Diurnal Leaf Starch Content: An Orphan Trait in Forage Legumes

Abstract

Abstract:Forage legumes have a relatively high biomass yield and crude protein content, but their grazed and harvested biomass lacks the high-energy carbohydrates required to meet the productivity potential of modern livestock breeds. Because of their low carbohydrate content, forage legume diets are typically supplemented with starch rich cereal grains or maize (Zea mays), leading to the disruption of local nutrient cycles. Although plant leaves were first reported to accumulate starch in a diurnal pattern over a century ago, leaf starch content has yet to be exploited as an agronomic trait in forage crops. Forage legumes such as red clover (Trifolium pratense) have the genetic potential to accumulate up to one third of their leaf dry mass as starch, but this starch is typically degraded at night to support nighttime growth and respiration. Even when diurnal accumulation is considered with regard to the time the crop is harvested, only limited gains are realized due to environmental effects and post-harvest losses from respiration. Here we present original data for starch metabolism in red clover and place it in the broader context of other forage legumes such as, white clover (T. repens), and alfalfa (Medicago sativa). We review the application of recent advances in molecular breeding, plant biology, and crop phenotyping, to forage legumes to improve and exploit a potentially valuable trait for sustainable ruminant livestock production.

Starch Biosynthesis in the Developing Endosperms of Grasses and Cereals

“…Not only do these leaf carbon stores play a major role in determining grain yield in cereals, but they also determine forage grass quality. Recent studies in other pasture plants such as white clover (Trifolium repens L.) point to the importance of leaf starch reserves in augmenting forage quality [47]. Details of leaf carbon metabolism of the various cereals and grasses and its relationship to plant growth is beyond the scope of this review, but it is an important consideration when examining the inter-relationships between sources (leaves, stems, and floral structures) and sinks (endosperm) in these different crops, and is covered elsewhere [48][49][50].…”

Genetic Diversity of Diurnal Carbohydrate Accumulation in White Clover (Trifolium repens L.)

“…Glasshouse growth conditions were the same as described previously for red clover [13]. Briefly, to simulate partially sunny growth conditions, direct intense sunlight was shaded to produce uniform light across the experimental growth area.…”

“…Carbohydrate contents were quantified as described previously for red clover [13]. In short, leaves were harvested, flash frozen in liquid nitrogen, lyophilized, weighed, and homogenized into powder using a Mixer Mill MM 400 (Retsch, Haan, Germany).…”

“…Although returning to traditional perennial pastures and locally produced roughage is increasingly being viewed as a way to improve the overall sustainability of ruminant-livestock production, currently 30%, depending on environment, harvesting time, post-harvest treatment, genotypic potential, and precision of quantitation [13]. Environmental factors that influence NSC content in model systems have been intensively studied, but little is known with respect to forage legumes [6,28].…”

“…Starch can make up 25-35% of the DW biomass in some forage legumes such as red clover (Trifolium pretense L.) and alfalfa (Medicago sativa L.). Because starch is a product closely linked to photosynthesis, this accumulation is highly dependent on environmental factors such as light intensity, harvest time, temperature, and day length [13,14]. At night, starch mobilization in white clover (T. repens L.) is reduced under low temperatures and starch accumulates [15].Because of the high degree at which both WSC and starch contents are influenced by the environment, such contents have been elusive for breeders of forage crops to select.…”