The tiller population was maximum at 90 DAP and it was significant among the crops and the varieties studied. Results obtained from the first ratoon and second ratoon crop showed 17.0% and 28.1% reduction in tiller production and 16.5% and 15.7% reduction in NMC over the plant crop. However, the reduction was less than 15% in varieties Co 86032, Co 97008, Co 95020, Co 2000-10 and Co 85019 (Figures 1(a) and (c)). In the first ratoon crop, tiller mortality (30.0%) was almost similar to that of plant crop (29.6%), while in the II ratoon crop, the tiller mortality was 28.50% (Figure 1(b)). Varieties, Co 8021 and Co 97009 recorded significantly higher tiller mortality of 48.3% and 44.0% in first ratoon and 47.7% and 40.0% in second ratoon crop respectively. The results suggested that the production of tiller itself limiting factor for poor NMC in ratoon crops than the tiller mortality per se. Tilllering behaviour of ratoon crops was extensively studied under sub-tropical condition [9,13- 15]. According to them, tillering of ratoon crops was earlier and more profuse than in corresponding plant crop and the early formed tillers have greater chance of survival than the later shoots. Further they reported that the tiller mortality was determined by the time of emergence of individual tillers and tiller growth type. However, the early formed tillers have grater chance of survival than the later shoots. It was even suggested that varietal variation in tiller production was up to 120 - 135 DARI in ratoon crops; however, in plant crop exhibited more such variation during 85 - 90 days after planting [14].

The reduction in tiller production in ratoon crops was

associated with gappiness in ratoons particularly in varieties Co 8021, Co 94012, Co 97009 and Co C 671. Tiller production was highly associated with yield of first ratoon (r = 0.630^*) and second ratoon crops (0.553^*). (Table 1). A significant difference in association of NMC with yield of plant and ratoon crops was also observed. NMC was highly and significantly associated with yield in first (r = 0.750^**) and second ratoon crops (r = 0.809^**). A similar result was reported by [7].

Table 1. Correlation coefficients between physiological parameters with yield of plant and ratoon crops.

^*FPFormative phase, ^*GGPGrand growth phase, NS-Not significant; ^*P = 0.05; ^**P = 0.01.

Data on plant height, leaf production, leaf size and LAI at different growth phases of plant and ratoon crop indicated that the differences in growth parameters at formative phase was narrow than at grand growth and maturity phases. At grand growth phase, the first and second ratoon crop recorded an average reduction in plant height of 12.8% and 22.0%, respectively, over plant crop, while at maturity phase the reduction was 12.42% and 17.8%, respectively (Table 2). Reports were available that the ratoon shoots in initial stages grew faster than the plant crop but the plant crop had overtaken during monsoon season [13,15-17]. Significant variation was observed among the varieties in plant height. Varieties, Co 97008, Co 99004, Co 95020, Co 2000-10 and Co 86032 maintained better stem growth than the other varieties studied. Similar varietal variation in stem growth was reported in sugarcane [7,9]. The ability of varieties to develop shoots quickly and rapidly early in the season in the ratoon crop had a distinct advantage over the plant crop.

Leaf development in sugarcane is important for the study of photosynthesis, canopy closure and light interception. Among the physiological attributes, the leaf area had significant positive association with cane yield and dry matter [18]. In present study, leaf growth parameters viz., LAI, leaf production (data not sown) and individual leaf size was recorded at important growth phases of plant and ratoon crops. However, the magnitude of variation of these parameters in plant and ratoon crops was significant at grand growth phase and maturity phase. LAI of first and second ratoon crop at grand growth phase showed 13.0% and 26.6% mean reduction over plant crop (Figure 2(a)). This might be due to the difference in leaf growth pattern between plant and ratoon crops, i.e., decline in LAI at maturity phase was more sharp in plant crop as compared to ratoon crops (Figure 2(b)). The higher reduction in LAI at grand growth phase was mainly due to the reduction in individual leaf size (29.7%) and leaf number (17.20%) over plant crop. Varietal variation in individual leaf size was highly significant in ratoon crops than the plant crop (Figure 2(c)). Varieties Co 99008, Co 94012 and Co 97009 showed higher reduction in leaf size of 44.3%, 41.5% and 36.5% over plant crop. However, varieties Co 99004, Co 97008, Co 95020 and Co 86032 maintained better leaf growth in terms of LAI, leaf size and leaf production (Figures 2(a) and (c)). Similar varietal variation in leaf growth parameters was reported in sugarcane [7,19]. The rate of leaf production of ratoon crop increased as age advanced and it continued even during the maturity phase of the crop as that of earlier report [17]. A significant association of leaf production with yield between plant and ratoon crops was observed at all the growth phases. In ratoon crops, leaf production (r = −0.415^* and −0.506^*) and LAI

Table 2. Plant height (cm) at different growth phases of plant and ratoon crops.

^*P = 0.05; ^**P = 0.01.

(r = −0.310^* and −0.436^*) were negatively but significantly associated with yield while the plant crop showed non-significant results (Table 1). Hence, the differences in leaf growth among the varieties in both plant and ratoon crops significantly influenced the source-sink relationship.

Total chlorophyll content was estimated at different physiological stages of ratoon crop. A significant reduction in total chlorophyll content was noticed over plant crop; however the reduction was high at early growth phase (26.50%) compared to grand growth phase (22.45%) and maturity phase (18.45%). This is also evident in prominent expression of yellowing symptoms in poor ratooners CoC 671, Co 97009, Co 94012 and Co 99008 (Figure 3). However, in both the ratoons, varieties Co 86032, Co 99004, Co 97008 and Co 95020 maintained higher chlorophyll content. The total chlorophyll content at formative phase was highly significantly associated with yield of first (r = 0.785^**) and second ratoon (r = 0.810^**) crops. Nitrate reductase (NRase) activity at 120 days in the first and second ratoon crops showed 20.5% and 23.5% reduction over plant crop respectively. Varieties Co 97008, Co 85019, Co 99004 and Co 86032 possessed higher NRase activity than the other varieties (Figure 4). Similar varietal variation in nitrate reductase activity between poor ratooners (CoC 671 & Co 775) and good ratooners (Co 8021 and Co 1148) was reported in sugarcane [20].

At early growth phase, production of biomass was comparable in both plant crop and ratoon crops because of faster early growth. However, at the later stages of the crop, the difference in biomass production was marked between plant and ratoon crops (Table 3). At grand growth phase, the reduction in total dry matter production was 23.4% and 32.5% in first and second ratoon crops, respectively. At maturity phase the reduction was 25.1% and 31.0% respectively. Higher reduction in total dry matter production in ratoon crops might be due to higher reduction in tiller production, shoot growth, LAI and higher tiller mortality over plant crop particularly in varieties Co 97009, Co 99008 and Co 94012. A similar varietal difference in biomass production was reported in sugarcane [7,19]. At maturity phase both the ratoon crops showed higher leaf and sheath partitioning efficiency compared to plant crop (Figure 5). However, the partitioning efficiency towards stem was less in first (79.0%)

Table 3. Total Dry matter production (kg·m⁻²) at different growth phases of plant and ratoon crops.

^*P = 0.05; ^**P = 0.01.

and second (72.0%) ratoon crop than the plant crop (89.0%). In both the ratoon crops, varieties Co 86032, Co 97008, Co 99004 and Co 95020 showed higher partitioning efficiency towards stem.

Data on yield and its components viz., NMC, cane length, inernodal length, cane girth, single cane weight and cane yield were recorded at harvest. First ratoon crop showed 16.5, 15.06, 12.5, 6.32, 20.4 & 22.20 mean reductions in NMC, cane length, inernodal length, cane girth, single cane weight, respectively and 27.38% reduction in cane yield. The reduction in yield and yield components was comparatively higher in second ratoon crop, with a mean reduction of 16.0%, 22.4%, 22.3%, 15.2% and 32.4% in NMC, cane length, inernodal length, cane girth, single cane weight. Varieties varied significantly in cane yield and yield components excepting cane girth of both first and second ratoon crops. Varieties, Co 86032, Co 97008, Co 95020 Co 99004 and Co 2000-10 showed better physiological efficiency in terms of plant height, shoot population, leaf size, TDMP, partitioning efficiency, chlorophyll content and nitrate reductase activity and significantly higher yield components (NMC, SCW, cane length, internodal length) and cane yield as compared to poor ratooners Co 99008, Co 94012, Co 8021 and Co 97009. Reports are available that the good ratooning cultivars are those which had attributes for rapid canopy development, early development of adequate stalk numbers for increased interception of light in the early growth, stability of harvested stalk weights to maintain yields over ratoon cycles [8,21,22]. In the subtropical region, sugarcane is harvested under low temperature (early harvesting) and high temperature (late harvest) conditions. The yield of ratoon crop is affected and much influenced by environmental factors [14]. However, in present study Co 94012, Co 97009, Co 671 and Co 99008 were appeared stunted in all the growth phases of ratoon crops which resulted in higher reduction of SCW (Tables 2 and 4). Further, higher reduction in ratoon yield in Co 99008, Co 94012, Co 8021, and Co 97009 (>35.00% reduction) due higher reduction in tiller production associated with stunted plant growth and root system, reduction in individual leaf size and LAI, TDMP, total chlorophyll content, poor NMC, intermodal length and SCW. The result obtained in correlation analysis suggested that the variation in the growth, physiological and yield attributing parameters such as plant height, TDMP, stem partitioning efficiency, leaf size, total chlorophyll content, SCW, cane length and cane girth) were highly associated with yield of first and second ratoon crops than that of plant crop (Table 1). Hence, the difference in the association between physiological parameters with yield of plant and ratoon crops therefore decides the ratooning potential of the crop.