Environmental effects on growth and development of cassava (Manihot esculenta Crantz) : with special reference to photoperiod and temperature

Abstract

Cassava is under consideration as a crop for northern Australia. The main interest lies in the use of storage roots as an alternative starch source to cereals and as a feedstock in the production of alcohol as a liquid fuel. The work reported here was aimed at developing an understanding of environmental effects on growth and development of cassava, as a pre-requisite to the delineation of suitable growing areas and to the development of efficient agronomic practices and yield improvement programs.Patterns of growth and development of the cultivar MAUS10 planted atntwelve monthly intervals were studied under non-limiting moisture andnnutritional conditions at Redland Bay (lat. 27o37'S, long. 153o17'E). Thenchoice of this relatively high latitude site made it possible to examinengrowth and development under a broad range of seasonal conditions. Sequential harvests carried out at 45 day intervals, over a one year period for eachnplanting date indicated important effects of environment, particularly ofntemperature and photoperiod, on patterns of assimilate distribution.nControlled environment studies were undertaken to study these effects.Time to 50 percent of final emergence ranged from 13 to 15 days fornplantings from November to February to 135 days for a May planting. Finalnemergence percent was correspondingly reduced from 98-100 percent to twonpercent respectively. These seasonal differences were related to soilntemperatures and linear heat sums developed for prediction of emergence timesnand selection of optimum planting dates.Canopy development was also highly seasonal with leaf area indicesnranging from above 10.0 for early planted crops in March and April to zeronfor all planting dates in August and September. Controlled environmentnstudies indicated that low temperatures and short photoperiods restrictednleaf production and expansion and, this y combined with shedding of existingnleaf, resulted in reduced LAI over the winter months. Likewisey highntemperatures and long photoperiods promoted leaf production and, expansionnand hence the high levels of canopy growth recorded from January to March.nRegrowth began in September and October and was accompanied by a drop innthe starch or total fermejitable levels in storage roots. First forkingn(i.e. branching associated with improductive development) generally indicated nthat this was due to the long photoperiods of these months. Flowernbuds always accompanied forking but floral development only proceeded in thenfield from March to June and controlled environment studies showed that thisnwas favoured by lower temperatures and shorter photoperiods.Maximum crop growth rates (CGR) calculated from fitted logistic curvesnranged from 23.8 g m-2 day-1 to 2.4 g m-2 day-1 for the various planting datesnand occurred from February to early June, becoming progressively smaller andnlater for later planting dates. Crop growth rate declined to zero or nearnzero from July to September and this, being the period of maximum starch ornfermentable content in storage roots, represented the optimum harvest time.nMaximum CGR's were higher than those reported for cassava at lower latitudesnbut, because of the restricted growing season, annual total biomass yieldsnwere similar and of the order of 30 t ha-1 DW yr-1Multiple regression models were developed which could account fornmuch of the variation in CGR in terms of mean air temperature or solarnradiation and total dry matter (TDM) or leaf area index (LAI). Temperaturenand solar radiation are highly correlated in the Redland Bay environment andnit was not possible to distinguish their separate effects on CGR. Leaf areanindex was considered to have more general application than TDM in such models.Important seasonal differences in the proportion of TDM assimilationnpartitioned to storage organs (referred to as distribution ratio, DR) werenobserved with low DR over the January to March period (0.1 to 0.3) when CGR wasnat a maximum compared with 0.4 to 0.5 in November and December and 0.5 ton1.0 from April to July. This period of low DR restricted storage organ yields which were generally lower (6-9 t ha-1 DW yr-1) than those reportednfor adapted germplasm at lower latitudes. n n n nMultiple regression models were developed which accounted for muchnof the variation in DR in terms of mean air temperature or photoperiod andnLAI. High temperatures, long photoperiods and high leaf area indices were associated with reduced DR. Mean air temperature and photoperiod arencorrelated tn this environment and their separate effects on DR could not bendistinguished. n n n n n n n