Improved understanding of cotton plant response to water stress.

Experiments to establish the response of cotton plant to soil water stress under different soil types,

climatic conditions and fruiting loads have shown that (i) the response of cotton to water stress was

different on different soils (eg heavy clay vs. sandy-loam) (ii) these differences can be accounted for

when soil moisture content is normalised for water holding capacity, expressed as the fraction of

transpirable soil water (FTSW) (iii) that climate, especially evaporative demand, can cause plant

stress even when the crop has adequate soil moisture and (iv) there was no difference in soil water

extraction and therefore root development by crops with different levels of fruit retention.

Field experiments were run over three cotton seasons at three sites with widely different soil types

around Narrabri NSW. The response to the cotton plant to moisture stress, imposed by skipping

irrigations around flowering, was measured as leaf water potential using a pressure chamber. Cotton

plants were found to behave in the same way to moisture stress on all soil types when the soil water

holding capacity of the soil was taken in to account and expressed as a percentage or fraction of

transpirable soil water (FTSW). Over the three seasons, prevailing climatic conditions have a large

effect on the ability of the plant to cope with a given level of soil moisture deficit. Even under low

levels of soil moisture deficit, on high evaporative demand days plants often experienced stress which

would impact on yield. There are some climatic conditions under which cotton plant is unable to take

up enough moisture even from a soil profile with readily available water that the plant will become

stress no matter if more water is applied.

The results of this research will provide a basis for refined irrigation management through

understanding the effect of climate and soil type to reduce water stress and provide decision points for

future management. This information will also be included in all extension methods, especially

decision support systems through inclusion in future versions of HydroLOGIC

A separate experiment conducted over two seasons also in Narrabri showed no difference in soil

moisture extraction and therefore extent of root development between crops that had high and low

levels of fruit retention before cutout. High retention crops (such as BG II®) should be irrigated in a

similar manner to lower retention cotton. The high level of early reproductive development did not

affect root development - activity.

A preliminary experiment was also conducted to investigate partial rootzone drying in cotton. This

showed no benefit from partial rootzone drying in terms of cotton plant stomatal control, biomass

production or yield.

This project has significantly improved our understanding of basic responses of cotton to soil

moisture stress and how this is influenced by climate and soil type. This knowledge is vital in

developing improved irrigation strategies for cotton and achieving maximum water use efficiency.