Height‐to‐Node Ratio as an Index of Early Season Cotton Growth

Because cotton ( Gossypium hirsutum L.) is an indeterminate, tropical perennial plant that is grown commercially as a temperate annual, proper management of vegetative and reproductive growth may produce substantial yield response. In regions such as the semi‐arid and arid West, characterized by long seasons and substantial control over the environment by means of irrigation and fertilization, field‐implementable indicators of early season vegetative source strength can provide a useful management tool. To be useable, baseline data must exist that provide values for these indicators under unstressed conditions. We present arguments in support of the concept of height‐to‐node ratio (HNR) and growth rate (GR) as indicators of the strength of vegetative growth during the early season (prior to anthesis). We analyzed relationships derived from the experimental controls in 13 separate replicated experiments conducted between 1982 and 1989. Relationships between plant height and number of mainstem nodes (MSN) were analyzed and summarized using linear and nonlinear regression. These analyses provide baseline values under unstressed conditions for Acala ‘SJ‐2’ cotton grown in the San Joaquin Valley, California. Research Question Cotton vigor is influenced by the balance between vegetative and reproductive sinks. Evidence suggests that advances can be made to improve seasonal management of vegetative growth and its relationship to hit yield. These advances must take into account the indeterminate nature of cotton development. Plant responses to various environmental factors suggest that the relationship of plant vegetative growth, particularly height and node development, to environmental factors may provide useful indices for crop management. The objective of this study was to establish reference data for two indices, height‐to‐node ratio (HNR) and growth rate (GR), in the San Joaquin Valley, California. Literature Summary During the early season, prior to anthesis, vegetative growth consisting of root and shoot development and leaf growth provides the dominant carbohydrate sink of the cotton plant. The rate of nodal development in cotton broadly supports the concept of temperature‐based physiological time. During the period of plant growth prior to anthesis mainstem nodes (MSN) are produced at an approximately constant rate averaging 1.0 node per 43 degree days. Anthesis begins at approximately 830 degree days and the rate of production of new nodes declines markedly due to increased competition from fruiting structures for carbohydrates. In unstressed Acala cotton, growth in height follows a pattern qualitatively similar to nodal development. At 330 degree days the average internode length is 0.8 in.; this increases steadily for the next 1000 degree days at which point average internode length is 2.3 in. This indicates that a substantial portion of increase in height is due to elongation of previously developed internodes. Average radiation penetration into the canopy is highly correlated with both leaf area index (LAI) and plant height, but more closely correlated with height. Since height is easier and more accurately measured than LAI, this is the better predictor of radiation penetration. Studies reported in the literature support the concept that, during the early season, the rate of nodal development in cotton is less affected by environmental stress than is the rate of growth in height. Leaf removal experiments involving removal of one or both cotyledons and removal of zero, one, or two first true leaves at varying days after emergence indicate that many removal combinations significantly affect plant height, whereas only the most severe removal combinations significantly affect number of MSN. In irrigated production systems, delaying the first irrigation significantly reduces plant height. Study Description Experiments were conducted to establish the number of MSN as a measure of stage of development. In these experiments there were no observable growth limitations due to plant or soil pathogens, insects, salinity, soil compaction, moisture or fertility (i.e., the status of these parameters was monitored and found to be within the guidelines established by the University of California). A reference relationship between height and number of MSN was established by combining results from 13 replicated tests conducted from 1982 to 1989 where Acala ‘SJ‐2’ was grown under these nonstressed conditions. The reference relationship is expressed in terms of two quantities, the HNR and GR. The HNR is computed as height / nodes, where height is the plant height, and nodes is the number of MSN. The GR is then computed as (height 2 , − height 1 ) / (nodes 2 − nodes 1 ), where height, represents the height measured at time i, and similarly for nodes. Applied Questions What is the relationship between height and node development in unstressed cotton? Number of MSN was related to accumulated heat units (R = 0.987) with a mean rate of 43 heat units (base 60°F) per node prior to anthesis. Recommendations Evaluating HNR and GR relative to reference values such as those established here for California can provide a common reference to document the impact of cultivars and stress factors in any environment. Research should be encouraged to document the impact of various forms of stress on these important measures of vegetative growth.