Drought Effects on Photosynthesis of Short-Season Cotton Cultivars

T.J. Gerik and K.L. Faver


 
ABSTRACT

Observations of experimental field trials conducted by the Texas Agricultural Experiment Station System indicated that cotton (Gossypium hirsutum) cultivar, TAMCOT HQ95, maintained greater green leaf area under drought than other commercially available cultivars. This study was conducted to determine if TAMCOT HQ95 maintained greater green leaf area under water stress than a commercially available short-season-cotton cultivar (GP74+), and if this were true, to determine if there were differences between the cultivars in carbon assimilation (A) and related components.

The study was conducted at the Blackland Research Center Sheltered Lysimeter Facility near Temple, TX. The shelter houses 16 2.25m3 non-weighing lysimeters containing a Pedernales fine sandy loam soil (fine, mixed, thermic, udic Paleustalf). Two 0.75m rows of HQ95 and GP74+ were planted in each lysimeter with a final plant density of 13 plants m-2. Replicated irrigation treatments of 100, 50, and 0% replacement of depleted soil water were initiated after flowering. Fertilization, and weed and insect control were maintained at optimum levels.

Leaf area development was not different between HQ95 and GP74+ before initiation of water stress. After imposing water stress, GP74+ maintained greater green leaf area than HQ95 under well-watered conditions. Under water stress, HQ95 maintained higher green leaf area for both the 0 and 50% replacement treatments. Larger plant canopies for GP74+ under non-stressed conditions, and maintenance of green leaf area during stress for HQ95 mimic field trial observations.

The relationships betweenA and stomatal conductance of CO2 (g) with leaf water potential (LWP) were highly linear for both cultivars, and as LWP declined, HQ95 was able to maintain significantly higher rates ofA andg than was GP74+, even thoughA was more sensitive to decliningLWP in HQ95.

There were no differences between HQ95 and CP74+ in the response ofA and intercellular CO2 (Ci) tog. For both cultivars, there was an uncoupling ofA fromg under stress, and when this occurred,Ci was lower in HQ95 leaves. Given the fact that HQ95 had higherA andg and a lowerCi under stress than GP74+ suggests that HQ95 had a greater assimilation capacity during conditions of water stress.

The response of leaf water use efficiency (WUE) and intercellular/ambient CO2 (Ci/Ca) ratios withLWP were not different between HQ95 and GP74+. However,WUE was slightly higher, andCi/Ca ratios were slightly lower in water stressed leaves of HQ95, further indication that HQ95 has a greater assimilation capacity under stressed conditions.

Assimilation/intercellular CO2 (A/Ci) measurements were made throughout the experiment to gain a better understanding of how assimilation capacity of the two cultivars was affected by water stress. Initial slopes and maximum assimilation rates from A/Ci curves, collectively, provide an indirect indication of assimilation capacity. Regression analyses revealed that HQ95 maintained significantly higher initial slopes and maximum assimilation rates in A/Ci response across the range of measured leaf water potentials from 1.1 to -3 .2 MPa. Also, the decline in A/Ci initial slopes with decreasingLWP was greater in GP74+ than in HQ95, again suggesting that assimilation capacity is less in GP74+, and is more sensitive to water stress as well.

In summary, GP74+ and HQ95 did not differ in leaf area development before the imposition of water stress. Under water stress, HQ9S maintained higher green leaf area than GP74+; however, GP74+ retained greater green leaf area under non-stressed conditions. These relationships confirm field trial observations. BothA andg were higher in HQ95 than in GP74+ across all measured leaf water potentials, even though assimilation was more sensitive to decliningLWP in HQ95. Assimilation capacity was greater in HQ95 across all measured leaf water potentials, and was also less sensitive to decliningLWP. Thus, the higher assimilation capacity andg of HQ95 allowed for higher rates ofA over the range of leaf water potentials measured.



Reprinted from 1991 Proceedings Beltwide Cotton Conferences pg. 838
©National Cotton Council, Memphis TN

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Document last modified Sunday, Dec 6 1998