Genotype x environment interactions and optimum resource allocation for sugarcane yield trials in Swaziland

To account for genotype x environment (G x E) interactions, sugarcane varieties are tested in multi-environment trials (METs) across multiple crop-years and seasons (early vs. late) under fully irrigated conditions in Swaziland. This study aimed to quantify the main sources of variation relevant to the MET network, identify mega-environments, evaluate adaptability of imported sugarcane varieties, and optimize resource use within the testing network. Nine varieties (eight imported Mauritian and one South African variety) were tested at two sites (Mhlume – M and Simunye – S), harvested early (E) and late (L) in the season across five crop-years in replicated (8) Latin square designs. A linear mixed model analysis of cane yield (tons of cane per ha; TCH), sucrose content (SUC%), sucrose yield (tons of sugar per ha; TSH), and fiber content (FIB%) showed that variety x season interactions were highly significant (P < 0.001) and larger than variety x site interactions for most traits. Genotype + genotype x environment (GGE) biplots revealed clustering of environments into two mega-environments by season rather than by site. Two Mauritian varieties were superior in each of the early and late mega-environments. Broad-sense heritability (H²) for all traits was larger than 0.79, with the highest value observed for FIB% (0.89). Resource optimization analysis, using variance components, suggested that future testing be conducted at a single site, across two seasons, with replicate numbers >5, and harvested across approximately five crop-years. Strong variety x season interactions should be exploited more aggressively in irrigated sugarcane selection programs.     

Interpretation of genotype × environment interactions of sugarcane: Identifying significant environmental factors

An understanding of the causes of genotype × environment (G × E) interactions is essential for the implementation of efficient selection and evaluation networks. Currently, studies involving the interpretation of sugarcane (Saccharum spp.) G × E interactions are limited. The objective of this study was to investigate the relative influence of environmental factors on the G × E interactions of sugarcane under rainfed conditions in South Africa through a comprehensive analysis of a multi-environment trial (MET) dataset. Fifteen commercial cultivars were evaluated over 147 environments (trial × ratoon combinations) across the coastal (C), hinterland (H) and midlands (M) regions of the sugar industry. Environments were characterized according to five site covariates (soil depth, clay percentage, organic matter percentage, nitrogen mineralization category, and total available moisture) and nine seasonal covariates (time of harvest, age at harvest, average daily heat units, solar radiation, rainfall, evaporation, and three derived water stress indices).Additive main effects and multiplicative interaction (AMMI) biplots for cane yield (TCANE), estimated recoverable crystal percent (ERC) and tons ERC (TERC) revealed overlapping of C and H environments, while M environments formed unique clusters characterized by specific cultivar adaptabilities. Principal components analysis (PCA) allowed visualization of the covariates determining the regional separation patterns. AMMI interaction principal components axes (IPCA) 1 and 2 scores were correlated to the covariates and showed that harvest age, temperature, and water stress were mainly responsible for separation of M environments from C and H environments on the TCANE and TERC biplots. Time of harvest was identified as an important covariate influencing ERC G × E patterns in the C and H regions. The third water stress index (based on a ratio of observed yields to simulated irrigated yields) was a dominant factor influencing G × E patterns within the C and H regions and was identified as a superior indicator of water deficient environments for future studies. The M trials were characterized by shallower soils with lower total available moisture and greater variability in this regard compared with the C and H trials. Nitrogen mineralization category, organic matter percent, and clay percent were not significantly correlated to IPCA scores, while soil depth was identified as a major site selection criterion in the M region. The M region should be treated as a single mega-environment, while the C and H regions could be combined for future interpretive studies, where covariates should be summarized within growth phases. The results of this study will assist in restructuring the MET network through exploitation and targeting of the relevant environmental factors within the different regions.     

Agronomic comparisons of sugarcane varieties derived from tissue culture (NovaCane®) and conventional propagation under rainfed conditions

Tissue-cultured (TC) sugarcane (Saccharum spp.) plants have been shown to possess an altered phenotype (high tillering and thin stalks) relative to conventionally propagated sugarcane from setts (Conv) in the first vegetative propagation stage (Stage 1). It is currently unclear if these effects are genotype specific, and whether they would persist in ratoon crops and the subsequent vegetative propagation stage (Stage 2). A field experiment was conducted to ascertain growth and yield differences between TC and Conv plants for different varieties (N12, N31, N41, and N48) across two crops (plant and first ratoon), and two plant spacings (30 cm and 50 cm) in Stage 1. A second field experiment evaluated agronomic performance of TC and Conv crops in Stage 2, established at three planting rates (PRs) (low, medium, and high).

Plants produced from TC in both Stage 1 and Stage 2 produced significantly higher number of stalks that were thinner compared with Conv plants for selected varieties. Cane and estimated recoverable crystal (ERC) yields were statistically similar between TC (Stage 1) and Conv plants, whereas TC (Stage 2) plants significantly improved cane and ERC yields compared with Conv Varieties responded differently to the TC process (N41 and N48 TC plants did not show phenotypic variation relative to Conv). Plant spacing (Stage 1) and PRs (Stage 2) had no significant effects on agronomic traits. The TC plants can be used as planting materials for commercial production without any negative effects on productivity. This is despite the persistence of the reduced stalk diameter and increased stalk population phenotype. Routine screening for phenotypic responses to TC is recommended to sensitize growers to expected phenotypic changes of new variety releases.