Carbohydrate starvation is a major determinant of the loss of greening capacity in cotyledons of dark-grown sugar beet seedlings

Abstract

1994, Carbohydrate starvation is a major determinant of the loss of greening capacity in cotyledons of dark-grown sugar beet seedlings, -Physiol, Plant, 91: 56-64.The transition of cotyledons from heterotrophy to autotrophy is a critical step of seedling establishment. We have studied the greening capacity of sugar beet (Beta vulgaris L. cv, Vega) cotyledons in relation to carbohydrate and energy metabohsm during dark growth. During early growth, sugar beet cotyledons behaved mainly as a lipid-mobilizing and gluconeogenic tissue providing substrates to the seedling. Reserve mobilization was followed by a maximum of the adenine nucleotide pool on day 6 in strict correlation with the maximum of greening capacity. This was immediately followed by the onset of a typical situation of carbohydrate starvation characterized by substrate limitation of respiration, a decrease in the adenine nucleotide pool and, as shown by the respiratory quotient and the loss of proteins, a probable utilization of cellular proteins and lipids to sustain respiration. The conversion from etioplast to chloroplast, as determined by the rate of chlorophyll synthesis, was less and less efficient as carbohydrate starvation continued, finally leading to incomplete and heterogeneous greening on day 12, The relationship of the loss of greening capacity with carbohydrate starvation is discussed.

Modifications of Etioplasts in Cotyledons during Prolonged Dark Growth of Sugar Beet Seedlings (Identification of Etiolation-Related Plastidial Aminopeptidase Activities)

“…After d 6, whereas the amount of carotenoid per cotyledon remained constant, the rate of carotenoid synthesis upon illumination, whether i n the absence or in the presence of exogenous Glc, started to decrease (Fig. 2), in strict accordance with the decrease in the rate of Chl synthesis (E1 Amrani et al, 1994). Thus, the profile of carotenoid accumulation and synthesis confirmed the previously described (E1 Amrani et al, 1994) irreversible decrease of greening and the effects of Glc limitation.…”

“…The amount of carotenoid per cotyledon ( Fig. 2) and the greening capacity, as determined by the rate of production of Chl (E1 Amrani et al, 1994) or by that of carotenoids ( Fig. 2) after illumination, increased during proplastid-etioplast transition.…”

“…Any of these mechanisms could function improperly during prolonged etiolation either through the lack of energy substrate when the conversion is initiated by light or through intrinsic damage. In the case of sugar beet, we showed the detrimental effects of prolonged dark growth, such as the loss of the greening capacity, not to be fully compensated by the exogenous addition of Glc (El Amrani et al, 1994).…”

“…Seeds of sugar beet (Beta vulgaris L., cv Véga) were sown in perlite-vermiculite (50-50%, w/w) and watered with a mineral nutrient solution for neutrophilic plants as previously described (El Amrani et al, 1994). Germination and growth were carried out in the dark at 2OOC.…”

“…Carotertoids were determined as described by Jeffrey et al (1974). Total Chl was extracted and measured as described previously (El Amrani et al, 1994).…”