Rice

Morphology

Morphological And Physiological Aspect

• A layer of thin walled cells on the outer most surface forms the root epidermis.
• Root hairs arise from certain epidermal cells.
• Internal to epidermis is layers of parenchymatic cells forming the cortex.
• Rice roots contain air-filled pore spaces that allow diffusion of oxygen, to help respiration under submerged, partially allophytic situations.
• Well-grown roots of rice are of two types, the ordinary roots and mat roots.
• Mat roots are shallow, and develop when oxygen tension is low in soil, during the latter stages of the crop. Vertical root distribution at different crop growth stages influences the absorption of water and nutrients.
• The physical size of the root system of a rice genotype, such as IR 36, reaches maximum at 43 days after transplanting.
• The main stem of the rice plant commonly referred as culm, is erect, cylindrical and often hollow, except at nodal regions.
• Nodes are easily identified by the presence of distinct, thickened transverse septum.
• Leaves arise from regions below the node. Buds at the leaf axil provide new culms or tillers.
• The primary culm in a rice plant is not a tiller. Anthocyanin deposits ranging from pink to purple are often seen at the node and internodes.
• Nodal and internodal regions appear green or pale whitish until ripening.
• A ripened culm is ‘straw yellow’ coloured.
• A coleoptile emerges from the seed, followed by a primary leaf, then the secondary leaf with an expanded leaf blade, and other leaves.
• The last leaf (top most) is called the flag leaf. A leaf sheath is always present.
• A pink or purple coloured ligule appears at the top of the sheath.
• The leaf blade is usually long and narrow, but variable in length.
• It is pubescent, hispid or glabrous.
• Glabrous varieties are better suited to mechanical milling because trichomes are avoided.
• The rice inflorescence is a panicle with spikelets.
• Spikelets are oblong or lanceolate, borne on a short pedicel.
• The terminal floret, among the three is fertile and develops as a rice grain.
• Two glumes are small and often obscure.
• The awn is just a tip or may be longer upto 10 cm.
• Flowers open from the tip of the panicle downwards. Pollen germinates between 30 and 35°C.
• Paddy is most often self-pollinated, however cross-pollination events may also occur.
• The grain (caryopsis) is enclosed within lemma and palea.
• The endosperm could be a white, chalky translucent or waxy depending on the genotype.
• The aleurone layer has a single row of parenchyma. Rice grain weight is considered a stable varietal character, with certain exceptions.
• Grains that remain submerged in a solution of specific gravity > 1.20 are termed high-density grains.
• High-density grains occupy nearly 70 to 85 % of a spikelet, in most rice genotypes, regardless of crop duration.
• It has been suggested that selection of greater percentage of high-density grain can lead to higher yield.
• Plant stature obviously depends on the genotype.
• The shorter stature reduces susceptibility to lodging and increases the harvest index.
• At the same time, shorter culms demand less respiratory energy, but stouter culms need greater accumulation of photosynthates.
• Under specific situations rapidly elongating stems are a preferred trait, such as in deep-water varieties.
• Recent rice varieties yield greater biomass, linearly increasing with longer crop duration from 90 to 135 days.
• Maximum yield benefits are realized in tropics, from a crop maturing in 120 days.
• Obviously, shortening duration, beyond a limit can decrease yields.
• However, early genotypes (100 days) have increased grain production, calculated per day.
• While a medium duration rice genotype (110 to 120 days) can produce higher total yields.
• A variation in crop duration obviously affects vegetative growth phase.
• Therefore, a shortened duration in early maturing genotypes provides for less time for spikelet formation, grain filling and maturation process.
• It sometimes lowers yield per plant.
• For practical rice breeding programs, change in duration from panicle initiation till grain maturity is still debatable.
• Rice yield is a function of total dry matter and harvest index, but the grain yield is not always related to higher dry matter production.
• Among C3 plants, net photosynthetic rate is comparatively higher in rice, ranging between 25 to 32 m mol CO2/m2/sec.
• Maximum growth rate ranges from 30 to 36 g/m2/day to 40 g/m2/day in Southeast Asian tropics.
• Greater biomass of rice is possible by mimicking greater solar radiation or as it occurs naturally in longer day length situations.
• Generally, harvest index (HI) reduces with longer vegetative phase.
• High yield genotypes possess between 40% and 55% harvest index depending on the season.
• The present yield barrier is 10 t/ha for rice with 50% HI.
• This would increase to 12 to 13 t/ha assuming a higher HI of 60%, which is possible with other cereals such as wheat.
• Yield of 15 t/ha at 14% grain moisture content is set for an irrigated dry season crop.
• This would require 25 t/ha of total biomass at an HI of 53%.