Leaf Structure and Photosynthetic Relationships in Cucumis sativus var. sativus and Cucumis sativus var. hardwickii

Cucurbit Genetics Cooperative Report 16:10-13 (article 5) 1993

J.E. Staub and B.E. Struckmeyer
Vegetable Crops Research, USDA/ARS, Department of Horticulture, University of Wisconsin-Madison, WI 53076 U.S.A.

Cucumis sativus var. sativus L. (cucumber) is widely cultivated in many parts of the world. Cucumis sativus var. hardwickii (r.) Alef. is a fully cross-compatible botanical variety of var. sativus. Variety hardwickii is of potential economic importance since it posseses a multiple lateral branching and sequential fruiting habit not found in var. sativus.

We have been using var. hardwickii as a source of genes for increasing yield potential in commercial cucumber. We have reported that there are morphological differences between these botanical varieties (Schuman et al., 1985). Three morphological plant characteristics and leaf anatomy were evaluated in two different seasons in the greenhouse for each of five var. sativus and five var. hardwickii cultigents. We found inter- and intravarietal differences which were, in the main, consistent acrossed growing environments.

Because leaf anatomy and morphology could be important factors associated with productivity differences between var. hardwickii and var. sativus, we designed a series of studies to more completely define varietal leaf variation and photosynthetic rresponse.

We used the var. hardwickii accessions PI 215589, PI 462369, PI 486336, PI 183967, PI 273648. and :K 91176, and the var. sativus breeding lines WI 1983 and WI 2238. In addition, we also evaluated the var. sativus x var. hardwickii derived (F7) line WI 2863. Accessions and lines were grown in a field nursery and leaf thickness, maximum photosynthetic potential and chorophyll content measurements were taken from each of four plants arranged in a randomized complete block design grown on a m2 spacing. Samples were taken during first flowering, but before the fruits had developed beyond approximately 1 cm in diameter (approximately 45 to 50 days after planting). Maximum pghotosynthetic rate was estimated from leaf slices (discs) prepared from the fourth leaf from the terminal whorl using the technique of Jones and Osmond (1973).

Mating were also made between the var. sativus (P2) breeding line WI 1606 and the var. hardwickii (P1) PI 215589 to produce reciprocal F1 and BC1 progeny. Studies were conducted in a greenhouse and field nursery. Maximum photosynthetic potential and chorophyll content measurements of these cultigens and their cross-progeny were taken from each of four plants arranged in a randomized complete block design on a m2 spacing. Parents, F1 and BC1 plants were flowering and fruits were enlarging (approximately 2 to 2.5 cm in diameter) on the sampling days.

The results of comparisons between var. hardwickii accessions, var. sativus lines and a derived line are given in Table 1 and Figure 1. Microscopic measurements indicate that leafs of the var. sativus lines examined were thicker than var. hardwickii, and the cells of var. sativus were larger with a more open arrangement. Maximum photosynthetic rates was greater in var. hardwickii when compared to var. sativus. LIkewise, the dry weight of chlorophyll (mg/g) of var. hardwickii was often higher (three of five cases) than that of var. sativus. The derived line was usually closer to var. sativus for the traits examined.

Cross-progeny comparisons indicated that measurements taken in the field were higher than those in the greenhouse (Table 2). Reciprocal F1 differences (P1 X P2 . P2 X P1) were also obswerved in the BC1 indicating the cytoplasmic nature of this trait. There appears to be a relationship between the compressed leaf structure of var. hardwickii and its high maximum photosynthetic rate.

Table 1. Means (means) and standard errors (Sx) of leaf thickness, maximum photosynthetic rate ( μ m02 evolved/mg chlorophyll/hr), and chlorophyll concentration of several Cucumis sativus var. sativus and var. hardwickii cultigens and a var. sativus x var. hardwickii derived line [(WI 1606 x PI 215589) F7].

Botanical variety	Line or PI	Thickness (mm)		Photosyn. rate		Chlorophyll (mg/g dry wt.)
						a		b		a + b
		Mean	Sx	Mean	Sx	Mean	Sx	Mean	Sx	Mean	Sx
hardwickii	215589	8.0	0.17	56.50	4.50	8.71	1.36	1.56	0.23	10.27	0.38
	462369	9.3	0.26	51.30	4.27	8.37	0.67	3.11	0.30	11.48	0.21
	LJ 91176	11.9	0.26	48.94	4.54	8.50	0.36	2.16	0.61	10.66	0.07
	486336	10.6	0.06	45.65	2.10	7.20	1.33	1.61	0.77	8.81	0.42
	183967	11.2	0.46	58.80	5.29	10.7	0.04	3.44	0.60	14.14	0.17
	273648	9.6	0.14	46.65	4.14	—	—	—	—	—	—
sativus	WI 1983	16.2	0.55	36.56	3.60	8.60	0.14	1.86	0.02	10.46	0.05
	WI 2238	13.5	0.32	28.30	0.75	7.59	0.65	2.75	0.14	10.36	0.19
Derivative	WI 2963	18.2	0.40	37.96	4.00	8.27	0.83	2.10	0.11	10.37	0.72

Table 2. field and greenhouse mean maximum photosynthgetic rates ( μ m02 evolved/mg chlorophyll/hr) of Cucumis sativus var. hardwickii (PI 215589) and var. sativus line WI 1606 and their F₁ and BC₁ progeny.

	Parents		Maximum photosynthetic rate
			Cross-progeny
Days					BC1	BC1	BC1	BC1
from	P1	P2	F1a	F1b	(P1 x F1a)	(F1a x P2)	(F1b x P1)	(F1b x P2)
plant	215589	1606	P1 x P2	P2 x P1
Field
55	54.24	39.10	105.41	56.28	35.63	31.41	38.45	54.24
56	59.00	36.86	80.77	41.84	32.46	29.30	27.49	30.00
Greenhouse
42	97.14	34.62	—	—	98.26	162.59	86.75	123.50
43	62.17	42.59	91.64	71.52	112.78	143.39	47.04	75.57
50	68.42	39.26	83.78	31.27	126.36	141.07	—	—

Figure 1. A comparison of the leaf structure of Cucumis sativusI var. Ihardwickii (A; PI 486336), cucumis sativus var. sativus (B; WI 2238), and a derivative (C; F7) of the cross between var. hardwickii (PI 215589) and var. sativus (WI 1606) (10 X).

Literature Cited

1. Jones, H.G. and C.B. Osmond. 1973. Photosynthesis by thin leaf slices in solution. I. Properties of leaf slices and comparison with whole leaves. Aust. J. Biol. Sci. 26: 15-24.
2. Schuman, D.A., J.E. Staub and B.E. struckmeyer. 1985. Morphological and anatomical comparisons between two botanical varieties of Cucumis sativus: hardwickii and sativus. CGC Rpt. 8:15-17.