulação estomática, ao passo que o genótipo CBD apresentou reduções mais acentuadas, provocadas provavelmente por deficiências adicionais no mesofilo e no metabolismo bioquímico. Houve diminuição da eficiência quântica máxima do fotossistema II associada ao aumento da fluorescência inicial da clorofila a, evidenciando sinais claros de fotoinibição do FSII nas plantas sob seca. Quanto a produtividade sob déficit hídrico, 0 genótipo THC manteve maior índice de colheita e concentração de canabinoides totais, resultando em uma biomassa de inflorescências com maior valor energético. Sob mesma condição, o genótipo CBD apresentou menores retornos produtivos e uma partição de biomassa menos eficiente. De modo geral, a resiliência de Cannabis sativa ao déficit hídrico imposto no inicio da floração parece depender menos da manutenção das relações hídricas e mais da proteção do desempenho fotossintético, de uma parti¢ao eficiente do carbono fixado entre os órgãos da planta e da manutenção do metabolismo secundário. Esses mecanismos representam alvos promissores para o melhoramento de genótipos mais adaptados a ambientes com baixa disponibilidade hídrica. Palavras-Chave: Cannabis sativa, Déficit Hídrico; Fotossíntese; Índice de colheita; Rendimento de canabinoides.
Cannabis is one of the oldest plant species domesticated by humans and, throughout its broad geographic dispersion, it has adapted to highly diverse edaphoclimatic conditions. Recent studies indicate that cultivation conditions can alter the chemotypic expression of the plants, directly impacting the final product. In the context of climate change, drought stands out as one of the major abiotic stresses, limiting growth and the production of secondary metabolism compounds such as cannabinoids. This study emerges from the scarcity of data regarding the physiological mechanisms underlying the variation in drought responses among Cannabis genotypes. We evaluated the effects of water deficit in two chemotypically contrasting genotypes (referred to as CBD and THC), subjected to drought (30% of field capacity) during the flowering stage and compared with continuously irrigated controls (100% of field capacity). Leaf water potential was similarly reduced in both genotypes under drought, with no evidence of osmotic or elastic adjustment. Water deficit reduced the photosynthetic rate of both genotypes; however, THC plants relied primarily on stomatal regulation, whereas the CBD genotype displayed stronger reductions probably triggered by additional limitations in the mesophyll and biochemical metabolism. A decrease in the maximum quantum efficiency of photosystem II, together with an increase in the initial chlorophyll a fluorescence, indicated clear signs of PSII photoinhibition in drought-stressed plants. With respect to productivity under water deficit, the THC genotype maintained a higher harvest index and higher total cannabinoid concentration, resulting in inflorescence biomass with greater energetic content. Under the same conditions, the CBD genotype showed lower productive performance and less efficient biomass partitioning. Overall, the resilience of Cannabis sativa to water deficit imposed at the onset of flowering appears to depend less on the maintenance of plant water relations and more on the protection of photosynthetic performance, efficient partitioning of fixed carbon among plant organs, and the maintenance of secondary metabolism. These mechanisms represent promising targets for breeding genotypes better adapted to environments with low water availability. Keywords: Cannabinoid yield; Cannabis sativa; Drought stress; Harvest Index; Photosynthesis.
Cannabis is one of the oldest plant species domesticated by humans and, throughout its broad geographic dispersion, it has adapted to highly diverse edaphoclimatic conditions. Recent studies indicate that cultivation conditions can alter the chemotypic expression of the plants, directly impacting the final product. In the context of climate change, drought stands out as one of the major abiotic stresses, limiting growth and the production of secondary metabolism compounds such as cannabinoids. This study emerges from the scarcity of data regarding the physiological mechanisms underlying the variation in drought responses among Cannabis genotypes. We evaluated the effects of water deficit in two chemotypically contrasting genotypes (referred to as CBD and THC), subjected to drought (30% of field capacity) during the flowering stage and compared with continuously irrigated controls (100% of field capacity). Leaf water potential was similarly reduced in both genotypes under drought, with no evidence of osmotic or elastic adjustment. Water deficit reduced the photosynthetic rate of both genotypes; however, THC plants relied primarily on stomatal regulation, whereas the CBD genotype displayed stronger reductions probably triggered by additional limitations in the mesophyll and biochemical metabolism. A decrease in the maximum quantum efficiency of photosystem II, together with an increase in the initial chlorophyll a fluorescence, indicated clear signs of PSII photoinhibition in drought-stressed plants. With respect to productivity under water deficit, the THC genotype maintained a higher harvest index and higher total cannabinoid concentration, resulting in inflorescence biomass with greater energetic content. Under the same conditions, the CBD genotype showed lower productive performance and less efficient biomass partitioning. Overall, the resilience of Cannabis sativa to water deficit imposed at the onset of flowering appears to depend less on the maintenance of plant water relations and more on the protection of photosynthetic performance, efficient partitioning of fixed carbon among plant organs, and the maintenance of secondary metabolism. These mechanisms represent promising targets for breeding genotypes better adapted to environments with low water availability. Keywords: Cannabinoid yield; Cannabis sativa; Drought stress; Harvest Index; Photosynthesis.