GROWTH AND PERFORMANCE OF COTTON IN A DESERT ENVIRONMENT .2. DRY MATTER PRODUCTION

Variety, water and spacing were treatments in two experiments with cotton in 1963 and 1964 in which fruiting points, flowers and bolls were counted and the dry weights and leaf areas of plants were measured at intervals during the season. Until leaf-area index, L,started to decrease, the equation described how dry weight,W,changed. The equation gave smoothed estimates of crop growth rate, C,which were consistent with estimates of photosynthesis made with de Wit's (1965) model. The relationship between G and L conformed to,derivedfrom Beer's Law, rather than C = aL - bL2derived from the linear regression of E on L. When L 3 the crop appeared to use most of the available light, so that C approached a maximum. Treatments initially affected dry-matter production through the numbers and types of branches and nodes, which in turn affected the sinks available and thus the proportion.of dry matter reinvested in new leaf. This initial period, when growth was simple to describe in conventional terms, was denned as the vegetative phase of growth. The start of the reproductive phase of growth overlapped the vegetative phase.The change from one to the other was completed when therateof dry weight increase of the bolls, CB,equalled C. This indicated that the sink formed by thebollshad increased sufficiently in size to use all the assimilates available for growth. Sink size increased as thecrop flowered and was estimated from the product of the number of bolls and the growth rate ofa single boll. When CB equalled C, bolls were shed which prevented the size of the sink to increasebeyond the ability of the plant to supply it with assimilates. This agrees with Mason's nutritional theory of boll shedding.Because of the crop's morphology and because age decreased the photosynthesis of the crop,the size of the sink inevitably increased out of phase with the supply of assimilates.The extent to which this was so determined when CB equalled C. It is postulated that environment,genotype and agronomic practice affect yield according to whether they increase or decrease the extent to which the sink size and the supply of assimilates are out of phase. © 1969, Cambridge University Press. All rights reserved.