The objectives of this study were to investigate the effect of seasonal variation on canopy size, and the effect of canopy size on light penetration of ‘Rayong 9′ cassava under irrigated and rainfed conditions. Rayong 9 was planted under two water regimes in a randomized complete block design with four replications in May and November for two years. At final harvest, years were significantly different (p ≤ 0.05) for biomass, shoot dry weight, and harvest index and contributed to large portions of total variations in shoot dry weight (56.8%) and HI (44.5%). Planting date was a significant source of variations in all measured characters, and it contributed to the largest portions of variations in biomass, storage root dry weight and storage root fresh weight (46.1–60.9%). Water regimes were not significantly different for most characters except for harvest index (p ≤ 0.01). The canopy of the crop planted in May grew rapidly in early growth stages in the rainy season and then slowly after the rainy season. The canopy of the crop planted in November grew rapidly in the middle to the late growth stages. Irrigation did not significantly increase root yield although it slightly increased canopy development, leaf area index (LAI), light penetration and photosynthesis. Irrigation at the late growth stages of the crop planted in May significantly increased storage root yield. Irrigation at these growth stages helped maintain canopy development, LAI and light penetration.

The objective of this study was to investigate the effect of canopy size (CS) on light penetration and leaf photosynthesis of three cassava genotypes (Kasetsart 50 (KU50), Rayong 11 and CMR38-125-77) under two planting dates in two years. Data were recorded for CS, leaf area index (LAI), leaf photosynthesis, biomass (BM), storage root yield (SRY), starch content (SC) and harvest index (HI). The variation of CS depended on temperature, solar radiation (SR), relative humidity (RH) and day length (DL). In early growth stages, the crops planted in May had higher CS than the crops planted in November, because they were subjected to higher temperature, more SR, higher RH and longer DL. In contrast, the storage root accumulation of the crops planted in November was under better climatic factors than the crops planted in May. Therefore, the crops planted in November had higher BM and SRY than the crops planted in May for KU50 and CMR38-125-77. However, Rayong 11 in the May planting date maintained higher CS during storage root accumulation than the other genotypes, and BM and SRY of Rayong 11 were not different for the two planting dates in both years. KU50 and CMR38-125-77 had higher BM and SRY in the November planting, whereas in the May planting, the SRY and BM of the three genotypes were not significantly different. Genotypes that maintained high CS at the storage root accumulation stage could be indirectly selected for high BM and SRY in cassava breeding programs.

Determining the effect of irrigated and rainfed conditions on light interception, light extinction coefficient (k), radiation use efficiency (RUE), biomass, and storage root accumulation of cassava was the objective of this study. The field experiment was arranged in a randomized complete block design (RCBD) with four replications. The effect of irrigated and rainfed water conditions in cassava were evaluated under two planting dates for two years. Light interception depended on k and LAI which affected solar radiation accumulation and thus biomass production for cassava. The k values ranged from 0.49 to 0.93 and 0.46 to 0.86 for irrigated and rainfed crops, respectively. The RUEbi and RUEsr depended on water conditions and crop growth stages and seasons, whereas rainfed crops in the May planting were slightly lower in RUEbi than irrigated crops. RUEbi of the crop planted in November was not significantly different for irrigated and rainfed crops. Irrigation at the late growth stage could maintain higher LAI, light interception, and RUE for the crop planted in May, whereas those in November planting were not significantly different.

The yield potential of cassava might be increased by enhancing light interception and the ability to convert energy into biomass and yield, which is described as radiation use efficiency (RUE). The objective of this study was to determine light interception, extinction coefficient (k), and RUE of three cassava genotypes (Kasetsart 50 (KU50), Rayong 11 (RY11), and CMR38-125-77) under seasonal variations. The field experiments were conducted in a randomized complete block design with four replications, using two planting dates for 2 years at Khon Kaen, Thailand. Data were recorded for weather conditions, light interception, leaf area index (LAI), and biomass. Solar radiation interception, RUE, and k were calculated. Light interception of the crop planted in May sharply increased in the early growth stage, whereas the crop planted in November slowly increased and could maintain higher light interception from the mid–late growth stages. Light interception and LAI had a moderate to high coefficient of determination (R2 = 0.61–0.89) for three cassava genotypes and all planting dates. The k values ranged from 0.59 to 0.94, varying by genotypes and planting dates, indicating that the leaf orientation of the three cassava genotypes was horizontally oriented. The relationship between biomass accumulation and cumulative solar radiation produced a high value of R2 (0.86–0.99). The RUE for biomass (RUEbi) varied by genotype and planting date, ranging from 0.66 g MJ−1 to 0.97 g MJ−1. However, the RUE for storage root dry weight (RUEsr) ranged from 0.29 g MJ−1 to 0.66 g MJ−1. The RUEbi and RUEsr in each genotype on each planting date were significantly different. The highest RUEbi and RUEsr were found at 4–6 and 7–9 MAP for almost all genotypes and planting dates, except for the crop planted in November 2015, when both RY11 and CMR38-125-77 had the highest RUEbi at 10–12 MAP. RY11 had a lower LAI compared to other genotypes, which contributed to lower light disruption and lower RUEbi and RUEsr. KU50 and CMR38-125-77 could maintain canopy light interception during canopy development and storage root accumulation stages and had high RUEbi and RUEsr, resulting in high biomass and crop yield.