Characterization of physiological traits in advanced rice (Oryza sativa L.) breeding lines

Introduction

Rice (Oryza sativa L.) is a staple food crop for a large proportion of the global population, particularly in Asia. India occupies a prominent position in rice cultivation with extensive acreage and substantial production; however, national productivity remains lower than the global average. This gap between production potential and realized yield necessitates focused efforts to improve rice productivity through physiological and genetic interventions. Among the rice growing countries, India has the largest area (41.27 mha) and production (109.24 m t) next to China (147 m t). With an average productivity of 2.49 t ha -1 , which shows increase marginal is still well below the world's average yield of 4.36 t ha -1 . At the current population growth rate (1.5%), rice requirement of India by 2025 would be around 125 m t [1] . To safeguard and sustain the food security in India, it is quite important to increase the productivity of rice under limited resources, especially land and water. Hence, the major challenges have been to produce more rice per unit amount of natural resource. Efforts were made in this study to characterize morphologically the-EMS-induced dwarf and early flowering mutants of rice variety Nagina22 and to study their mode of inheritance. Nine true breeding mutants generated earlier by EMS treatment were analysed for differences in their phenotypic characteristics recorded according to the national guidelines for Distinctness, Uniformity and Stability (DUS) [2] . The increasing scarcity of water for agriculture is becoming a major problem in many countries, particularly the leading riceproducing countries like China and India, where competition for freshwater and growing demands for other sectors are increasing in the future [3] . With the increasing pressure on land and water resources, enhancing crop productivity per unit of input has become a major challenge for sustainable rice cultivation. Physiological traits such as leaf area index (LAI), leaf area duration (LAD), and crop growth rate (CGR) play a critical role in determining photosynthetic efficiency and biomass accumulation, thereby influencing final grain yield. These parameters provide valuable insight into the growth dynamics and adaptability of rice genotypes under varying environmental conditions.

Materials and Methods

Growth parameters

Growth parameters were calculated by using the following formulae described [4,5] .

Leaf Area

Leaf area was measured by using LI-3100 Leaf area meter (LICOR-Lincoln, Nebraska, USA). Five hills in the third row of every plot were uprooted and leaves were separated and area was measured. From the leaf area of these five hills LAI was calculated.

Leaf Area Index (LAI)

The LAI was worked out using the formula proposed [6] .

Where, L = Length of 3 rd leaf from the top (cm) W = Maximum width of the same leaf (cm) K = Correction factor (0.75)

Leaf Area Duration (LAD)

The LAD during the crop growth period was calculated.

Where, L1 = Leaf Area at time T1 L2 = Leaf Area at time T2 2 d -1 ) CGR = (W2-W1) / (T2-T1) x (1/P) g m -2 day -1 Where W1 &W2 are total dry weight of plant at times T1 and T2 and p is the land area.

Crop Growth Rate (CGR) (g m -

Results and Discussion 1. Leaf Area Index (LAI) at active tillering stage, panicle initiation stage, physiological maturity stage and at harvest stage Leaf area index increased progressively from the active tillering stage to panicle initiation and declined towards harvest across all genotypes. Significant variation was observed among the evaluated breeding lines during both seasons and in pooled analysis. Among the genotypes, SP-08 and SP-72 consistently recorded higher LAI values at all growth stages, indicating superior canopy development and enhanced photosynthetic surface area. In contrast, IR-64 exhibited comparatively lower LAI values throughout the crop growth period. Higher LAI observed in SP-08 and SP-72 suggests their greater capacity for intercepting solar radiation and assimilating photosynthates, which is essential for biomass production. Similar associations between LAI and productivity have been reported earlier, highlighting the importance of canopy structure in determining crop performance [7] . In experiments of increasing in nitrogen fertilizer rice LAI were raised. LAI trend in different rice cultivars is illustrated in Khazar late maturity cultivar (70 days after transplanting) had the highest LAI and after that Ali-Kazemi and Hashemi early-maturity cultivars (60 days after transplanting). One of the important growth indicators which have been being used as a photosynthetic system measurement is leaf area index (LAI). LAI is related to the biologic and economic yields and increase in LAI causes higher yield [8] . The genotypes SP-08 (2.19) and SP-72 (2.00) showed maximum LAI compared to quality check BPT-5204. While, IR-64 (0.63) showed minimum during the first season. In second season genotype SP-08 (2.54) and SP-72 (2.03) showed maximum value compared to quality check BPT-5204. While, IR-64 (0.64) showed the minimum during flowering stage. Pooled data revealed that the genotype SP-08 (2.34) and SP-72 (2.01) recorded maximum LAI compared to quality check BPT-5204. While, IR-64 recorded minimum (0.33) respectively.

Leaf Area Duration (LAD)

It sounds that nitrogen shortage with reduction in vegetative growth and light use efficiency and as a result reduces in leaf area extension decreased LAD. Maximum LAD in each treatment was obtained about 65 days after sowing. LAD which shows the value of leaf area and photosynthetic tissues durability can be appropriate indicator of production. Correlation of LAD and grain yield is positive and so high and compared to the LAR has more correlation with grain yield [9] since produced leaf area is important for the plant when it has capability to photosynthesis for a long time and a leaf which has not durability is not beneficial for the plant and a plant consumes more energy and photosynthesis assimilates for leaf production so the leaves which have longer life are more capable to compensate consumed photosynthesis assimilates for its production.

Leaf Area Duration (LAD) at active tillering stage, panicle initiation stage at harvest stage and harvesting stage

Significant difference was observed in the genotypes for leaf area duration at active tillering stage (Table 2). The genotypes SP-08 (59 days) and SP-72 (56 days) showed maximum leaf area duration compared to quality check BPT-5204 respectively while, minimum leaf Leaf area duration differed significantly among genotypes at all growth stages. Genotypes SP-08 and SP-72 recorded the maximum LAD values, reflecting prolonged maintenance of functional leaf area. Conversely, IR-64 showed the lowest LAD, indicating faster senescence of foliage. LAD represents the persistence of photosynthetically active leaf area over time and is closely associated with dry matter accumulation. Genotypes with higher LAD are better able to sustain photosynthesis for longer durations, thereby contributing positively to crop productivity. The present findings emphasize the importance of extended leaf longevity in achieving superior growth performance.

Crop Growth Rate (CGR) g m -2 day -1

The phenomena of CGR, RGR, NAR and PR tend to be low again during later stage and negative towards maturity considerably due to several reasons like leaves shading owing to early closure of canopy which hinder solar radiation absorbed by the leaves therefore, less photosynthetic assimilates produced which causes lowering the net assimilation rate, excessive leaf senescence after reproductive stage diminishing photosynthesis rate [9] .

Crop Growth Rate (CGR) g m -2

day -1 at active tillering, panicle initiation stage, physiological maturity stage and harvest stage 30 genotypes showed an increasing trend in the crop growth rate at active tillering stage (Table 3 and depicted Figure 2). Significant difference was recorded among the genotypes for crop growth rate in both the seasons as well as in pooled data. The genotype SP-08 (12.17 g m -2 day -1 ) and SP-72 (10.77 g m -2 day -1 ) showed maximum crop growth rate compared to quality check BPT-5204. While, IR-64 (8.23 g m -2 day -1 ) showed minimum during the active tillering stage in first season. In second season genotype SP-08 (12.47 g m -2 day -1 ) and SP-72 (11.00 g m -2 day -1 ) showed maximum value compared to quality check BPT-5204. While, IR-64 (8.50 g m -2 day -1 ) showed the minimum value. Crop growth rate increased from active tillering to panicle initiation and declined towards maturity in all genotypes. Significant differences were observed among the breeding lines, with SP-08 and SP-72 consistently recording higher CGR values compared to the quality check. The lowest CGR values were observed in IR-64. The reduction in CGR at later growth stages may be attributed to leaf senescence, reduced photosynthetic efficiency, and increased allocation of assimilates towards reproductive structures. Higher CGR in superior genotypes indicates efficient biomass production during critical growth phases, which is essential for achieving higher yield potential.

References

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