Evaluation of sugar beet plant height variations to growth-degree days (GDD) and its relation to light absorbtion physiological indices.

To study the effect of agronomical (sowing date and plant density) and genetical factors on radiation interception components, two separate experiments were carried out in Motahari Agricultural Research Station (Kamalabad, Karaj, Iran), Sugar Beet Seed Institute (SBSI) during 2005-2007. The first experiment was conducted in 2005 and 2006 on split-split plots based on Randomized Completely Block Design (CRBD) with three replications with two sowing dates as the main plots (as-soon-aspossible date including 04-April and 22-April in 2005 and 2006 and 40 days later including 23-May and 31-May in 2005 and 2006, respectively), four plant densities as the sub-plot (50000, 75000, 100000 and 125000 plants/ha) and three sugar beet monogerm cultivars as the sub-sub-plot (Jolgah, Hybrid 428 and DS 4027). To evaluate the variations of growth components, sampling from the plots was commenced about one week after thinning. The plots were sampled five times during growth period excluding final harvest; each time, leaf area and dry weight of plant parts (including root, crown, petiole and leaf) were measured. Radiation intensity above and below canopy was measured eight times by sun scan meter between 11:00-13:00 in growth period. The results showed that the maximum crop growth rate (CGRmax), root growth rate (RCGRmax) and shoot growth rate (SCGRmax) in Karadj region are 17.15, 13.83 and 5.07 g.m-2.d-1. RCGRmax was gained about seven weeks after gaining SCGRmax which indicated that shoot growth was in priority for crop. The maximum leaf dry weight (282.58 g.m-2), crown dry weight (157.67 g.m-2) and root dry weight (951.14 g.m-2) were obtained about 139, 173 and 165 days after sowing (DAS) following receiving 2926, 3378 and 3248 growth degree days (GDD). The SDWmax (469.23 g.m-2), RtDWmax (1107.26 g.m-2) and TDWmax (1537.38 g.m-2) were gained about 136, 166 and 161 DAS (19 September, 19 October and 14 October, respectively) following receiving 2866, 3266 and 3195 GDD. In the study, the maximum R/S ratio (2.30) was obtained about 182 DASs after receiving 3593 GDD. Also, intercepted photosynthetically active radiation (iPAR) rate (fi) had a significant relation with LAI through an exponential equation (the variation scope of equation R2 was in the range of 0.46-0.67). The response of fi to intercepted GDD was positive and significant (r2=0.99**) but to DAS was non-significant (r2=0.84ns). Mean radiation extinction coefficient (K) was 0.605 in whole experiment and radiation use efficiency (RUE) was estimated as 1.455 g.m-2.MJ-1. Variation trend of LAI, TDW and RUE showed that the increase in LAI led to the increase in TDW and RUE. But the maximum RUE was obtained about 300-600 GDD after obtaining maximum LAI. The results showed that there was a significant linear regression between sugar yield and total iPAR during growth period (r2=0.88*) as well as between total product of mean daily radiation and growth period (number of days) (r=0.90*). But there was not similar relation between sugar yield and GDD and number of days. In this study, the increase in leaf area led to the increase in intercepted PAR. The relation between radiation interception rate and LAI was significant in 2005 (r2=0.82**) and 2006 (r2=0.64*). The maximum radiation interception rate in 2005 (85%) and 2006 (93%) were obtained after receiving 2884 and 2589 GDD (Mid-Sptember) in Karadj region with LAI of 2.5-3.5. The results showed that to intercept 95% of radiation, LAI must be greater than 3.5. In terms of effect of planting date, it was shown that sugar beet sowing in Karadj at the as soon as possible date (Mid-March) compared to delaying it for 40 days only led to increase in intercepted radiation by 425 MJ.m-2; while RUE for producing dry matter which was in the range of 1.426-1.483 g.m-2 did not have a significant difference between these two planting dates. The increase in planting density from 6.0 to 10.5 plants.m-2 led to an non-significant increase in intercepted radiation. RUE did not show a linear response to plant density, too; while the increase in planting density led to the decrease in radiation extinction coefficient and improvement of radiation distribution in canopy. Foreign cultivars had better RUE than domestic ones (1.434 and 1.853 g.m-2.MJ-1, respectively). The main reason was higher RUE of foreign cultivars (1.536 g.m-2.MJ-1) in producing root system than domestic ones (1.098 g.m-2.MJ-1). In terms of RUE in producing shoot system, there was no significant difference between domestic cultivars (0.336 g.m-2.MJ-1) and foreign one (0.317 g.m-2.MJ-1). According to the results it can be drived that considering the superiority of diploid cultivars compared to other ploidy levels, the breeding researches in moderate regions (like Karadj) should focus on breeding diploid cultivars. On the other hand, in these researches improving the main shortage of domestic cultivars i.e. low R/S ratio should be put in priority. Agronomically, appropriate sowing date of sugar beet in moderate regions from early May through late April will help developing canopy and intercepting much radiation in peak radiation period. The most suitable plant density for realizing the maximum root and sugar yield is 90000-105000 plants.ha-1. At last, with comparison of DAS and GDD indices it seems that by applying DAS × PAR index in sugar beet growth models, it will be possible to better adjust yield variations.